The Fall of the Viscount: The crash of Capital Airlines flight 20
On the 18th of January 1960, a four-engine turboprop Vickers Viscount fell from the night sky over eastern Virginia, pancaking into a ravine and taking with it the lives of 50 people. The scene of the disaster provided a number of strange clues, including trees that had punched directly through the plane from below yet remained standing, as though the aircraft had fallen straight down with no forward momentum. Evidence suggested that some, perhaps all, of the engines had failed — and yet none showed any sign of a malfunction. The aircraft didn’t carry any black boxes, and with no survivors to explain what happened during the flight’s final plunge, investigators decided to work backwards from what they had — a motley collection of seemingly random clues — in order to piece together the harrowing events that most likely occurred aboard Capital Airlines flight 20. Their findings would ultimately help reshape several aspects of the way all turbine-powered aircraft are certified, even as the disaster itself has been largely forgotten, along with the pioneering yet troubled airline that it befell.
◊◊◊
The post-war aviation boom in the late 1940s and 1950s was a time of spectacular growth in America’s air travel industry, when many of the nation’s legacy airlines got their wings, transforming from small, ad-hoc organizations into transportation titans. But while some of the major carriers of this period still exist today, others may be less familiar to modern readers — including Capital Airlines, which in the mid-1950s was the fifth largest airline in the United States by passengers carried (a ranking held by Alaska Airlines today). Based at National Airport in Washington, D.C., Capital Airlines pioneered some of the most ubiquitous aspects of modern air travel, including the fold-down tray table and the no-frills economy ticket. And yet, by the end of 1961, it was gone forever.
The story of Capital Airlines’ rise and fall is closely tied to an extraordinary airplane, the Vickers Viscount. Designed in the United Kingdom by Vickers-Armstrongs in the 1940s, the four-engine Viscount was intended to revitalize the British aircraft manufacturing industry and transform the world’s airways — and by many measures it proved successful at both. The Viscount was the first passenger airliner to be equipped with turboprop engines, which use a turbine, similar to a jet engine, in order to drive a propeller. At that time, all airliners were powered by radial piston engines, similar in concept to the internal combustion engines in traditional automobiles. However, these engines, driven by an array of pistons slamming up and down, were loud, inefficient, and unreliable. In contrast, the turbines powering the Viscount’s four Rolls Royce Dart turboprop engines only needed to spin in place very fast, providing greater mechanical energy to turn the propeller with fewer moving parts.
The Dart, like all turboprop engines, has a large, open inlet that that draws air into a compressor section, where the air is pressurized before being fed into the combustion chamber. In the combustion chamber, the pressurized air is mixed with fuel and ignited, causing it to expand. Within the constricted space of the combustion chamber, this expansion translates into acceleration, propelling the air at high speed through the turbine section. The energy from this air spins the turbine, which is connected by a shaft to the compressors and to the propeller gearbox, from which rotational energy is transmitted to the propeller. The propeller is responsible for almost all the thrust generated by a turboprop engine, in contrast to a turbojet engine, which is largely similar but generates thrust directly from the turbine exhaust. The advantage of a turboprop engine over a true jet engine is that a propeller is more efficient at lower speeds and altitudes, making turboprops ideal for short, regional flights.
When the Vickers Viscount entered service in 1953, it was the first turboprop aircraft to carry passengers, and only the second turbine-powered airliner, after the ill-fated de Havilland Comet. In contrast to its pure-jet contemporary, however, the Viscount was highly successful, with nearly 450 examples produced by the time production ended ten years later. The type proved popular with airlines, who praised its efficiency and ease of maintenance, and with passengers, who described it as quiet and comfortable in comparison to piston airliners. Both parties were also enamored with its speed, which was considerably greater than its predecessors.
In 1955, Capital Airlines staked its fortunes on the Viscount in a bid to become the first US airline to operate a turbine-powered aircraft. That year the airline placed a record-breaking order for more than 60 Viscounts, and records show that it ultimately ordered a total of 81, although not all of them were delivered. The arrival of the Viscount on American shores was treated with great fanfare, and the first aircraft to enter service was christened with a champagne smash by second lady (and future first lady) Pat Nixon.
However, while the 48-passenger Viscounts enabled Capital Airlines to rapidly increase its passenger turnover, the airline may have bitten off more than it could chew with such a large purchase. By the start of 1960, it was already clear that the company would struggle to complete the required payments to Vickers-Armstrongs, and to make matters worse, the airline’s Viscount fleet began to suffer from a series of tragic accidents. In April 1958, a Viscount was lost with all 47 passengers and crew near Saginaw, Michigan due to icing of the horizontal stabilizer. Only one month later, twelve were killed in a mid-air collision between a Capital Airlines Viscount and an Air National Guard Lockheed T-33 near Brunswick, Maryland. And finally, in May 1959, another Viscount was lost, along with 31 passengers and crew, due to a thunderstorm-induced in-flight breakup near Chase, Maryland — and on that very same day, two Capital Airlines passengers also died in a runway overrun involving a Lockheed Constellation in Charleston, West Virginia. Even by 1950s standards, such an accident rate was abnormal and clearly unsustainable.
◊◊◊
The last and worst accident to befall Capital Airlines unfolded on the evening of the 18th of January, 1960, aboard a Viscount built in 1957 with registration N7462. Operating as Capital Airlines flight 20, this aircraft was assigned to the second leg of a scheduled Chicago-Washington-Norfolk service departing National Airport at 21:40. A total of 46 passengers boarded, along with four crew, consisting of two stewardesses and two flight crew. In command was 50-year-old Captain James Fornasero, a highly experienced pilot with over 20,800 flying hours, including over 3,500 on the Viscount; he was assisted by 36-year-old First Officer Phillip Cullom, Jr., who had 5,200 total hours and 2,900 on the Viscount. Although the Viscount was the first turboprop either pilot had flown, neither was new to the type and they would have had extensive systems experience.
Prior to departure, the pilots received a detailed weather briefing from a company dispatcher, which included an area forecast issued at 20:30, a flash advisory valid until 22:10, and the 21:00 observed weather at their destination and at intermediate airports. All these reports highlighted poor weather over eastern Virginia, featuring cloud ceilings between 100 and 400 feet with visibility ranging from 1/8 mile (200 m) to 2 miles (3,200 m) in light drizzle and fog. The freezing level ranged from 4,000 to 6,000 feet elevation, and both forecasts and pilot reports warned of moderate to heavy ice accumulation in clouds.
Following takeoff, the flight climbed to an initial altitude of 5,000 feet over Springfield, Virginia, after which Washington area control cleared them to a final cruising altitude of 8,000 feet. From there, flight 20 was to follow a series of victor airways via waypoints Brooke, Tappahannock, and Hopewell, direct to Norfolk, Virginia.
En route, the crew reported their position normally over Brooke and Tappahannock, at which point Washington area control instructed the flight to contact Norfolk over Hopewell at 8,000 feet. The pilot acknowledged the instruction at 22:05. This was the last time anyone heard from flight 20.
Minutes later, in the vicinity of Holdcroft, around half way between Richmond and Norfolk, witnesses heard the sound of a low-flying airplane making several wide, counterclockwise descending circles amid dense clouds and fog. The sound of the engines was considered abnormal by all, as sounds similar to backfiring were heard, as well as at least three abrupt starts and stops, as though some of the engines were cutting in and out. Finally, at 22:19, one last roar of power was heard, followed suddenly by an eerie silence.
◊◊◊
Local farmer Robert Tench was the closest person to the crash — so close, in fact, that smoke drifted past his house, and the glow of flames was visible only about 300 meters distant. Nevertheless, it took him 30 minutes to locate the wreckage of the Viscount in a swampy ravine on his property, close to the Chickahominy River between Holdcroft and Binns Hall.
Several locals made it to the scene before the first responders, where they found the airplane almost completely consumed in flames, lying flat on its belly amid the trees. Some bodies could be seen intact in the wreckage but it was obvious that none of the 50 people on board had survived.
Reaching the site with heavy equipment proved difficult, and the fire was only extinguished some ten hours later, when it burnt itself out, leaving behind only the tail, the nose, the wings, and a pile of charred debris. However, what most drew onlookers’ attention was the fact that the airplane appeared not to have damaged a single tree other than those directly within its own footprint. Furthermore, two tree trunks had punched through each of the wings from below, and another had penetrated the tail section, all of which were still standing straight up, as though the airplane had been dropped onto the forest from a crane. It didn’t take a trained expert to understand that the flight must have had no forward momentum when it hit the ground. But how could that have happened? And how was the plane’s unusual fate connected to the odd behavior observed by witnesses before the crash?
◊◊◊
The crash of flight 20 was the worst loss of life in the history of Capital Airlines, and at that time it was the worst crash in the state of Virginia. Many prominent members of the Norfolk intelligentsia were lost, including bankers, lawyers, and military officers. The local community surrounding the crash site was also left stunned: “That’s too many people to die on one man’s place,” said the farmer, Robert Tench, the following day.
Nevertheless, for investigators with the Civil Aeronautics Board, all of that was secondary. Their primary mission was to solve a complex technical mystery, using only the most rudimentary methods. The airplane didn’t have a flight data recorder or a cockpit voice recorder, nor was it required to have any — at that time, only jets carried FDRs. As for witnesses, those who were there for the start of the flight’s troubles were all dead, and the dense fog prevented ground witnesses from directly observing the airplane in its final minutes.
One clue that the CAB did have was the location of the wreckage itself — some 6.3 nautical miles (11.7 km) east of victor airway 213, which flight 20 should have been following. That the plane deviated so far off course before the crash precluded the possibility of a sudden, devastating event, instead favoring an emergency that lasted for some period of time — which was consistent with witness reports that the plane circled the area at least twice before it came down. And yet, no distress call was issued, so whatever happened to the flight must have occupied the pilots’ attention to an exceptional degree.
Given the witness statements about strange engine sounds, one of the first things investigators checked was the condition of the engines. Although reports of abnormal engine operation from earwitnesses are frequently incorrect, in this case their observations were substantiated right away. In fact, while engines 3 and 4 on the right wing were evidently operating at the time of the crash, the turbines in engines 1 and 2 on the left wing showed no rotational damage, indicating that neither of these engines was developing power when the plane hit the ground. Furthermore, the propellers on both of these engines were found in the feathered position.
As I’ve discussed in many previous articles, the amount of thrust produced by a propeller depends in part on the angle of the blades as they slice through the air. At an angle of zero degrees, the chord of the blades is aligned with the plane of rotation, and no thrust is provided. As the angle increases, thrust also increases, up to an optimum point, above which it again decreases toward zero. The highest possible blade pitch is the “feathered” position, where the blades face the oncoming air edge-first. This is usually around 90 degrees, although on the Viscount the maximum achievable blade pitch was 84 degrees.
Normally, a propeller should be moved to the feathered position in the event of an engine failure in flight. When the turbine stops powering the propeller, the oncoming air will catch the propeller blades, causing it to start windmilling, driving the turbine in reverse. This causes enormous drag that imposes a severe performance penalty. However, this drag can be avoided by feathering the propeller so that the blades face the oncoming airflow edge-first, preventing the air from “catching” the blades. In the clip shown below, filmed on board a Viscount during a demonstration flight, you can see how the propeller immediately stops windmilling when the blades are feathered.
On the Viscount, the propellers can be feathered either automatically or manually.
In the event of an engine failure at low altitude, it’s critical that the associated propeller is feathered as quickly as possible in order to minimize drag and avoid a critical loss of airspeed. For this reason, every turboprop aircraft is equipped with auto-feathering systems in order to feather a propeller immediately following an engine failure. On the Viscount, the propellers would auto-feather if their associated engine’s torque output dropped below 50 pound-feet while the thrust lever was set to an RPM (revolutions per minute) of 13,400 or greater. (For reference, engine rotation speed in the takeoff and cruise phases ranged from 13,400 to 14,500 RPM, while a normal torque output at these values would have been in the vicinity of 160 pound-feet. At an RPM in this range it would not be possible to produce so little torque; thus, a torque output below 50 was consistent with engine failure.) The system worked by transmitting a feather command to the associated propeller control unit (PCU), which would open a control valve, allowing pressurized engine oil to push a piston affixed to the blade pitch change mechanism into the fully aft position, forcing the propeller blades to feather.
Should the need arise, the pilot could also manually open the PCU control valve by moving the associated fuel shutoff lever (the condition lever on modern turboprops) to the “feather” position. This would also close that engine’s high pressure fuel cock, cutting off fuel flow. Alternatively, the pilot could shut off fuel flow and then depress the feather button, which activated an electric feathering pump that boosted oil pressure to drive the piston into the feathered position.
These two methods existed in part because hydraulic oil pressure within the PCU was provided by a separate engine-driven pump that would cease to operate if propeller RPM was too low. Without this pump, opening the PCU control valve wouldn’t supply enough oil pressure to move the piston into or out of the feathered position. The separate electrically-controlled feather pump was there to overcome this vulnerability and provide an alternate means of feathering or unfeathering the propeller.
If the pilot wanted to unfeather the propeller and restart the engine, there were two ways to do that as well. Both were manual methods as there was no such thing as auto-unfeather. The normal way would be to pull the associated feather button outward, which commanded the feathering pump in reverse, pulling the piston out of the feathered position. This action also engaged the air-relight circuit, supplying power to the igniters for 30 seconds in order to re-initiate combustion. Alternatively, at high airspeeds even a feathered propeller could windmill at sufficient RPM to allow the engine-driven oil pump to push the piston out of the feathered position as long as the shutoff lever was not set to “feather.” This allowed unfeathering of the propeller without the electrical power that was required when using the feathering pump.
It must also be noted that the propeller would not unfeather unless the high pressure fuel cock was closed and the thrust lever was pulled back to idle. My sources don’t explicitly state why this was the case but I presume it was because the auto-feather conditions will continue to be met during the restart attempt if the thrust lever is still at the original cruise or takeoff setting.
◊◊◊
With all this in mind, let’s return to the investigative process.
By carefully examining the wreckage of N7462, Civil Aeronautics Board investigators observed that the high pressure fuel cocks on engines 1 and 2 were open. If the pilots had manually feathered these propellers, then the fuel cocks would have been closed. Therefore, it was considered likely that both engines on the left wing auto-feathered while the plane was in flight. However, a detailed teardown of both engines revealed no pre-impact mechanical failures or damage that would have caused a loss of thrust. Furthermore, the Viscount was certified to maintain altitude safely with two engines running and two propellers feathered. So why did flight 20 come down anyway? To answer that question, investigators needed to narrow down what the nature of the engine problem actually was.
In the CAB’s view, the presumably near-simultaneous failure of two engines in cruise flight, without detectable damage on either, had only three plausible explanations: fuel starvation, fuel contamination, or engine icing. However, these first two theories were ruled out because there was plenty of fuel at the scene, the valves allowing that fuel into the engines were open and unobstructed, and the fuel source that was used during the airplane’s stop in Washington tested negative for any contaminants. That left ice as the only reasonably probable explanation.
While many of my articles have talked about the danger of airframe icing, ice can also pose a danger to the engines of a turbine-powered aircraft. In-flight, ice can build up around the leading edges of the air intakes, until it reaches a certain volume and breaks off, plunging into the engine. The type of ice that builds up in flight is unlikely to cause mechanical damage to any part of the engine, but if the water content is great enough, its sudden ingestion into the engine can extinguish the burners in the combustion chamber, causing the engine to flame out like a candle.
Although the actual loss of power in such cases is often transient — the engines can be restarted just fine, and will sometimes do so all on their own — the resultant drop in torque would trigger the Viscount’s auto-feather system. Furthermore, an environmental condition such as ice should affect all the engines almost equally, explaining why multiple engines failed at the same time. In fact, there was no reason to believe that an ice encounter would have affected engines 1 and 2 only — and if engines 3 and 4 also flamed out, then that would also explain why flight 20 was unable to maintain altitude. If so, then the pilots must have gotten these engines running again later, but perhaps they did so too late to prevent the crash.
The weather conditions at the time of the accident further supported this hypothesis. Both the forecast and observed conditions included moderate icing in clouds, and at flight 20’s cruise altitude of 8,000 feet the temperature was -4˚C, well within the most dangerous icing range. Calculations based on Weather Bureau data suggested that an ice buildup of ¼ to ½ inch (6.4–12.7 mm) could have occurred on the engine inlets in the minutes leading up to the crash. Separately, ground tests proved that this amount of ice, if ingested all at once, would be sufficient to cause flame-out in a Rolls Royce Dart engine.
However, such conditions are encountered routinely, and for that reason the Viscount was equipped with an engine anti-icing system designed to prevent ice from building up on the inlets. This system, when manually activated, provided power to a heating circuit within the engine inlets that would melt any ice immediately upon contact. If this system was armed prior to entering icing conditions, then no ice would have built up and the engines would not have flamed out. On the other hand, if it was armed too late, then it could have heated up the bottom of an existing layer of ice, causing it to slough off all at once, which absolutely would lead to flame-out.
In order to understand whether the crew of flight 20 might have delayed arming the system, investigators examined the procedures controlling its use and found some notable discrepancies. Originally, the manufacturer’s procedure called for the crew to arm engine anti-ice when the outside temperature was below 5˚C and there was visible moisture in the air. However, in practice the inlet temperature sensors had a tendency to read too high due to aerodynamic effects, resulting in failure to arm the system at the appropriate time. As a result, in 1958 Vickers-Armstrongs modified the procedure to increase the threshold for arming the engine anti-ice to 10˚C indicated temperature — even though it’s not clear whether this would have made any difference, as investigators pointed out that the system was inhibited above 5˚C indicated temperature even when armed, in order to prevent overheating, and no change to the system’s design had occurred.
In addition to this issue, investigators also noted that Capital Airlines had failed to incorporate the new advice, included under an official revision entitled change 15, into its training manual or its emergency or routine checklists. The airline had received a copy of change 15 some 19 months before the accident, but it simply inserted the revision into the flight manuals carried on its aircraft without ensuring that new pilots were trained on the modified procedure, or that existing pilots were aware of it. Interviews with other Capital Airlines Viscount pilots confirmed that most of them were unaware of the requirement to arm the engine anti-ice at 10˚C. Furthermore, there was some indication that pilots were monitoring ice buildup on the windshield wipers rather than the presence of moisture as the other criterion for arming the system. This could have potentially dangerous effects, because studies had shown that ice on the windshield wipers might not become obvious to the pilot until a considerable amount had already built up on the engine inlets, resulting in delayed arming of the engine anti-ice.
Additionally, there had been a previous accident in Denmark in 1957 involving a British European Airways Viscount operating a cargo flight, in which three of the four engines failed simultaneously while flying in icing conditions. The pilots of that flight managed to make a forced landing in a darkened field, and neither was injured, although the airplane was written off. Much like Capital Airlines flight 20, no pre-impact damage was found that would explain the engine failures, nor was there a fuel issue, so ice was suspected. However, the pilots claimed that they activated the engine anti-ice as soon as they entered icing conditions. In the absence of evidence to the contrary, investigators concluded that there could have been an intermittent failure of the power supply to the anti-icing system that temporarily prevented the heating element from warming up, allowing ice to accumulate. When the system started working again, it could then have dislodged the accreted ice all at once, causing the flame-outs. (There was no specific evidence for this scenario, but in those days it was not considered couth to speculate that the pilot’s memory might be mistaken, even though that explanation was probably equally likely. A modern investigation using the same evidence would probably have left the reason for the ice ingestion undetermined.)
Regardless, the accident in Denmark was yet more evidence of the safety risk posed by delayed activation of anti-ice on the Viscount. And while that was the most prominent event, investigators also identified eight other incidents in which Viscounts lost two or more engines simultaneously due to ice ingestion, caused in every case by delayed anti-ice activation. Clearly this was not a rare or isolated issue.
Vickers-Armstrongs was well aware of these recurring incidents, and as a result, change 15 included several other procedural revisions intended to mitigate the effects of this type of event. Most notably, the new procedures stated that if icing conditions were encountered before the anti-ice was armed, the pilots should arm engine anti-ice in engines 1 and 3 only, then verify normal operation before arming anti-ice in engines 2 and 4. That way, if ice had built up prior to arming the system, it would only be ingested by two engines instead of all four.
Additionally, existing procedures called for the crew to descend to a warmer altitude before trying to relight engines that had flamed out due to ice. However, this provision was not justified by the facts, since the engines could be relit regardless of the temperature. Furthermore, descending required reducing power on the working engines, which would also reduce the amount of ice required to flame these out, too. As a result, change 15 removed this provision and advised that any failed engines could be relit while remaining at cruise altitude.
Of course, as I already mentioned, Capital Airlines pilots weren’t made aware of change 15, nor were their checklists updated, so the crew of flight 20 wouldn’t have used the new procedures. For whatever reason, they presumably delayed arming engine anti-ice until ice had already built up, perhaps because the indicated temperature was erroneously high, or because they were waiting for ice to accrete on the windshield wipers, or because they just weren’t paying attention — we’ll never know. In any case, when they did arm the engine anti-ice, they would have done so on all four engines simultaneously — and if all four engines had already accumulated at least ¼ inch of inlet ice, then all four could have flamed out within seconds. Then, as soon as the engines’ torque outputs dropped below 50 foot-pounds, all four propellers would have independently auto-feathered, and the accident sequence would have begun.
In all previous incidents of multiple engine failures due to ice on the Viscount, the flight crews had no trouble unfeathering and restarting the failed engines. In fact, relighting a flamed-out engine should have been as simple as retarding the thrust lever to idle, pulling the unfeather button, and waiting for the engine to start, a process that should take only a few seconds. However, if all four engines had failed, then restarting the first one would have been quite a lot more difficult unless a successful attempt was made immediately. This was because the unfeather button relied on the electrically controlled feather pump.
The Viscount, like all turbine airliners, was equipped with DC generators that used energy from the turbines to supply electrical power to the aircraft via a series of distribution buses. If the turbine RPM was too low, then the generators wouldn’t be able to produce any electricity. A failed engine could continue to generate electrical power if the propeller was windmilling fast enough to keep spinning the turbine, but a feathered propeller won’t windmill very much, if at all, so if all four engines failed and their propellers feathered, then no electricity would be generated. In that case, the airplane’s electrical system would continue to run off battery power alone.
Tests conducted by the CAB showed that the electrical load on a Viscount flying at night was in the vicinity of 500 amps. That level of current draw would deplete the battery below the voltage required to run the feather pumps after only 90 to 120 seconds. That would have rendered the normal method of unfeathering the propellers by pulling the feather buttons ineffective.
If the pilots quickly selected the emergency power switch, then the electrical system would be run through the emergency bus, ensuring that only essential systems were powered while shedding high-demand equipment like exterior lighting. In that case, the battery’s charge would last for 30 minutes to an hour. But if the pilots were too late switching to emergency power, then they would be out of luck, because the Viscount had no auxiliary power unit.
This was where the outdated emergency checklist procedure to descend to warmer temperatures became so critical. If the pilots followed the checklist and waited to begin the relight process until they had descended to the freezing level, which was at 6,000 feet in the area of the accident site, then the battery would have already depleted before even a single engine relight attempt was made. The crew would not have had a lot of time to recognize the need to flip the emergency power switch. Today, aircraft have emergency checklists for failure of all engines, which would include essential steps such as this, but there was no such checklist in 1960.
The CAB believed that the above sequence of events was the most probable explanation for why the crew was not able to relight engines 1 and 2 immediately after they failed. But how did they get from a controlled descent with no working engines, to an uncontrolled descent with two?
With no electrical power on the aircraft, the unfeathering buttons wouldn’t have worked, and the only way to restart any of the engines would have been to use the previously described windmilling technique to unfeather the propellers. But getting a feathered propeller to windmill fast enough to create sufficient oil pressure to move the blade pitch piston wouldn’t be easy, because a feathered propeller by definition is resistant to windmilling. An airspeed of at least 150 knots was required to get the outboard propellers spinning with sufficient RPM, and 180 knots was required for the inboard propellers. A speed of 150 knots was well within the capabilities of a properly functioning Viscount, but with no engine thrust, the only way to achieve this speed was to enter a dive. Once a sufficient speed was achieved, the engine-driven oil pumps would have come online, and since the shutoff levers should have been in the normal flight setting, the resulting oil pressure should have driven the blade pitch pistons out of the feathered position all by itself. (Recall that moving the shutoff levers to “feather” commands oil pressure to move the piston to the feather position — thus, if the lever is not set to “feather,” the oil pressure should work the other way, when there is any.) Once the propeller was out of feather, it would have started windmilling fast enough to generate electrical power again, at which point the relight circuit could be energized and a restart attempted.
Obviously, this is a lot to think about while in a dive from a height of only 6,000 feet. Furthermore, this procedure would have to be accomplished twice before the crew could level off, because the Viscount was incapable of maintaining altitude on only one engine.
Investigators believed that the pilots likely managed to get the №4 engine started via this method as the aircraft was in a rapid descent toward the ground. The pilots would then have commanded maximum power on this engine in an attempt to slow their descent rate. However, with one engine on the right side at max thrust and no engines on the other side, the yawing moment would have caused the airplane to embark on a wide, counterclockwise circle. Test flights after the crash confirmed that in this configuration the plane would slowly turn left even with the application of full opposite aileron and rudder. This was likely why ground witnesses heard the plane circling counterclockwise overhead several times before it crashed.
With engine 4 running, electrical power would have been available to restart the other engines normally. However, with the plane spiraling steadily downward, the crew did not have much time to do so, perhaps a couple of minutes at most. Evidently, they managed to start engine 3, given that it was running at impact. However, the addition of another running engine on the same side would have made the controllability problems even worse.
The problem now was that the aircraft’s speed would by this point have decreased quite substantially as the pilots sought to avoid descending into the ground. Physics demand that when thrust is insufficient to maintain altitude, then any attempt to maintain altitude anyway will result in a loss of speed. As speed decreases, so does control authority, which depends on airflow velocity over the control surfaces. And decreased control authority means that the amount of asymmetric thrust that the controls can compensate for decreases. Therefore, at low airspeed, advancing two engines to full power on the same side, with two failed engines on the other side, will result in a loss of control instead of an increase in speed. Pilots call this a “VMC roll” (VMC being minimum control velocity) and it is essentially always fatal.
However, the evidence showed that flight 20 didn’t enter a VMC roll — instead, it crashed with the wings and nose approximately level. Investigators believed that the pilots, being aware of the risk of a VMC roll, likely decreased thrust to idle on both working engines in an attempt to prevent a loss of control from occurring before the other two engines could be restarted. If engines 1 and 2 could be started, then the risk of loss of control would go away; they could push all the engines to full power; and they could climb away from the ground in the nick of time.
The blade pitch measured on engine 3 was consistent with flight idle power at impact, and the pitch on engine 4 was only slightly above this, strongly supporting this hypothesis. There was no other plausible reason why the pilots would idle their working engines so close to the ground unless they feared a VMC roll.
Unfortunately, the evidence proves that the crew did not manage to start their remaining engines. Most likely, they just ran out of time.
As for the witness reports of sharp backfiring sounds, investigators believed that this was likely due to explosive ignition of accumulated fuel within the engines during the successful restart attempts of engines 3 and 4. During previous unsuccessful relight attempts — for example, while descending with no electrical power — fuel would have been supplied to the engines, but if it did not ignite, it required two minutes to drain away. In an emergency such as this one, the crew couldn’t have waited for two minutes, so they would have initiated another relight attempt right away, causing the accumulated fuel to combust explosively. This would have made a lot of noise but wouldn’t have damaged the engines or prevented them from starting.
As the aircraft descended below the cloud base somewhere beneath 400 feet, the pilots would have understood that ground impact was imminent. At that point they might have pulled up sharply in order to avoid impact, only to stall the aircraft, as they had insufficient thrust to climb. The airplane would then have lost speed, yawed, and pancaked to earth in a flat spin, where it was impaled by trees. The pilots probably increased thrust in engines 3 and 4 just before impact, which was heard by witnesses, but there was only enough time for the №4 propeller to move slightly above the flight idle pitch before the plane hit the ground, instantly killing everyone on board.
◊◊◊
The sequence of events outlined by the CAB painted a picture of a flight crew whose harrowing final moments must have been filled with both urgency and terror. A complex and unexpected series of problems sent their plane plummeting from 8,000 feet as they fought to bring systems back online in a desperate race against time. And yet, from beginning to end, there was nothing mechanically wrong with their aircraft.
The CAB’s version of events is necessarily speculative — while it’s consistent with the observed evidence, it can’t be conclusively proven. If the plane had been fitted with a cockpit voice recorder, it might even have revealed that the crew’s performance was less than perfect, and that they stalled the plane earlier in the timeline than investigators thought. But mistakes such as these were not necessary to explain what happened. Even a crew that followed their procedures perfectly still could have ended up pancaking into that benighted ravine by the Chickahominy River.
The fact that those procedures directly contributed to the accident was the fault of Capital Airlines, whose lazy implementation of change 15 allowed outdated and potentially hazardous checklists to remain in use. But much of the responsibility for the accident lay with Vickers-Armstrongs and the aviation authorities in the UK and the US who certified the Viscount.
The first issue with the Viscount was the isolation of its propeller auto-feathering systems. Although isolating safety systems is normally good practice, auto-feather systems are an exception. If a transient event affects torque output on all four engines, then independent auto-feathering systems will feather all four propellers simultaneously, leaving the plane with no thrust, even if the engines otherwise would have recovered by themselves. This is what probably happened to Capital Airlines flight 20.
On the other hand, US regulations at the time required that engine systems be isolated such that “a failure of one engine will not prevent the safe operation of the remaining engines.” Some interpretations of this rule would prohibit an auto-feathering system that can feather all the propellers during a common-cause event. One way to prevent this is to design an interconnected auto-feathering system that can only activate on one engine at a time. That way, in the event of a loss of torque on all engines, only the first engine to experience a power loss will auto-feather. The other engines will then recover, or if they don’t, the pilots can feather those propellers manually. Had the Viscount been designed in this manner, the crash of flight 20 almost certainly wouldn’t have happened.
The Viscount was originally certified by the British Air Registration Board, which had a similar regulatory provision. However, this potential area of non-compliance was not detected. The aircraft was subsequently approved for use in the United States even though this particular design requirement was arguably not met. This was presumably because the US Civil Aeronautics Authority (the predecessor to the FAA) had an agreement of mutual trust with the British Air Registration Board that eliminated any requirement for the CAA to conduct a full certification examination of aircraft types already certified by the ARB. Although such agreements are still in use, it’s unlikely that both agencies would miss something this significant today. And in any case, a limit on the number of propellers that can auto-feather has now been standard for decades.
The second design issue that contributed to the accident was the lack of any safeguard against the effects of delayed arming of the engine anti-ice system. Whether this delay was due to a mechanical failure, or inadequate procedures, or crew inattention was immaterial. Any ice protection system requiring crew activation is occasionally going to be left off when it should be turned on, or vice versa, and the airplane must be designed with this in mind. The fact that late activation of the engine anti-ice on the Viscount could cause all four propellers to feather was unacceptable by modern standards. However, the regulations weren’t always so clear-cut. Aircraft with radial piston engines weren’t seriously affected by engine ice because they didn’t require large air intakes to spin a turbine, and as such there were no regulations pertaining to engine ice ingestion when the Viscount entered service. Such regulations were eventually put in place in 1958, but by then hundreds of Viscounts had already been built and delivered.
Today, manufacturers are required to prove that there will be no hazardous effects if engine anti-ice activation is delayed by two minutes in the most adverse icing conditions. Furthermore, many airliners now have engine anti-ice systems that don’t require crew intervention at all. (The Boeing 737 is of course the one big exception, because its 1960s-era crew-activated engine anti-ice system has been repeatedly grandfathered in with no automatic mode for the last half century.)
◊◊◊
Although the crash of flight 20 brought attention to these safety issues, Capital Airlines itself didn’t survive to see their eventual solutions. In May 1960, just five months after the accident, the airline missed payments on its massive Viscount order, and Vickers-Armstrongs filed suit to foreclose on its fleet, ultimately seizing dozens of aircraft. The following year, Capital Airlines was sold to United in order to avoid bankruptcy, and the brand was dissolved.
Well over six decades have now passed since both the accident and the closure of the airline, consigning both to history as those who remember the tragedy grow ever fewer. It would however be a different sort of tragedy to forget the 50 lives that were lost, and the pilots’ doomed struggle to save them. It would also be irresponsible to forget the very real devastation that underpins the otherwise abstract design requirements discussed in this article. Most aviation regulations, if you trace them back far enough, eventually lead to the scene of a disaster — occasionally a well-known one, but more often something obscure, like a Vickers Viscount burning in a Virginia woodland. If there’s any use in retelling the story of that aircraft’s harrowing demise, then it must lie in the reminder that the safety we enjoy today was purchased with the blood of prior generations.
_______________________________________________________________
Don’t forget to listen to Controlled Pod Into Terrain, my podcast (with slides!), where I discuss aerospace disasters with my cohosts Ariadne and J! Check out our channel here, and listen to our latest episode, in which we join forces with an Airbus captain to discuss Air France flight 447. Alternatively, download audio-only versions via RSS.com, or look us up on Spotify!
_______________________________________________________________
Join the discussion of this article on Reddit
Support me on Patreon (Note: I do not earn money from views on Medium!)
Visit r/admiralcloudberg to read and discuss over 250 similar articles
