A Snowy Surprise: The crash of Air Canada flight 646

Admiral Cloudberg
28 min readMar 25


This striking image of a tree inside the cabin of Air Canada flight 646 was taken the day after the crash by press photographer Noel Chenier.

On the 16th of December 1997, an Air Canada regional jet approaching New Brunswick’s capital city attempted to abort a misaligned approach amid darkness and freezing fog. But as the pilots pulled up to climb away, the stall warning activated, the plane slewed hard to the right, and the nose slammed suddenly to the ground, sending the plane careening off the side of the runway and into a snowbound forest, where it slammed into a tree and ground to a halt. Inside the plane, the tree carved a path of destruction through several seat rows, trapping passengers amid the wreckage; outside, frigid temperatures and low visibility hampered rescuers’ efforts to reach the survivors. But when they finally located the plane, the first responders discovered that a miracle had occurred: despite the violence of the crash, all 42 people on board had survived.

The pilots, shaken but not badly injured, explained to investigators that as soon as they attempted to abandon the approach, events went sideways so fast that there was no time to take action. Flight data confirmed that moments from touchdown, the captain called out “Go around,” and just three seconds later, the plane was out of control. And yet, no malfunctions had occurred — so what went wrong? The answer was that the pilots of Air Canada flight 646 had unwittingly put themselves in a situation outside their airplane’s demonstrated maneuvering envelope, where ingrained procedures were inapplicable and the margin of error was greatly reduced. It was at that moment that an insidious but all-too-common danger­ — ice on the wings — did them in. But perhaps the most important conclusion was that the only way for the pilots to get out of the situation would have been to avoid getting into it in the first place. For that reason alone, the story of flight 646 is one that any safety-conscious pilot should keep in mind any time they make an approach in bad weather.


C-FSKI, the Air Canada Bombardier CRJ-100 involved in the accident. (Robert M. Campbell)

Air Canada flight 646 should never have been a remarkable flight. The trip was merely one of hundreds carried out every day by Canada’s largest airline and flag carrier, connecting cities across a vast and sparsely populated nation. In fact, the flight number 646 is still in use today on Air Canada flights from Toronto to Moncton, New Brunswick, now operated by an Airbus A220. In 1997, flight 646 was no less unassuming, but it was a little different: the route ran from Toronto not to Moncton, but to Fredericton, the provincial capital and third largest city in New Brunswick, using a Canadian-built Bombardier CRJ-100 twin rear-engine regional jet. With room for 50 passengers, the small jet was ideal for flights to Fredericton, whose population of only about 50,000 made it far from a mainline destination.

The approximate route of flight 646. (Own work, map by Google)

On the night of the 16th of December 1997, 39 passengers boarded a CRJ-100 at Toronto Pearson International Airport for the last flight of the day to Fredericton, scheduled to arrive shortly before midnight local time. They would be joined by a single flight attendant, seated in the forward galley, and two pilots, both of whom were still fairly early in their careers. At 34 years old, Captain Donald MacFarlane had already racked up 11,000 flying hours and had been a CRJ captain since October 1996. Experienced for his age, he was probably expecting a long career that would take him to the top of the company. His copilot, on the other hand, was rather green: 26-year-old First Officer Jeffrey Cyr had until recently flown only light aircraft, Pipers and Cessnas, before entering Air Canada’s CRJ training program. By December 16th he had just 60 hours on the CRJ, his first jet aircraft, and it had only been one week since his release for unsupervised flight.

The two pilots had already flown two legs together that day, alternating the roles of pilot flying and pilot monitoring. The two roles are well-defined in standard operating procedures and can be assumed by either pilot, so captains normally allow first officers to act as pilot flying on every other leg, conditions permitting, in order to gain experience. For flight 646 to Fredericton, it was First Officer Cyr’s turn to fly. The weather forecast at their destination called for a visibility of 1 to 3 miles (1.6 to 4.8 km) in light snow and fog, with a cloud base at 400 feet, but these were well above the minimum values for the approach to the airport, which called for a runway visual range (RVR) of at least 2,600 feet with a decision height of 200 feet.

At 21:24 local time, flight 646 departed Toronto and climbed to its cruising altitude, heading northeast across Ontario and Quebec. Later in the flight, however, the pilots began to receive word that weather conditions at Fredericton were deteriorating. About 45 minutes after takeoff, Air Canada dispatchers sent a message through the plane’s Aircraft Communications and Reporting System, or ACARS, informing flight 646 that the visibility at Fredericton was now ¼ mile (400 m) in fog, with a vertical visibility of 100 feet and a runway visual range of 1,000 feet. Minutes later, flight 646 made contact with the Flight Service Specialist at Fredericton Airport and received the latest weather update. The specialist, or FSS, was not a full controller and could not give clearances to aircraft, but he could provide weather information. The news he brought was more positive than negative: visibility had fallen to 1/8 mile in fog, but the RVR had increased to 1,200 feet. Although the minimum RVR shown on their chart was 2,600, an RVR of 1,200 was actually sufficient for them to attempt an approach, for legal reasons which will be explained in detail.

A breakdown of Category I, II, and III approaches, and how they relate to RVR. Note that according to international standards, the minimum RVR for a Category II approach is 350 m, not 300 m as indicated above. This chart appears to be referencing local standards in India, but otherwise it’s accurate enough for our purposes. (SP’s AirBuz)

First of all, it must be mentioned that in aviation, “visibility” and “runway visual range,” or RVR, have specific definitions and are not the same. The visibility refers to the maximum distance from which an object can be seen, while RVR refers specifically to the distance from which the runway lights can be seen, which is often higher than the generic visibility due to the intensity of the lighting.

The reported visibility and RVR are important from a procedural standpoint because they determine whether an approach to land can even be attempted. During an instrument landing system (ILS) approach, the exact minimum allowable visibility and RVR values depend on the approach category, which in turn is based on the precision of the equipment on the ground, the type of equipment on board the plane, and the pilots’ qualifications. A Category III approach, the strictest type, allows a properly equipped aircraft with qualified pilots to land in near-zero visibility, but can only be attempted at major international airports that have sufficiently precise ILS equipment. The Air Canada CRJ-100 and its pilots, on the other hand, were qualified for approaches up to Category II, which comes with a minimum RVR of 1,200 feet (350 m) and a decision height between 100 and 200 feet. (As the name implies, if the runway is not visible at the decision height, then the approach must be discontinued.) However, Fredericton International Airport had only the most basic instrument landing system, allowing only Category I approaches, which come with a minimum RVR between 1,800 and 2,600 feet (550 and 800 m) and a decision height of at least 200 feet.

In practice, many airports do not measure RVR, as specialized equipment is required. In contrast, any towered airport can measure standard visibility using either sensors or human weather observers. Approach charts therefore provide both a minimum RVR and a minimum standard visibility for a given approach. However, there is no law which says you can’t attempt an approach when the reported visibility or RVR is below the number on the chart, even if a successful landing would be improbable. Instead, most countries have a blanket legal minimum for all approaches of a certain category. For instance, in the United States at that time, no pilot could commence a Category I ILS approach unless the visibility (not RVR!) was at least 1,800 feet (550 m). However, in Canada in 1997, the only limit was on RVR: at an airport equipped with RVR sensors, a Category I approach could be attempted if the reported RVR was at least 1,200 feet (350 m). The standard visibility was advisory only, and if there was no RVR equipment at the airport, then there was also no minimum visibility for attempting an approach, regardless of the odds of success.

The red circle shows the location of the visibility minimums on the approach chart for runway 15 at Fredericton. “RVR 26 or ½” means “RVR 2,600 feet or visibility ½ mile.” (TSB)

In the case of flight 646, the pilots were planning to execute a Category I ILS approach to runway 15 at Fredericton. The visibility minimums for this approach were 1/2 mile (800 m) or an RVR of 2,600, but the weather update sent by the Flight Services Specialist indicated that the actual conditions at Fredericton were 1/8 mile visibility and 1,200 RVR. Airlines sometimes prohibit their pilots from attempting an approach when visibility is below the minimum indicated on their charts, but Air Canada was not one of them, so the legal minimum applied. Under Canadian law, with an RVR of 1,200 feet, they were allowed to attempt an approach in the hope that conditions might improve enough for them to see the runway. In the United States, by contrast, they could not have attempted an approach at all because the standard visibility was less than 1,800 feet (1/3 mile).

With all this in mind, Captain MacFarlane had to consider whether it was appropriate for his inexperienced First Officer to fly the approach. After all, with an RVR of only 1,200 feet, it would be difficult to land. But First Officer Cyr assured him that he had completed approaches in similarly poor visibility on his previous aircraft, and Captain MacFarlane had already judged him to be a competent pilot, so they agreed that Cyr would remain in the role of pilot flying.

Shortly after 23:30 local time, flight 646 descended toward Fredericton, lined up with runway 15, and commenced the ILS approach. The autopilot locked onto the localizer and glide slope, keeping the plane straight with a steady descent angle, aimed directly for the runway touchdown zone, which lay hidden beneath a layer of fog and low clouds.

A view like this one would have greeted the pilots as they neared the runway. (Luca Ventura)

As the flight approached the decision height of 200 feet, Captain MacFarlane began searching visually for the runway while First Officer Cyr continued to ensure that the plane was on course. Despite the bad visibility, MacFarlane managed to spot the high-intensity approach lights gleaming faintly through the fog, and at the decision height he called out “lights in sight.” First Officer Cyr looked up, spotted the lights, and then disconnected the autopilot at a height of 165 feet to finish the landing manually, which was the normal practice at Air Canada. It was then that things first started to go wrong.

As First Officer Cyr attempted to aim the plane at the touchdown zone, he faced several difficulties. For one, the airport was not equipped with high-intensity centerline and touchdown zone lighting, so it was difficult to tell through the fog whether he was lined up properly. And on top of that, the wind at decision height was about 10 knots from the right, transitioning to dead calm at ground level. After initially compensating for the wind with a slight right yaw, he soon found himself drifting too far to the right once the wind went away. Simultaneously, the plane began to drift above the glide slope, and MacFarlane urged Cyr to keep the nose down. Cyr responded with an initial pitch down; seconds later, the plane crossed the runway threshold, and he reduced engine power to idle for the imminent touchdown. But because the CRJ’s engines are mounted above the center of gravity, reducing thrust tends to result in a pitch up motion, so the plane’s pitch started to creep up once again. Observing that the nose was again too high, Captain MacFarlane repeated his order to keep it down. At the same time, realizing they were drifting right, First Officer Cyr used the rudder to steer left, briefly aligning with the centerline of the runway before the plane started to slip too far in the opposite direction.

“Fifty,” an automated voice called out, reading off their altitude above the runway.

The plane was too high, would likely overshoot the touchdown zone, and was significantly left of the centerline. Both pilots suddenly realized that it would be impossible to land safely, and First Officer Cyr was about to take action himself when Captain MacFarlane bit the bullet: “Go around,” he ordered.

An example of a flight director indicating 10 degrees pitch up. (aircraftsystemstech.com)

With their plane descending through 33 feet above the ground, the pilots took immediate action. First Officer Cyr acknowledged the Captain’s order and pressed the go-around switches, putting the flight computer in go-around mode, while Captain MacFarlane slammed the thrust levers to maximum power. Entering go-around mode, the flight director — an overlay on the pilots’ primary flight displays indicating whether to fly up, down, left or right — began commanding a pitch up to 10 degrees, the nominal pitch angle during a go-around on the CRJ-100. First Officer Cyr immediately pitched up to follow the flight director indication, in accordance with his training.

The trajectory of the aircraft across the ground, from first impact to final resting place. The numbers refer to items of debris; a corresponding key can be found appended to the end of the official accident report, available here: http://www.bst-tsb.gc.ca/eng/rapports-reports/aviation/1997/a97h0011/a97h0011.pdf (TSB)

Practically the instant he did so, events went sideways with astonishing speed. Barely one second after Cyr started pitching up, the stick shaker stall warning unexpectedly activated, literally shaking the pilots’ control columns to alert them of an impending stall. Captain MacFarlane announced that he was retracting the flaps in accordance with the standard go-around procedure, but before he could even address the sudden activation of the stick shaker, a repetitive warbling alarm sounded, indicating that if the pilots did not take immediate action to prevent the stall, an automatic safety system called the stick pusher would do it for them. But before even the stick pusher could activate, the right wing stalled, lost lift, and dropped toward the ground. Caught completely by surprise, the pilots could scarcely react as the plane suddenly banked 55 degrees to the right, causing the wingtip to strike the runway in a shower of sparks. First Officer Cyr attempted to regain control, but it was too late: the nose pivoted down, the wing lifted up, and the plane slammed head-first into the asphalt with a tremendous crunch. The nose gear ripped away, breaching the avionics compartment, and the right wingtip separated; the plane then veered hard to the right, skidding uncontrollably off the side of the runway.

Upright and with both engines at full power, flight 646 careened into the snow beside the runway and smashed hard into a ditch, causing the gear to collapse. The impact sent the plane bouncing momentarily back into the air, where it streaked across a field just above the ground, dangling equipment dragging through the snow beneath it. Moments later it clipped a small hill, slewed nose right, and finally plowed headlong into a forest, where it struck a large tree and ground abruptly to a halt.


The tree inside the cabin made for quite an astonishing sight. (Noel Chenier)

In the passenger cabin, there was no warning that the plane was about to crash — only the frightening screech of the wingtip hitting the runway, followed by the flight attendant’s frantic shouts to “brace, brace, brace!” At the first big impact the lights went out, plunging everyone into darkness as the plane skidded across the field, until at last it came to a stop, and the dim emergency lighting blinked on. It illuminated a shocking scene: a tree 56 cm (22 in) in diameter had entered the fuselage near the front left passenger door, from which it tore a path of destruction through the first several rows on the left side, ripping seats out of the floor, before coming to a halt part way into the center aisle in the vicinity of row 4. Several passengers who had been struck by the tree were seriously injured, and some were trapped amid the overturned seats and mangled panels.

Up front, Captain MacFarlane attempted to shut down the engines using the emergency shutoff switches, but the switches didn’t work because the plane had no electrical power. Instead, he tried retarding the thrust levers to the shutoff position, but only the right lever would move. The left throttle cable was wrapped around the tree, putting it in tension, and MacFarlane was unable to move it off of full power.

Without waiting for a command from the Captain, the flight attendant ordered an evacuation, and those passengers who could do so made their way out of the plane, mostly through the overwing exits, despite the proximity of the still-running left engine. The flight attendant followed them out into the snowy night, where he could see the distant lights of rescue vehicles moving up and down the runway, accompanied by sirens. He tried to signal to them using a flashlight, but his efforts went unheeded.

An aerial view of the plane in the forest. (Bureau of Aircraft Accidents Archives)

As soon as the plane failed to announce its landing and stopped responding to radio calls, the Flight Services Specialist sent the airport’s lone firefighter to search the runway for the aircraft using the airport’s sole fire truck. The airport normally had two firefighters, but the second one had gone home about 20 minutes earlier due to illness — truly impeccable timing — so the airport’s maintenance foreman went out with a second vehicle to assist. But after driving all the way down the runway and back — at low speed to avoid running over any survivors — they found no sign of the plane. Therefore, at 23:58, about 10 minutes after the crash, the full emergency response plan was initiated, and firefighters and police officers from Fredericton rushed to the scene to join the search for the missing airplane.

Meanwhile, Captain MacFarlane finally managed to shut down the left engine by bracing his foot against the instrument panel and using his entire body weight to pull the thrust lever. With that difficult task finally out of the way, the pilots left the cockpit and entered the cabin, where they encountered several passengers who were trapped in the wreckage. One passenger in particular was stuck with one hand pinned between a seat and the side of the fuselage; the pilots attempted to free it by prying the seat away from the wall using the handle of a crash axe, but the handle swiftly broke.

Outside the plane, the primary flight attendant was joined by an off-duty flight attendant who happened to have been riding as a passenger, providing some much-needed assistance. The off-duty flight attendant conducted a head count while the on-duty flight attendant gave his flashlight to a group of passengers and instructed them to head toward the runway in search of help. Leaving the off-duty flight attendant in charge, he then went inside the plane to assist the pilots with the rescue.

Rescue workers at the scene of the crash. (CBC News)

It was not until 00:06, around 18 minutes after the crash, that an RCMP officer arriving on the scene encountered a passenger walking through the snow near the runway, followed shortly thereafter by a large group of 15–20 survivors, including a woman with a baby, none of whom were dressed for winter conditions. While the officer assisted the passengers to the terminal, the airport firefighter continued onward to the plane, where he encountered all three crewmembers attempting to free seven trapped passengers. Word went out over the radio calling for the jaws of life, but before sophisticated rescue equipment could reach the plane, a path needed to be cleared through the snow. It took some time for a snowblower to make its way to the scene, but once it did, the rescue began in earnest, as firefighters used the jaws of life to pry away the wreckage and free those still on board. The last survivor was extracted at 2:34, almost three hours after the crash. No victims were located, and although a headcount came up two people short, it was eventually discovered that the missing passengers had simply gone home, and everyone was accounted for. Indeed, after a long and harrowing night, the crew and the rescuers alike were relieved to learn that although nine people were seriously injured, everyone aboard Air Canada flight 646 had survived.


Noel Chenier’s image of the tree in the cabin ended up on the front pages of several papers, including the New Brunswick Telegraph Journal. (Noel Chenier)

The following morning, investigators from the Transportation Safety Board of Canada arrived at the scene to begin a major investigation into the causes of the near-disaster. It was apparent that the plane had struck the runway twice, then veered to the right through a field and into a forest, for reasons unknown. But when the pilots provided their testimony, it only raised more questions. According to them, the plane was at about 50 feet when they realized they would land too far down the runway and too far to the left, at which point Captain MacFarlane ordered a go-around. Then, as soon as they tried to climb away, the stick shaker activated, the right wing stalled, and the plane went out of control. As for why this had happened, the pilots were as mystified as anyone else — as far as they knew, they had followed standard procedures to the letter.

Only after many months of flight data analysis, simulator testing, and mathematical modeling would investigators be able to resolve this question. Their eventual answer would bear lessons for the pilots of every passenger jet.

The tail of flight 646 lies atop a pile of snow. (CBC News)

At the core of the question was why the plane stalled during the go-around, even though First Officer Cyr had simply been following the pitch commands indicated by his flight director overlay.

A stall occurs when the angle of attack, or the angle formed between the wings and the oncoming airstream, exceeds a critical value. As angle of attack (or AOA) increases, lift increases, up to the critical value, at which point the air can no longer flow smoothly over the wings, resulting in separation of the airflow and a catastrophic loss of lift, known as a stall. For a given aircraft type and configuration, the stall angle of attack is always the same, and in the case of a CRJ-100 with the landing gear and flaps extended, it should have been somewhere north of 13.5 degrees. The stick shaker stall warning is designed to activate at a substantially lower AOA, with the exact threshold depending on the rate of AOA increase, in order to provide the pilots with adequate advance notice of the danger. Furthermore, the CRJ-100 is a T-tail aircraft which can experience an unrecoverable “deep stall,” in which disrupted airflow behind the wings blanks out the elevators, so failure by the pilot to intervene is potentially catastrophic. For that reason, the CRJ is also equipped with a stick pusher, which automatically pushes the nose down if the AOA reaches a value somewhat above the stick shaker activation threshold.

None of this should matter during a go-around, however. The normal go-around procedure calls for the pilots to pitch up toward the ten degrees indicated by the flight director, while advancing thrust to go-around power and retracting the flaps and landing gear. At no point during this maneuver, if carried out correctly, should the AOA reach the stick shaker activation threshold, and if it does, the pilots should be able to easily recover by reducing pitch. But on flight 646, the stick shaker activated just one second after First Officer Cyr started pitching up, well before reaching the 10 degrees indicated by the flight director.

Another aerial view of the scene. (Canadian Press)

The reason for this unexpected behavior was simple — the engines weren’t producing enough power. Normally, a go-around on the CRJ-100 is conducted with the engines at approach power, or about 68% of the fan red line speed (N1). However, in this case, the go-around was initiated after First Officer Cyr pulled back power to idle for touchdown. Therefore, the actual power at the start of the go-around was only 29%. Furthermore, it takes fewer than three seconds for the engines to accelerate from approach power to go-around power, but it takes eight seconds to go from idle power to go-around power. Therefore, when Captain MacFarlane snapped the thrust levers to maximum power for the go-around, it would have taken eight seconds for the engines to catch up. In the event, however, the plane stalled after only three seconds, and the engines didn’t reach go-around power until about the time the plane was slamming nose-first into the runway.

Because the go-around was initiated with the engines at low power, the plane did not actually have enough energy to sustain a climb. Therefore, when First Officer Cyr pitched up, the airplane actually continued to descend at a rate of about 350 feet per minute. As the pitch of the airplane increased but its trajectory remained generally downward, the angle of attack increased rapidly, whereas in a normal go-around, the increase in pitch would send the plane onto a corresponding upward trajectory, resulting in a minimal AOA increase. This is why the stall warning activated so quickly on flight 646, when its activation would not be expected during a nominal go-around. The plane simply wasn’t ready to climb.

An alternate angle of the tree in the cabin. One can image how passengers might have wedged limbs in various awkward places. (CBC News)

As it turned out, the pilots attempted the go-around in a phase of flight during which there was no guarantee that the aircraft would behave according to any particular certification requirements. Certification criteria of an airplane’s go-around performance assumed that a go-around would be conducted with the engines at approach power, and that the act of rolling back power to idle for touchdown was tantamount to making the decision to land. A go-around initiated later than this would involve a number of special considerations. First of all, the pilot would need to wait several seconds before pitching up toward the flight director command arrow, so that the engines would have time to reach a power level sufficient to sustain a climb; and second, the pilot would need to be aware that during those few seconds, the aircraft would almost certainly touch down on the runway, before lifting off again later.

This type of go-around — referred to as a “rejected landing with power at idle” — was not required to be demonstrated in certification testing, and by extension, no guarantees were placed on the plane’s performance during such a maneuver. This was not a major concern for regulators because the maneuver is rarely executed in practice; in fact, it only becomes necessary if something prevents a safe landing after the decision to land has already been made. In the case of flight 646, the plane drifted too far to the left of the centerline as First Officer Cyr was rolling back power to land, forcing the pilots to make just such a rejected landing with power at idle. But because this was not a “demonstrated” maneuver, they had not been taught that there was any distinction between a “rejected landing with power at idle” and a normal go-around.

In fact, there was no requirement to include this type of go-around in aircraft documentation or in pilot training, and Air Canada did not train its pilots to follow a different procedure depending on whether a go-around is initiated before or after reducing thrust for touchdown. All go-arounds conducted during training at Air Canada were initiated with the engines at approach power, and the flight manual listed only one go-around procedure. At the same time, however, there was no indication in the manual that a go-around at this phase of flight was prohibited — in fact such a maneuver is permitted, and for good reason. Certainly pilots must be able to abandon the approach at low altitude if, say, a snowplow suddenly enters the runway ahead of them. But under the existing system, they were expected to recognize, in the split second after spotting the hypothetical snowplow, that they actually needed to deviate from the normal go-around procedure or risk a potentially dangerous increase in AOA.

This problem had actually been recognized in 1996, after an inspection report found that trainee pilots at Canadian airlines were pitching up too quickly, leading to stick shaker activation, during simulated single-engine go-arounds. The considerations in a single-engine go-around and a rejected landing with power at idle are actually fairly similar, in that both types of go-around are initiated with less than expected engine power available. In order to reduce the likelihood that pilots would make this mistake, the CRJ-100 go-around procedure was changed in late 1996 in order to de-emphasize following the flight director’s pitch commands and promote awareness of airspeed instead. The idea was that the pilot should use the flight director command arrow as “initial guidance” in order to establish a positive rate of climb, then refer to the airspeed indicator only. Because increased airspeed allows the wings to generate the same amount of lift at a lower angle of attack, ensuring adequate airspeed also ensures that the AOA will not increase dangerously.

A timeline of events leading up to the stall illustrates just how quickly the accident unfolded. (TSB)

However, it was clear that this change to the procedure did little to help the crew of flight 646. In the event, the stick shaker activated almost immediately after First Officer Cyr started pitching up, and in fact a positive rate of climb was not established until just before the onset of the stall, so he never had an opportunity to shift attention to his airspeed indicator. Events simply escalated too quickly.

This finding raised eyebrows at the TSB and at Bombardier, the manufacturer of the aircraft. The problem was that under nominal conditions, the plane should have been able to establish a positive rate of climb before the stick shaker activated, all else being equal. That is to say, the updated procedure should have worked — First Officer Cyr should have been able to pitch toward the flight director command arrow, glance at his airspeed indicator, realize that their speed was too low, and then decrease the AOA, all before the airplane actually stalled. It would have been dodgy, but they shouldn’t have crashed. Analyzing the flight data, however, investigators observed that the plane stalled much earlier than expected, at an AOA of only about 9.0 degrees, rather 13.5 degrees, the CRJ-100’s normal stall AOA in the landing configuration. This was the reason that the pilots lost control so quickly, before they could even attempt to correct the situation. In fact the stall occurred so early that it caught out the stick pusher too, as the AOA never reached the system’s activation threshold.

The effect of ice on a wing on the stall AOA. (Boldmethod)

There was one very obvious possible reason for this discrepancy: ice on the wings. The formation of even a very thin layer of ice on the wings of an aircraft can significantly affect its performance, and in particular it does this by decreasing the AOA at which the airflow begins to separate from the top of the wings. By altering the stall AOA in this manner, it also reduces or even eliminates the margins between the stick shaker and stick pusher activation and the stall itself.

In order to determine whether ice could account for the difference between the expected and actual stall AOA on flight 646, the TSB commissioned an ice accretion study from the Institute of Aviation Research, which eventually determined that the plane was in icing conditions for only about 60 seconds before the go-around, having descended into the clouds at about 500 feet above ground level. However, once in cloud, the conditions of freezing fog were highly conducive to ice formation, and although the study came with large error bars, the researchers concluded that the plane could have accumulated enough ice during those 60 seconds to fully account for the stall AOA discrepancy.

I have to imagine that responders and investigators made a lot of off-the-record jokes about the tree in the cabin. Guilt-free, of course, because no one died. (Bureau of Aircraft Accidents Archives)

At this point investigators noted that the CRJ-100 was equipped not only with sophisticated ice removal and prevention systems, but also with an ice detection system which could identify ice accumulations as thin as 0.02 in (0.5 mm) and illuminate an amber caution light in the cockpit. If the caution light illuminated, the pilots were required to turn on the anti-icing systems. However, the light never illuminated on the accident flight, even though the total ice accumulation was surely more than this, because the caution was inhibited by design below 400 feet above ground level. The intention behind this feature was to prevent an ice indication from distracting the crew just before landing, given that ice would no longer be an issue once the plane was on the ground. But in this case, the ice accumulation didn’t reach the indication threshold until after the plane descended through 400 feet, so the pilots were never notified of an ice buildup, even though the presence of ice suddenly became very important when they attempted a go-around.

The lack of wisdom behind this design feature was easily recognizable, and in fact regulations in the United States required that the ice warning remain active all the way to the ground. Frustratingly, that meant that if the accident airplane had been registered in the US, the pilots would have been warned of the ice buildup, might have turned on the anti-icing systems, and might have avoided the stall.

A close-up of the forward fuselage shows the neat slice cut into it by the tree. (Stephen MacGillivray)

Just to cover all their bases, however, the TSB also conducted studies into other possible factors, and found that the stall AOA might have already been somewhat degraded just because the airplane was old. Most notably, the sealant used to fill the tiny gaps between wing panels was starting to wear away, and in some places it had been extruded upward, creating tiny imperfections which further interfered with the smooth airflow over the tops of the wings. The TSB’s study found that these sealant issues alone could have reduced the stall AOA by about two degrees — not enough by itself to cause the 4.5-degree discrepancy on flight 646, but a noticeable amount nonetheless. After being made aware of the problem, Air Canada improved its maintenance practices in order to better catch and fix age-related degradation of the wing surface.


Incidentally, the bands of snow running up one side of every tree testify to the dismal conditions on the night of the accident. (CBC News)

Taken in total, the combination of circumstances leading to flight 646’s stall and crash would have been hard for the crew to predict and virtually impossible to avoid once the go-around was initiated. Not having been trained to conduct go-arounds from idle power, and unaware that ice was building up on their wings, the pilots were caught out by a situation that began to make sense to them only in hindsight. In the event, they had no idea that they were about to attempt a maneuver outside the plane’s demonstrated flight envelope under conditions which made the maneuver’s failure near-certain.

In fact, the best and probably only way to prevent the accident was to avoid getting into this situation in the first place. And it is here that we circle back to all that discussion at the start of this article about visibility, RVR, and landing minimums.

First of all, the TSB noted, the accident would not have happened if Fredericton were located in the US, because America’s 1,800-foot legal minimum visibility would have barred the pilots from even attempting the approach. In Canada, however, they were allowed to make an attempt under vertical visibility and RVR conditions corresponding to the Category II minimums, despite only having Category I equipment. In that case, one has to wonder, what was even the point of the stricter equipment requirements for a Category II approach? In the TSB’s view, this rule was unacceptably lax, in that it encouraged pilots to attempt approaches which had low odds of success and elevated levels of risk. In that sense, the loose regulations didn’t directly cause the crash, but they did create the circumstances for the crash to occur, and when it comes to aviation safety, avoiding those circumstances is half the battle.

Crash survivor Allyson Vaughan returned to the crash site the following day to look it over. (Stephen MacGillivray)

The second issue which the TSB noted was Captain MacFarlane’s decision to let the First Officer fly the approach. Air Canada’s CRJ-100 flight operations manual recommended (but did not require) that the captain fly the approach if the reported or forecast RVR was less than the minimum RVR indicated on the chart, unless the airport was equipped with high-intensity approach lights, touchdown zone lights, and centerline lights. Of these, Fredericton had only the high-intensity approach lights. On the accident flight, this recommendation was shown to be well-founded. Although First Officer Cyr had previously landed a Cessna in similar conditions, his 60 hours in the CRJ-100 were not enough for him to handle the approach on the night of the accident. In fact, as soon as he disconnected the autopilot at 165 feet, his lack of experience began to show, as the shifting wind and absence of high-intensity centerline lighting left him struggling to keep the plane aligned with the runway. Furthermore, he twice allowed the plane to deviate above the glide slope, probably in part due to an optical illusion. The CRJ has an unusually high approach speed, and consequently an unusually low pitch angle on approach; therefore, in conditions of low visibility with few reference points other than the runway itself, it can feel as though the plane is pointed rather alarmingly at the ground. An inexperienced pilot might instinctively react by pitching up. Both pilots were aware of this illusion, but it takes practice to overcome it — practice which First Officer Cyr didn’t have.

Because the approach was beyond the First Officer’s abilities, he was unable to maneuver into the proper position for landing, and the late go-around became necessary. Much like the unacceptably low legal minimum visibility, then, the decision to let the First Officer fly the approach in spite of Air Canada’s recommendation did not cause the crash, but did set the stage for it to occur.


A crane removes the wrecked airplane from the forest. (Stephen MacGillivray)

As a result of the accident, several changes were made at both the company and national levels. Air Canada and Bombardier updated their procedures so that the anti-ice must be turned on below 400 feet when in suspected icing conditions, regardless of the status of the ice warning, effectively circumventing the inhibition, although the TSB still recommended that the inhibition be removed altogether. Air Canada also added low-energy go-arounds to its training curriculum, and a series of publications were issued to raise awareness of the topic, followed by renewed efforts to make the new training mandatory. More concretely, Transport Canada increased the minimum visibility required to attempt a Category I ILS approach, bringing Canadian regulations into line with those of the rest of the world; and in the field of passenger survival, Canada began requiring that all turbine-powered transport aircraft carry an emergency locator transmitter, regardless of where they are flying.

All of this having been said, while pilots today receive better training on the differences between normal and low-energy go-arounds, awareness of the issue remains key to preventing a similar accident from happening in the future. The pilots of Air Canada flight 646 were qualified, competent, and conscientious; they never violated any procedures or broke any rules. And yet they were caught by surprise all the same, left stunned by events that built up slowly amid an environment of elevated risk. What happened to them could have happened to anyone, which is what makes the story of flight 646 still so valuable more than a quarter-century later. It prompts the attentive pilot to constantly ask themselves: could ice affect my plane’s performance during the next maneuver? If I have to go around at an awkward point in the approach or landing, how will I do it? Simple questions like these make us all safer, because this is a type of accident which is difficult to entirely prevent and could in theory happen again if pilots are not paying attention. On the other hand, we should also be thankful that no one died in the crash of flight 646, creating an excellent chance to learn these lessons without loss of life. Hopefully, good judgment, risk awareness, and training will ensure that it stays that way.


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Admiral Cloudberg

Kyra Dempsey, analyzer of plane crashes. Contact me via @Admiral_Cloudberg on Reddit, @KyraCloudy on Twitter, or by email at kyracloudy97@gmail.com.