Antonov’s Curse: The crash of Sepahan Airlines flight 5915 and the story of the An-140
On the 10th of August 2014, an Iranian airliner lost height and crashed shortly after takeoff from Tehran, killing 40 people and bringing the safety of an aircraft type into question. The plane involved was a little known HESA IrAn-140, an Iranian license-built version of the Ukrainian Antonov An-140 regional turboprop — a model that was seemingly cursed from the moment the first airframe rolled off the assembly line in 1997. Suffering from a string of accidents, poor sales, and premature groundings, the An-140 and its Iranian spinoff gathered such a disastrous reputation that most of the airlines willing to fly them were captive companies owned by the type’s own manufacturers — including the short-lived Sepahan Airlines, which was wholly owned by HESA, Iran’s state aviation company. So when the world learned that a Sepahan Airlines IrAn-140 had gone down in Tehran, there was little assurance that the investigation would be objective — and indeed it was not. The cause of the crash became the subject of a three-way dispute between Iran’s Civil Aviation Organization, Ukrainian air crash investigators, and the independent Interstate Aviation Committee. At the center of the debate were two critical questions: why did the plane’s right engine fail almost at the moment of liftoff, and why couldn’t the pilots maintain altitude afterward? Amid competing arguments from sometimes unreliable actors, the truth is difficult to discern — but there is plenty of interesting drama to be dissected along the way.
Throughout the mid-to-late 20th century, one of the most prolific and renowned global aircraft designers was the Soviet Union’s Antonov Design Bureau. Based in Kyiv, Ukraine, Antonov’s engineers produced numerous transport aircraft that have since become icons of the Eastern Bloc, including the ubiquitous An-2 biplane, the An-124 heavy lift cargo plane, and of course, the mighty one-off An-225 Mriya, which was the largest airplane in the world until it was tragically destroyed in the opening hours of the Russian invasion of Ukraine.
One of the less glamorous Antonov products was the An-24 twin turboprop, which was once the most common regional airliner in the Soviet Union. Over 1,000 were built between 1959 and 1979, and dozens remain in service around the world, especially in Africa, where airlines appreciate the model’s ability to operate out of unimproved airports with minimal or no ground services. But even 30 years ago, it was obvious that the An-24, long out of production, would not be around forever — and so the Antonov Company, now the largest aircraft manufacturer in the newly independent Ukraine, resolved to design and build a successor. The result was the An-140: an airplane that unfortunately turned out to be cursed from the very beginning.
The An-140 was conceived to fulfill a role similar to the An-24 that it would replace, and although the two planes were genealogically unrelated, the new airplane’s general shape and layout mirrored its predecessor. It featured two-by-two seating with room for 52 passenger seats (including four rear-facing seats in row 1); a high-wing design to minimize the risk of foreign object damage; and two Motor Sich Al-30 turboprop engines, which were essentially souped-up, license-built versions of the Soviet Klimov TV3–117 that powered nearly all Soviet helicopters built since 1974. In most respects, the An-140 can be compared to the French-built ATR-42, which is similar in appearance and role. Relative to the ATR-42, the An-140 carries slightly more passengers at a slightly higher cruising speed, but is comparatively underpowered, achieving only 70% of the ATR’s climb rate with 10–25% less horsepower than comparable ATR-42 variants. These statistics probably help explain why the world’s regional airlines didn’t ditch their ATRs in favor of the Antonov, but as we will soon see, there were plenty of other reasons as well.
Although the first An-140 rolled off the assembly line in Kharkiv, Ukraine, in 1997, the production rate never exceeded 3 airframes per year, and the first three were all retained by Antonov itself. One of these was badly damaged during a test flight in 1999, but was repaired. Over the next several years, a number of additional airframes were delivered to various Ukrainian regional carriers, including Motor Sich airlines, a company wholly owned by the An-140’s engine manufacturer. But where things really began to go south was when Antonov attempted to export the An-140 to customers abroad.
Shortly before the turn of the millennium, Antonov negotiated a deal that would allow the state-owned Iran Aircraft Manufacturing Industrial Company, known by its Persian acronym HESA, to assemble license-built An-140s in Iran. As the primary aerospace manufacturer in the Islamic Republic of Iran, HESA is probably best known today as the company behind the Shahed-131 drone, which has become notorious for its use by Russian forces against civilian targets in Ukraine. The company has been under strict sanctions by the US and the EU for many years, but in the early 2000s the world was a different place, and not only were sanctions not an impediment to Ukraine’s partnership with HESA, but the future role the company would play in the destruction of Ukrainian cities could not possibly have been predicted.
In fact, the partnership seemed to begin auspiciously — at least, until the first actual aircraft rolled off the assembly line. In December 2002, HESA had just completed its first license-built An-140 (granted the amusing official designation “IrAn-140”), and a delegation from Antonov was invited to attend a ceremony marking its inauguration. To transport its engineers to the HESA plant in Isfahan, Antonov chartered an An-140 (but of course!) from the newly created Ukrainian airline Aeromist-Kharkiv. Flown by two Antonov test pilots and packed with important Antonov employees, the plane departed Ukraine on December 23rd, 2002 — but tragically, it never reached its destination. Relying on an unapproved and unreliable GPS, the pilots flew off course on approach to Isfahan, and the plane crashed into a mountain, killing all 44 passengers and crew.
Nevertheless, production of the HESA IrAn-140 forged ahead. Several were delivered to the Iranian state police — probably a captive buyer — followed by three for Safiran Airlines, an obscure Iranian cargo carrier. Meanwhile, Antonov delivered three Ukrainian-built An-140s to Azerbaijan Airlines, the flag carrier of Azerbaijan, with plans for one more. But both of these airlines soon got a bad case of buyer’s regret. The planes proved unreliable in service, and in August 2005 a Safiran Airlines IrAn-140 was substantially damaged when it suffered an engine failure followed by a runway overrun during the subsequent emergency landing. Records indicate that after the accident, Safiran gave all its An-140s back to HESA. Azerbaijan Airlines suffered even worse for its purchase: in December 2005, one of its An-140s experienced a triple gyroscope failure shortly after takeoff from Baku, causing the pilots to become disoriented and lose control; the plane crashed into the Caspian Sea, killing all 23 passengers and crew. News reports indicate that Azerbaijan Airlines grounded its An-140s after the accident, and registration records show that the fourth airframe was never delivered.
Still, production continued. Despite the accidents, Antonov concluded an agreement with Aviakor, a Russian aerospace manufacturing plant, to produce An-140s in Samara for sale to Russian customers, and the first airframe rolled off the new assembly line in 2006. Several more followed in 2009, 2011, and 2012, which were sold to the Russian Navy, Russian Air Force, and Russian regional carrier Yakutia Airlines. Simultaneously, HESA continued to produce additional IrAn-140s in Isfahan, but apparently found few buyers. It certainly didn’t help that in 2009, one of HESA’s own airframes crashed during a training flight with the loss of all 5 crew. After that, possibly for no other reason than to make back some money on the airframes it had already built, in 2010 the company founded “HESA Airlines,” a wholly owned subsidiary that would carry passengers on short flights throughout Iran using a fleet of 6 IrAn-140s. The name of the company was changed to Sepahan Airlines in late 2013. (Perhaps the name was changed to make it sound more like a normal airline. Would you have flown on McDonnell Douglas Air?)
Shortly after that, buyer’s remorse turned to seller’s remorse for Antonov. In the spring of 2014, the Ukrainian revolution led to a break in ties with Russia that ultimately spiraled into interstate conflict, and the two countries have been in a state of war ever since. The last Aviakor An-140 was completed in 2013, and records suggest that no more Ukrainian or Iranian An-140s were built after that date either. Furthermore, given that Antonov built its An-140s in Kharkiv, a city in Ukraine’s east that was devastated by the Russian invasion in 2022, any near-term attempt to restart production appears unlikely.
It is after this final airframe departed the factory that we pick up the story of yet another disaster involving the An-140. The tale begins on the morning of the 10th of August 2014, at Mehrabad International Airport, the main domestic hub in Tehran, where a Sepahan Airlines HESA IrAn-140 prepared to depart on a routine scheduled flight to Tabas in northeastern Iran.
In command of the flight, designated flight 5915, were two pilots whose identities have not been revealed, consisting of a 63-year-old Captain and a 32-year-old First Officer. The Captain had about 9,500 flying hours, including a very respectable 2,000 on the An-140, but the First Officer was relatively green, with only 572 total hours. The An-140 appears to have been the first aircraft to which he was assigned after finishing flight school.
The conditions that day made for poor flying weather — not because it was wet or cloudy, but rather the opposite. By 9:00 that morning, the temperature at Mehrabad had already reached a blistering 36˚C (97˚F) and was continuing to rise rapidly, a fact which the pilots knew would harm their takeoff performance, because hotter air is less dense and thus reduces lift. The elevation of the airport, at 3,962 feet (1,208 m) above sea level, only compounded this effect. And with 42 passengers and six crew aboard, plus luggage and fuel, the weight and balance calculations promised to be tight — so tight, in fact, that at 19,866 kilograms, the final total at which the crew arrived was clearly above the maximum takeoff weight, although by how much depends on who you ask (more on that later).
Nevertheless, the pilots went ahead with the flight, selecting a takeoff flap setting of 10 degrees, and they probably determined VR — the speed at which they would rotate for liftoff — to be 224 km/h, or 121 knots. (The An-140 uses fully metric instrumentation, so km/h will be used hereinafter.) Or at least, that’s what VR should have been — no actual discussion of the rotation speed was recorded.
At 9:12, they were ready to taxi to the takeoff runway 29L. The parallel runway 29R was not in use that day, so aircraft were taxiing up 29R before turning around to take off on 29L, and flight 5915 would do the same.
“SPN 5915, take E6, A3,” the controller said.
“E6, backtrack 29L, SPN 5915,” the First Officer replied.
But the First Officer, not being a native speaker of English, had made a minor error. “No, vice versa, backtrack 29R, hold short 29L,” the controller explained.
“You said all sentences wrong,” the Captain admonished.
“Backtrack 29R, hold short 29L, SPN 5915,” the First Officer read back, correctly this time.
“You said all sentences wrong, all!!” the Captain exclaimed again. The official cockpit voice recorder transcript appends multiple exclamation points to this line, suggesting that the Captain was rather agitated, although it’s hard to understand why.
Less than two minutes later, at the First Officer’s prompting, the Captain led the takeoff briefing, explaining that if an engine failure occurred, they would get the plane into the air, turn left to waypoint KAZ, and come back into the traffic pattern for landing. There was no discussion of the specifics of the engine failure on takeoff procedures.
Now holding short of an intersection alongside runway 29L, the First Officer commented, “If the aircraft was light and empty, we could arrange departure from here.” Intersection takeoffs are sometimes permissible when the available takeoff distance is much longer than needed, but today they would need the whole runway.
“Yes, once we did that in Dubai Airport, but on that time we had not so much payload,” said the Captain.
At 9:18, flight 5915 was cleared onto the runway behind a departing MD-88. “MD-88 just running. It is so heavy, same as us. Run, run till tomorrow,” the Captain said, referring to the length of the heavily laden jet’s takeoff run.
“Just now its nose gear is lifting off,” said the First Officer.
Finally, at 9:20, it was the Antonov’s turn. “SPN 5915, clear for takeoff,” said the tower.
The pilots ran up the engines, and observed that all indications were normal. Its two Al-30 engines whirring at maximum power, the plane accelerated down the runway, reaching its decision speed at 9:21 and 2 seconds. “Decision speed, continue,” the First Officer called out. It was now too late to abort the takeoff — if an engine failed now, they would have to take the plane into the air.
Shortly after the decision speed callout, the Captain began rotating the nose for liftoff, but this was premature: their speed was in fact only 219 km/h, not 224 as required. Consequently, he had to pull the nose a few degrees higher than usual to become airborne, which would not have been a major issue if not for the fact that the right engine unexpectedly failed at 9:21 and 6 seconds, moments after the Captain began rotating and only two seconds before liftoff.
Why exactly the right engine failed is subject to dispute, and will be discussed later. But what is known is that instead of a proper failure warning, which should come with a continuous repetitive chime, there was only a single chime, followed by a brief blast from a warning horn, then two more chimes. The right engine immediately began rolling back, its thrust dropping precipitously. Despite the absence of the expected warnings, however, the Captain identified the failure immediately, and within five seconds he said to the First Officer: “It’s the engine, please watch the engine.” Five seconds later, he repeated his command again: “Watch the engine!”
“Engine fuel rate failed,” the First Officer said, presumably reading a caution message on the engine display. “Engine fuel chips.” He then turned to the Captain and asked, “May I request turnback?”
“Yes,” the Captain replied. “Declare emergency.” At that moment, the repetitive chime finally sounded, a full 17 seconds after the engine actually failed.
What the pilots did not appear to realize was that during those 17 seconds, their situation had deteriorated substantially. Considering their excess weight, the high temperature, and the altitude, the plane’s climb performance with one engine inoperative was already marginal — but several additional factors were tipping their situation over the line from serious to critical. One of these was that amid the surprise of the failure, the pilots had not remembered to raise the landing gear, which was greatly increasing drag on the airplane (a mistake that they were neither the first nor the last to make, as some of my previous articles have illustrated).
To make matters even worse, however, the electronic engine control system, or EEC, which should have automatically feathered the propeller as soon as it detected the engine failure, did not do so. On propeller aircraft, thrust and drag alike are heavily dependent on blade pitch — the angle of the propeller blades relative to the plane of rotation. When the blades are aligned with the plane of rotation, the blade pitch is 0 degrees, and when the blades are positioned perpendicular to the plane of rotation, their pitch is 90 degrees, or fully “feathered.” During normal engine operation, a blade pitch somewhere in between these two extremes allows the propeller to take a “bite” out of the air, accelerating the air backward to generate thrust. But when an engine fails, this relationship will reverse, as the oncoming airflow pushes the propeller around in circles, creating drag instead of thrust. For this reason, when an engine fails on a propeller-driven airplane, it’s critical that the blade pitch is quickly increased to 90 degrees, so that the airflow strikes the blades edge-first and passes smoothly around them, instead of catching the faces of the blades and driving the propeller in reverse.
This explanation, while simplified, should be sufficient to understand why the failure of the EEC to feather the propeller was a serious problem. In fact, the possibility of such a failure was serious enough that standard procedures called for the pilots to immediately press the “propeller feather” button as soon as they identified an engine failure, in order to be extra sure that the propeller actually feathered — but on flight 5915, the pilots never did so. Instead, the blade pitch remained at 46 degrees, where it was held by the propeller’s overspeed governor, causing substantial drag. It was not until 17 seconds after the failure, when the EEC finally seemed to wake up, that the engine failure warning was generated and the automatic feather command was issued. The propeller blade pitch immediately increased toward the 90-degree feathered position, but by then, it was already too late.
The problem was that flight 5915 had been losing speed ever since it lifted off, due to various factors. With one engine inoperative, the pilots needed to maintain an airspeed not less than the takeoff safety speed, or V2, which at an aircraft weight of 19,866 kg, a temperature of 36˚C, and an altitude of 1,208 m, should have been 234 km/h. Failing to maintain this speed after an engine failure on takeoff could result in an inability to gain sufficient altitude. But flight 5915 never reached an airspeed of 234 km/h at any point, and in fact its speed peaked at liftoff and thereafter decreased continuously. The Captain created this snowballing crisis when he rotated for liftoff 5 km/h too early, resulting in a higher angle of attack than on a normal takeoff. At any given altitude and aircraft configuration, lift is primarily a function of airspeed and angle of attack, or the angle of the lifting surfaces into the airstream, so to become airborne at a lower airspeed, a higher angle of attack is necessary. But a higher angle of attack presents more of the airplane to the oncoming airflow, causing increased drag. If the total drag on the airplane is greater than the available thrust, the airspeed will decrease, and since maintaining lift at a lower airspeed requires a higher angle of attack, the angle of attack will increase further, creating additional drag, and so on, until the plane stalls and crashes.
And as if that wasn’t enough, drag was also being created by the plane’s sideslip angle — the angle between the direction the nose was pointing and the actual direction of travel. When an engine fails, asymmetric thrust will cause an airplane to enter a sideslip, which must be countered by the pilot using the rudder in order to keep the plane flying straight. But the Captain was consistently applying less force on the rudder than was required to neutralize the sideslip, so the plane began to drift nose right, presenting more of the left side of the fuselage to the oncoming airstream, which of course created still more drag.
Now consider all of the above factors together. Again, because of the weight of the airplane and the high temperature and altitude, an airspeed not less than 234 km/h was required to climb safely with thrust from only one engine. But the landing gear was extended, the right propeller was not feathered, the plane was in a sideslip, and the Captain’s early rotation had caused the angle of attack to rise past 10 degrees, well above the normal value. With all of these sources of drag, the aircraft was unable to reach the takeoff safety speed of 234 km/h, and its airspeed slowly fell as it climbed toward a maximum height of just 40 meters above the ground. Survival required immediate corrective action by retracting the landing gear, feathering the windmilling right-hand propeller, zeroing the sideslip, and pitching down to reduce the angle of attack. But the pilots did none of these things, and while the electronic engine control system did eventually feather the propeller automatically, this was insufficient by itself to prevent the plane from decelerating. Within seconds, a crash became inevitable.
And yet, as the plane hurtled toward the edge of a stall, the First Officer was busy making a radio call: “Mehrabad radar, SPN 5915,” he said.
Captain let out a Persian expletive, probably due to his increasing control difficulties.
“Mehrabad radar, left turn immediately,” the tower controller said, observing that flight 5915 was deviating to the right of course.
“Left turn immediately,” the First Officer read back. The Captain was in fact already attempting to steer left using the control column, but this did nothing to help the situation. The only way to get back on course was to counter the sideslip, while turning left with the ailerons actually hastened the onset of the stall.
“Engine number two failed,” the First Officer repeated.
“Turn left!” the Captain said.
At that moment, with an angle of attack and a sideslip both greater than 15 degrees, and at an airspeed less than 180 km/h, the right wing ceased to generate lift, and the aircraft entered a stall-spin from a height of just 40 meters. The right wing dipped and the plane plunged to ground in seconds, clipping the tops off several trees near the parking lot of an industrial complex before crashing to earth in a right-wing-down attitude. The plane impacted heavily, breaching the fuel tanks, and a massive fireball erupted as the fuselage slid across the ground and into the perimeter wall of the industrial complex. The wall collapsed, the fuselage broke apart, and the tail section continued past the disintegrating wreckage, before coming to a halt in the middle of the eight lane Azadi Stadium Boulevard.
For most of the occupants, the fiery crash was almost instantly fatal. The fire consumed most of the fuselage before any survivors could even dream of escaping, but a few who were seated near breaks in the cabin or who were thrown from the plane during the crash managed to escape, albeit not without suffering serious burns. Unconfirmed reports also suggest that some motorists on the boulevard may have sustained injuries as well.
When the tower controller saw the plane go down, he activated the crash alarm, dispatching airport fire crews to the scene, but no notification to outside emergency services was issued. Tehran city fire crews learned of the disaster separately, upon receiving calls from witnesses, and arrived at the scene before the airport firefighters did, discovering several badly injured survivors near the burning aircraft. In total, 11 people were rushed to hospital, but one died en route, and two others died while receiving treatment, bringing the number of survivors down to just eight. Forty others perished in the crash, including all six crewmembers.
With the crash having taken place just a short distance from the headquarters of the Iranian Civil Aviation Organization’s Aircraft Accident Investigation Board, investigators arrived on the scene only 30 minutes after the accident, where they immediately set about gathering evidence. An invitation to join the investigation was also sent to the Interstate Aviation Committee, or MAK, an international body that certifies equipment and investigates accidents in much of the former Soviet Union, and which had originally certified the An-140 in the late 1990s. Ukraine withdrew from the MAK in 2012 and set up its own National Bureau of Aircraft Accident Investigations, or NBAAI, so an invitation was sent to them as well, in order to represent the country of manufacture. However, just over three weeks earlier, Malaysia Airlines flight 17 was shot down near Donetsk in the worst air disaster ever to occur on Ukrainian soil, and all the NBAAI investigators were otherwise occupied, so the agency did not send any representatives to Iran. Nevertheless, they did participate in tests conducted at Antonov’s facilities in Ukraine, and they were provided with the opportunity to comment on the draft final report, as required under international law.
The first question faced by investigators was why the right engine failed shortly before liftoff. The flight data clearly showed a drop in all relevant parameters at 9:21:06, 2 seconds before liftoff, including a sudden drop in combustion chamber pressure. In search of a fault, the wreckage of the right-hand engine was taken to the Klimov engine laboratory in Russia, where a possible problem was found: a poor-quality weld in the mounting flange for the air conditioning bleed air duct. Bleed air is used to pressurize the cabin via the air conditioning system, among other purposes, and this air is siphoned off from the engines via the bleed air duct, which are mounted in the compressor section just forward of the combustion chamber. If the duct separated at its mounting point, pressurized air could have escaped out of the engine instead of passing into the combustion chamber, causing the engine to flame out. But was this what actually happened? Or did the poor weld fail as a result of the accident? Bizarrely, this depends on who you ask.
In its final report, the Iranian AAIB wrote that the most probable cause of the weld failure, was, in their view, the impact itself, despite its pre-existing deficiencies. No particular evidence for this conclusion was provided. The Ukrainian NBAAI didn’t comment on the matter, but the MAK in its comments was quite unequivocal in its rejection of this reasoning. In the MAK’s opinion, the fractured weld showed clear evidence of metal fatigue leading to its failure, and that the separation could have and most likely did cause the loss of combustion chamber pressure and subsequent engine flameout that were captured by the flight data recorder.
The Iran AAIB proposed a completely different theory: that the engine flameout was caused by a failure of the RED-2000 Electronic Engine Control system, which they believed sent an erroneous fuel shutoff command to the fuel control unit. The fact that the EEC was not operating correctly was evident from the flight data, which showed that a number of parameters sourced from the EEC became unreliable at the moment of the failure, recording values that the AAIB described as “inconsistent with the principles of physics.” (For instance, the recorded inlet air pressure changed rapidly back and forth between 0.93 and 1.734 kg/cm2 every second, which is physically impossible.) Furthermore, the fact that neither the engine failure warning nor the automatic feather command took place until 17 seconds after the failure was also cited as evidence that the EEC was not working correctly until that point.
On the other hand, the MAK believed that the abnormal operation of the EEC occurred as a result of the engine failure. The agency’s comments don’t go into much detail about why they think this was the case, stating only that the “external conditions” created by the sudden release of hot, pressurized air into the engine accessories area could explain the system’s behavior. Instead, focusing on the scenario of the bleed air duct failure, the MAK noted that the accident airplane had previously experienced warnings about abnormal vibrations in the right engine due to unknown causes, which were not adequately explored — instead, HESA mechanics simply replaced the vibration sensor, which then recorded unreliable data for some time before it eventually stopped working entirely. The implication seems to be that because the vibration warnings stopped, the problem was considered fixed, regardless of whether this was actually the case. In the MAK’s view, however, the problem clearly was not fixed, and continued vibrations could have contributed to the premature failure of the poorly welded joint.
At the same time, the Iran AAIB noted that there was a history of problems with the RED-2000 EEC — a claim that the MAK called unfounded — and that An-140s overall suffered engine failures at an unacceptable rate, to which the MAK asked what an acceptable rate was, and who had defined it. The Iran AAIB did not reply to these questions. Nevertheless, given that the An-140 was (anecdotally) riddled with technical flaws of all sorts, I would not be particularly surprised if the Iranians’ claims were accurate. On the other hand, when it comes to the technical analysis of the engine failure, I tend to place more trust in the MAK, which has a much better investigative track record than Iran does.
Of course, the engine failure itself was only part of the story. The An-140 was certified to climb safely on one engine, so the investigation also focused on the weight of the aircraft and the actions of the crew during the critical seconds that flight 5915 was in the air.
That the aircraft was overweight, and that this probably contributed to the plane’s failure to maintain altitude, was agreed upon by all parties. The weight and balance logs showed that the aircraft weighed in at 19,866 kg, which was overweight no matter how one sliced it, and the pilots’ comments about their weight during taxi suggested that they might have been aware of this. But what was the actual maximum takeoff weight (MTOW) that day, after accounting for the temperature and altitude? Using the weight chart in the Aircraft Flight Manual (or AFM), the Iranians arrived at a max takeoff weight of 19,650 kg (making the aircraft 216 kg overweight, although the Iranian report uses the value of 190 kg throughout, in defiance of basic math), while the Ukrainians calculated a max takeoff weight of 19,500 kg (366 kg overweight). The difference probably comes from the Ukrainians’ interpretation that the MTOW must be rounded down to the nearest 500, although this is not explicitly stated. Both sides also noted that it would have been possible to bring the plane under the maximum by offloading fuel, as the plane was carrying 500 kg more fuel than it actually needed for the journey.
The MAK, however, arrived at a completely different MTOW figure, noting that while the temperature and altitude chart produced a MTOW above 19,000 kg, the limiting factor was actually wheel speed during the takeoff roll. The An-140’s tires and wheels were only rated to a speed of 250 km/h, and if a ground speed higher than this was needed to get the plane airborne, then the weight would need to be reduced. At the plane’s actual weight of 19,866 kg, and assuming correct rotation, liftoff should occur at an airspeed of 231 km/h. However, because of the reduced air density at Mehrabad that day relative to the standard conditions for which its sensors were calibrated, the plane would have needed to travel considerably faster across the ground in order to achieve this airspeed. Combined with a tailwind component that was present during the accident takeoff, the actual speed of the wheels at liftoff would have been 270 km/h, which was much too fast. In fact, using the ground speed limitation chart in the AFM, the MAK calculated that the weight of the airplane would have to have been reduced to 17,200 kg, with a resultant liftoff speed of 216 km/h, in order to satisfy the 250 km/h wheel speed limit. That meant that the plane was actually more than 2,600 kg overweight. Recognizing that the MAK’s observation was correct, the Iran AAIB revised its report to reflect this discovery.
Had the pilots also realized this, and had they actually kept their takeoff weight under 17,200 kg, the resulting performance improvement might have allowed them to power through the engine failure, avoiding the accident. However, Iranian and MAK investigators agreed that the AFM charts were confusing, insofar as they implied that 19,500 kg was an acceptable MTOW right up until one attempted to calculate the takeoff ground speed, which is rarely a limiting factor and could be overlooked. For their part, the Ukrainians disputed the claim that there was anything confusing about the charts, and that if there was, the Iran Civil Aviation Organization had an opportunity to bring up the issue when they certified the An-140 for production in Iran, but they did not.
Nevertheless, while taking off with 2,600 fewer kilograms of cargo, fuel, and passengers might have saved them, weight wasn’t the decisive factor. Wheel speed had nothing to do with takeoff performance, and if we were to ignore it, then the plane would have been only 366 kg overweight, which was not enough to explain why it failed to climb following the engine failure. The Ukraine NBAAI stated that simulator tests conducted by Antonov in the presence of investigators showed that even at that weight, the plane’s single-engine climb performance met international certification requirements, in that the V2 takeoff safety speed was achievable by the required height of 35 feet (10.7 m) with a resultant climb gradient greater than 2.4%. However, the Iran AAIB noted that this performance could only be achieved if the pilot rotated at the correct speed and retracted the landing gear after takeoff. In fact, they wrote, even if the rotation was performed correctly, V2 still would not be achieved by 35 feet — but they did not clearly mention that AFM procedures also require the crew to retract the landing gear before reaching this height. With the correct rotation technique and the landing gear retracted by 35 feet, it was possible to meet the above-mentioned performance requirements; and furthermore, if the gear was not retracted by 35 feet, then certification rules only required that the airplane achieve a positive climb gradient, which it would. In their comments, Ukrainian investigators admonished their Iranian counterparts for implying that the aircraft didn’t meet single-engine performance standards, when in fact it did, as explained above.
In the end, the performance testing showed that four main factors caused drag that prevented the plane from gaining altitude: the extended landing gear; the high angle of attack caused by the early rotation; the failure to feather the propeller for 17 seconds; and the airplane’s large sideslip angle. All of these factors together contributed to the crash, but the early rotation was by far the most significant, according to a drag chart appended to the Iranian report. The chart suggests that maintaining a lower angle of attack might have been enough by itself to prevent the loss of airspeed and subsequent stall, even with all the other factors present. Conversely, the chart suggests that after about 9:21 and 16 seconds (or 10 seconds after the engine failure), retracting the landing gear, neutralizing the sideslip angle, and immediately feathering the propeller might not have been enough to achieve a positive airspeed trend even when performed together, if the angle of attack was not also reduced. However, these actions might have been sufficient if performed immediately, because achieving a positive airspeed trend early on would have halted the increasing angle of attack trend, avoiding additional drag later in the flight.
The fact that none of these actions were actually performed at any point testified to the poor performance of the flight crew during the emergency. The pilots neglected almost every single tenet of the “engine failure on takeoff” procedure, as laid out in the manual, which called for them to retract the landing gear, press the propeller feather pushbutton, use the rudder to maintain as little sideslip as possible, and reduce the pitch angle as needed in order to maintain a speed no less than V2. In the event, no one ever pressed the feather pushbutton; nobody ever called “gear up” and the landing gear was not retracted; the Captain’s rudder inputs were insufficient to neutralize the sideslip; and the Captain never made any attempt to achieve an airspeed above V2. The First Officer, lacking in assertiveness and possibly worried by the Captain’s earlier unflattering comments about his abilities, never intervened either. Instead, during the approximately 45-second flight, the only notable task accomplished by either pilot was a call to air traffic control, which represented an abject failure of the principle of “aviate, navigate, communicate.” Communication with ATC should only ever happen after the aircraft is under control, which in this case it never was.
Despite the above, in its final report the Iran AAIB cited only the engine failure and high takeoff weight as being causal to the accident, with the early rotation, failure to feather the propeller, and excessive fuel as contributing factors. Both the MAK and the Ukraine NBAAI commented that the early rotation and other flight crew errors ought to have been elevated to the probable cause alongside the engine failure, but this suggestion was rejected, along with the vast majority of points made by the two agencies. Reading through their comments gives the impression that the Iranians did not have the best working relationship with their MAK and Ukrainian counterparts, which has since been reaffirmed by the NBAAI’s scathing commentary on the Iranian investigation into the 2020 shootdown of Ukraine International Airlines flight 752 over Tehran. As for who appeared to have the most clear-eyed view of what happened and why, I have to go with the MAK, as usual.
Nevertheless, many questions remain. Were the pilots adequately trained to handle an engine failure on the An-140? (Their actions suggest they were not.) Was the Iran CAO adequately monitoring the operations of the state-owned Sepahan Airlines? (The Iranian political system contains few safeguards against conflicts of interest.) And why did the Captain rotate early in the first place? (Was he trying to avoid overspeeding the tires? A 250 km/h speed limit on the tires is exceptionally low, and may have been a known problem among An-140 pilots. I’ll leave that one to my pilot readers to mull over; they would know better than me.) With these questions outstanding, the complete story of flight 5915 will probably never be told, even in the unlikely event that the disagreements over the direct causes of the disaster are resolved.
Beyond the immediate loss of life and property, the crash of flight 5915 also dealt yet another withering blow to the already poor reputation of the An-140. Whether the cause of the engine failure was the EEC or a poorly welded bleed air duct, low standards of manufacture were responsible either way, and the accident only reinforced the model’s history of dangerous and premature mechanical failures. Consequently, Iranian Prime Minister Hassan Rouhani ordered the grounding of all HESA IrAn-140s immediately after the crash, where they sat for almost two weeks until the CAO allowed them to fly again on August 23rd. But the damage was done: with its all-IrAn-140 fleet, Sepahan Airlines never flew again, and records suggest that its remaining aircraft were mothballed.
The same fate has since befallen most other An-140s. The AeroTransport Data Bank website indicates that only 6 An-140s were still flying in July 2023, including two with the Russian Air Force, two with the Russian navy, and two with Yakutia Airlines. One additional airframe belonging to Motor Sich Airlines has been grounded since the Russian invasion of Ukraine in February 2022 and could theoretically fly again if the war were to end, but as of this writing, that appears unlikely. All things considered, it’s not the end Antonov would have hoped for when it set out to build a successor to the 1950s-era An-24. In fact, right now there are still more An-24s flying around the world than the total number of An-140s that were ever built.
The crash of Sepahan Airlines flight 5915 also ought to stand as a cautionary tale about the importance of being ready to take swift and decisive action in the event of an engine failure on takeoff. It should not be overlooked simply because of where it happened, or what type of plane was involved. Similar accidents involving twin-engine turboprops have happened and continue to happen all over the world, most of which could have been avoided if the pilot’s instinct was to maintain V2 at all costs. If the pilots of flight 5915 had prepared for the worst, if they had done whatever it took to achieve this speed, then they and 38 others would still be alive. The next flight crew to face such a situation probably won’t be flying an An-140, and we’re all counting on them to recognize that such an event could happen to them, regardless of what they’re flying — before they, too, find themselves staring at the ground with a plane full of people behind them.
As for the seemingly cursed An-140, its fate is unfortunately similar to that of virtually every new airliner produced in the former USSR since its collapse, whether Russian or Ukrainian. None of these aircraft have managed to compete against Western models, sometimes because they lack comparable performance, or because spare parts are hard to find, or — as appears to have been the case with the An-140 — because they are just not very well made. And with its flagship An-225 in ruins, its assembly line out of commission, and its country fighting for survival, it’s hard to find a light at the end of the tunnel for the An-140’s beleaguered manufacturer. Nevertheless, we can hope that one day, in the skies of a more peaceful Europe, a renewed Antonov might yet replicate the glory of its past.
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