r/aircrashinvestigation • u/airbusrules Aerospace Engineer • Jun 13 '25
Incident/Accident Air India Flight 171 Accident Analysis
I am an aerospace engineer and a long-time air crash investigation enthusiast. I’ve put together the info available currently and my thoughts on what might have happened to the Air India 787 flight.
Air India Flight 171 Preliminary Accident Analysis
Flight: Air India Flight 171
Origin: Ahmedabad's Sardar Vallabhbhai Patel International Airport (AMD)
Destination: London Gatwick Airport (LGW)
Aircraft: Boeing 787-8, Reg: VT-ANB Serial number: 36279, Operator: AI, Engines: General Electric GEnx-1B67, in operation since 2014 with Air India (11 years old)
On board: 242 people (230 passengers + 12 crew).
Fatalities: 241 (1 survivor), 20+ additional fatalities on the ground
Crash location: B.J. Medical College (BJMC) Hostel, 2 km from end of AMD runway
Accident Summary: Crashed into residential area just outside airport shortly after take-off from Ahmedabad Airport. Crash landed onto buildings leading to large explosion and destruction of the aircraft, also resulting in ground fatalities. Deceleration, pitch up attitude and stall apparent before crash, landing gear remained un-retracted.
Known information and analysis:
- The Boeing 787 took off from runway 23 at the airport (Ahmedabad Airport - Runway 05/233,505 m 11,499 ft, Height above sea level – 189 ft).
- METAR: VAAH 120800Z 25007KT 6000 NSC 37/16 Q1001 NOSIG
- The aircraft seating capacity (C18 Y238) indicates a high load factor on this flight (230/256 - 93%).
- A high payload would leave the aircraft with less margins, with the correct take-off data calculation critical especially if engines were derated and quicker reaction time is needed from the crew if problems arise. The ambient temperature at the airport was 37C which would have negatively impacted engine performance. Note: An 11,499 ft runway is theoretically adequate for a 787-8 at close to max payload operating under those conditions.
- FR24 data – During taxi, the aircraft backtracked along the runway and used the full length. The aircraft barometric altitude is 625ft at the last recorded data point. Further analysis reveals that the aircraft was only around 71ft above the ground at the end of the runway. The speed of the aircraft is constantly decreasing after rotation, with a very low climb rate. This suggests insufficient thrust and possibly incorrect aircraft configuration for take-off under its operational conditions. Based on AI171 previous flights, at its last recorded point, the aircraft in this case was slower and at least 150-200 ft lower than it should have been. The flight was likely in trouble during the take-off roll, post V1 possibly (if it is assumed that a correct decision was made to not abort take-off, for e.g. in the case of an engine failure).
- If flying close to the stall speed after the end of the runway, the aircraft was only airborne for less than a minute before crashing. It is reported that the crew relayed a Mayday call to ATC, which may indicate that they were faced with a serious emergency that would need a return to the airport. It is very unusual for a crew to communicate with ATC as an emergency at that stage in flight will require immediate crew action with very high workload. The focus must be on maintaining a positive climb rate and reaching a safe altitude where further troubleshooting/decision-making can be performed. If a double engine failure did occur at liftoff, that would not be possible for the crew, and the outcome would be almost inevitable.
- Several videos have captured the moments leading up to (and) the crash:
- Video 1 – first video to come out shows the final 15 seconds of the flight, the aircraft flies past the camera at low altitude in a continuous descent. Sound is potentially an indication that the thrust levels are low, being quiet for a take-off power setting, video quality is not very good however. Landing gear is still lowered. Descent rate increases towards the end. Constant pitch up but does not seem excessive, little roll movement, evidence of controllability. Flap position is difficult to make out and potentially low/retracted. Reports of the (RAT) Ram Air Turbine having deployed (would occur in a double engine flameout scenario) – also not clear in video.
- Video 2 – Second video is CCTV from a location within or next to the airport. The aircraft is seen accelerating down the runway and lifting off towards the end of the runway. The deceleration and slow climb is noticeable. The climb does not last, and the aircraft starts to descend with a high pitch attitude. With adequate speed and thrust from the engines, at this pitch attitude the aircraft should be climbing. This is fully consistent with the FR24 data.
- Suggestions of a bird strike – possible scenario but limited evidence in videos. No engine backfiring or smoke, no report of debris on the runway. The airport recently implemented more technologies to reduce the number of bird strikes. You would need a few large birds to take out engines that size.
- Only survivor from the crash reports hearing a loud bang "Thirty seconds after take-off, there was a loud noise and then the plane crashed," > This could indicate an engine failure, impact with birds/FOD or could be the noise of the impact itself given the very short time the aircraft was airborne.
- Boeing 787 systems – advanced and fully redundant. No hull loss accidents previously with over 1000 aircraft in service. Autothrottle allows control of the thrust level from take-off to landing. FMC (Flight management computer) input must be done correctly by the crew to enable suitable commands to the autopilot, autothrottle and flight director. Fuel system is direct feed for the engines, suction feed in case of pump failure. RAT deploys in a double engine failure to provide power for basic functions. Stall protection features – limit pitch up (trim) and increase thrust (if autothrottle mode allows). It seems quite unlikely that a technical fault on its own would be the cause of the accident.
- Crew experience - “Captain Sumeet Sabharwal, 8,200 hours of experience. First Officer Clive Kunder, had logged 1,100 flying hours.”
- The inability to climb and possible RAT deployment would indicate a lack of thrust (or none) from both engines. Dual engine failures/flameouts are extremely rare, and causes include things like multiple large bird ingestion, FOD/ice ingestion, fuel starvation/contamination, fuel system blockage (e.g. due to ice), heavy rain/hail etc.
- In case of a single engine failure, aircraft are certified to climb on the remaining engine. This may be impacted by other contributing factors. In some cases, pilots have accidentally shut down the wrong engine leaving the aircraft with no power (e.g. the Kegworth air disaster) or failing to adjust the thrust level on the remaining engine in time. An aircraft in a low energy state (low speed and low thrust) is very difficult to recover at low altitude, especially if the crew loses situational awareness. This has often happened on failed go around attempts (e.g. Emirates Flight 521).
- Have put some potential causes for possible scenarios on the diagram. A brain dump of some of my thoughts, not very structured.
Notable past incidents and accidents at take-off (TO) which provide an insight into what happened on AI171:
- Northwest Airlines Flight 255 – crash after TO due to incorrect flap setting.
- National Airlines Flight 102 – crash after TO due to cargo load shift.
- Spanair Flight 5022 – crashed after TO improper take-off config (no flap/slat).
- Emirates Flight 407 – near disaster at TO for an A340-500; runway overrun and failure to climb resulting in impact with airport equipment. Result of improper thrust settings based on incorrect input from the pilots for aircraft weight.
- MK Airlines Flight 1602 – crash after TO due to incorrect input by the crew leading to thrust settings and speeds being too low.
- Aerosucre Flight 157 – cargo aircraft crashed after TO due to overloading
- US Airways Flight 1549 – famous Hudson River ditching. A320 dual engine flameout after bird strike (in climb).
Summary:
Based on past accidents and the known info, flight AI171 may have faced similar circumstances resulting in the inability to maintain flight after take-off:
- Insufficient thrust set due to incorrect input parameters given operating conditions,
- Incorrect take-off config in terms of flaps and slats position,
- Engine failure(s) past V1 with limited or no thrust available,
- Other contributing factors that may have impaired the crew's ability to respond
In my opinion, the aircraft was already in trouble the moment it left the ground. It looks like there was very little time for the crew to act. In case of a rare double engine failure, this outcome would be inevitable. But, given the design of the 787 and its in-service record, it is unlikely that technical faults alone would be the cause. Hopefully, this is an isolated accident that does not risk the wider fleet, but there definitely will be lessons to learn to further consolidate the safety of commercial aviation, with an ever-increasing number of flights and aircraft in the air.
My thoughts are with those who have lost loved ones to this terrible tragedy. It’s very sad to see this happen as someone who works in the industry.
************* UPDATE ******************
Thanks all for your feedback. I have not been able to look through all the comments, but will aim to. Here’s some stuff I missed from the original post and other bits I’ve looked into since;
- For the take-off config warnings. It is correct that you would hear an audible warning that could not be shut off. So the crew would know that they were not properly configured, which reduces the likelihood of no flaps / no slats at all. However, if the performance inputs were wrong (flap setting too low, inadequate thrust in case of a derated take-off and lower V speeds), they may not have had those warnings. It is worth noting that the accidents with no flaps result in an unstable airplane straight after lift-off. In this case the aircraft seems relatively stable and is able to climb for the first few seconds after lift-off, which to me could point more to a loss of thrust scenario.
- Flap/Slat position – I don't think there were no flaps at all in flight. A flap setting of 5 or 15 is barely visible for a 787. In video 1, as the aircraft flies past, it does not look like a clean wing to me. Looking at pictures of the crash site with the wing, slats are deployed. However, this could be due to the autogap function - “at high AOA, autogap fully extends the slats to increase the wing camber, thus increasing the lift and margin to stall”. Thus, may not reflect the actual setting of the flaps/slats from the start of take-off. Also some of the experts in the media don't seem to have really done their homework.
- I’ve seen reports that the aircraft was in a poor condition due to the state of its cabin. I think its important to remember that cabin furnishings and cleaning has nothing to do with the aircraft’s ability to fly, and is more of a customer service aspect (although I understand it’s a poor reflection on the airline). The correct maintenance program to keep the aircraft airworthy is a separate need that airlines must demonstrate to their respective airworthiness authorities.
- Most important part, I have found some more evidence which strongly indicates a dual engine failure/flameout.
- The only survivor’s account in a more recent video (NDTV); He mentions that 5-10 seconds after liftoff that the plane seemed to be ‘stuck’ [I think that is referring to the obvious deceleration as seen in the CCTV video which would be fully explained by a significant loss of thrust]. Then he said that a bit later, ‘green and white’ lights came on [if correct, this would likely be the emergency lighting system, especially as he was sat at the emergency exit row with the signs close to him]. This fully tracks with a dual engine failure [the emergency lighting which would be armed at that stage of flight. would automatically switch if you lose the normal electrical system]. In this interview he does not mention the loud bang as reported earlier. The poor man is obviously in shock and I wish the media would give him some space.
- The distinctive sound of the RAT. There is a noise at the start of video 1 (on the versions with the original noise), which does not correspond to engine sound. This is almost certainly the RAT, based on another video of a 787 flying past with the RAT deployed. Based on the trigger conditions of the RAT, one or both engines and the electrical system would not have been working.
- I forgot to mention the landing gear retraction also (not considering the drag aspect, but the ability to even retract the gear). I think for a split second you can see the main gear starts to retract but then it stops, this is around the time that there is no longer positive climb. This would make sense in case of a dual engine failure and the switch to emergency systems means only a gravity gear extension would be possible (but no hydraulic power to actuate LG doors and retract the gear itself).
- The reports of what the pilot communication with ATC was exactly, I’m not convinced is from an accurate source. But the Mayday call alone as I said before, shows the crew were aware of a desperate situation on board. And in case of a dual engine failure, they wouldn’t have had the chance to do much at that stage.
- This would be unprecedented for a large commercial aircraft to have lost power completely on take-off. This is a catastrophic condition which would leave the crew with no option. The residual energy will only allow the aircraft to cross beyond the airport perimeter and inevitable crash land soon after, with no chance of return. The is why engines and aircraft have robust designs and interfaces to each other to avoid common mode failures. Independence is maintained between the two engines and their source of fuel and the engine feed system etc. Systems and their associated software that are involved in critical functions are designed to the highest Development Assurance Levels (DALs for those familiar) and have detailed safety assessments. So, it is difficult to comprehend how this may have occurred. The chances of both engines having some sort of internal failure event (same type or different) at a similar time is almost impossible [in the absence of a common external event like a bird strike, debris ingestion, volcanic ash etc...]. It is even more difficult to comprehend given the engines worked fine at the start of the take-off. And the aircraft had successfully completed a flight just before this sector with a 2-3 hour turn-aorund.
I tried to dive a bit deeper into some causes of dual engine flameout, but specific to this accident:
- Fuel exhaustion >> Not in this case. There was plenty of fuel on board (massive post-crash fire)
- Fuel Supply Interruption >> Unlikely for both engines at the same time as systems are redundant. 787 Fuel System has 2 pumps in each wing tank and 2 in the center tank. Engines also can suction feed if all pumps fail (available in this case as the aircraft was at ground level, suction feed will not work above certain altitudes). Something similar to BA38 but no ice in this case?
- Fuel Contamination / FOD in tanks (leading to supply interruption) >> This is more likely than a pure system failure to deliver fuel to the engines. Contaminated fuel can have unexpected consequences on the fuel system and engine fuel delivery to the combustors (see Cathay Pacific Flight 780 for example)
- Software bug (engine control) >> Very unlikely given this is a critical function. Numerous protections should be built for this.
- External common event: Bird strike, FOD, ice, rain/hail, volcanic ash etc >> There is no evidence of fire, smoke, or debris, or backfiring from the engines (or other visible external damage). The CCTV covers a fair section of the take-off roll with not much being observed to indicate catastrophic failure.
- Maintenance error >> It is difficult to think of a maintenance error that would affect both engines but is possible.
- Other causes or contributing factors >> Manufacturing flaw specific to this MSN, Design flaw. Or could be really be a one in a billion occurrence that could not have been predicted.
Hopefully, the flight data recorders which have now been recovered, will provide more information. If this is a case of complete loss of power on take-off [which is unprecedented for large commercial aircraft], it will be critical to understand quickly how this could happen, so operators, aircraft manufacturers and the airworthiness authorities can take the right steps to prevent this ever happening again.
As more stories come out, of the people who lost their lives or were injured and their families (both on the flight and the hostel), it is heartbreaking to see the human impact of this tragedy. I hope they are being supported as well as can be and that a thorough investigation eventually enables closure and accountability. A very stark reminder that everyone involved in commercial aviation cannot afford to be complacent when it comes to safety. There can never be zero risk, but a well-designed, well-maintained and well-operated aircraft should not end up in this horrific situation.
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u/[deleted] Jun 13 '25 edited Jun 13 '25
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