AI 171 Accident - a tragedy that could, and should have been avoided

About the causes of tragedy of AI 171, plethora of videos and texts is available on social media, some attributing to deliberate or inadvertent action of pilots, some attributing to maintenance lapses on the part of the airlines and some attributing to quality control overlooks by Boeing manufacturer. One or two did address the tendency of profiteering, but none leading to clear understanding of the cause that triggered the event leading to the crash, and to a recommendation for efficient and cost effective steps to avoid recurrence of such tragedy in future. To demonstrate ad hominem one has to grasp the root of the matter. In 2019 at Osaka airport while landing ANA flight the Dreamliner suffered dual engine failure. Unfortunately the experts could not identify exact cause of dual engine failure and attributed it to error by pilot in applying full thrust reversal too quickly before the aircraft transitioned to ground mode which caused malfunction of TCMA (thrust Control Malfunction Accommodation system. And the matter closed at that Physicists, scientists and practicing specialist observe and analyse a phenomenon in isolation and tend to develop habit of missing the woods for the trees while ideological thinkers and philosophers tend to develop habit of missing the roots for the woods. But for holistic view innate skill of abstraction needs to be honed. Capitalist mode of production develops social consciousness predisposed to take higher risks for maximisation of profits. Society for SCIENCE (Socially Conscious Intellectuals ENlightenment and CEphalisation) is a platform, organising group discussions to hone skills in abstraction and publishing articles to spread scientific temper. On crash of AI 171 the author is of the view that low safety margins in operation procedure is the cause of this accident, and published three articles on social media, one before the release of the preliminary report by AAIB and two after the release of the report. Society for SCIENCE organised group discussion and put a video on YouTube. In this article and video, the author has summarised his analysis of the accident. Dreamliner Boeing 787-8 is considered one of the most efficient aircrafts, fitted with two turbofan engines and equipped with FADEC (Full Authority Digital Engine Control). A turbofan engine consists of three rotating machines, fan, multistage axial flow compressor and multistage axial flow turbine, all integrated and working in tandem. Compressed air is fed to ignition chamber where burning fuel generates heat to raise temperature of compressed air, which when guided to expand through various stages of turbine, generates enough power to run the fan and the compressor, to provide required thrust to fly the aircraft and to provide electrical and mechanical energy needed for auxiliaries. Characteristic curves of compressor and turbine are bell shaped, meaning they work very efficiently over a range of rotating speeds and air flow rates, and efficiency drops very sharply outside this range and then at some point outside this range compressor may stall and go into surge or choke, meaning they will stop performing the task they are supposed to perform. Problem with compressor is more complicated and serious because the flow of air is with adverse pressure gradient, that is, from low pressure to high pressure, while in turbine the flow is from high pressure to low pressure. Flow of air between compressor blades is like flow through a passage. For efficient functioning the flow must be laminar and not turbulent. In laminar flow the boundary layer (the layer next to the surface of blade) must follow blade contour and must not leave contact. If it leaves the contact, in the space between the boundary layer and blade surface air will become turbulent. Because of turbulence there will be energy loss and desired pressure will not be achieved. This is stalling. Stalling can be caused by sudden reduction in the speed of inlet air which will cause separation of boundary layer because of adverse pressure gradient. Sudden reduction in the speed of inlet air can be caused either by a back pressure event in the turbine downstream or reduction in the fan speed because of overloading of the turbine. If, with timely action, the flow is not increased or angle of flow is not changed by changing stator blade angle, the stall from one blade will lead to stall in other blades which ultimately will cascade into reversal of flow, that is compressor will go into surge. During stall, flow is uniform, but in surge the flow is in the form of rapid violent pulses with time periods of few milliseconds, sometimes violent enough to result in complete reverse flow with big bang. In complete reversal ignition chamber will not get any fresh air, flame will extinguish and the turbine will stop generating any power meaning by complete engine failure. Variable-pitch stators (to control flow angles), compressor bleeds and tip clearance controls are some measures used to reverse stall and to avoid going into surge. The designer and manufacturer of the engine (GE herein) knows limiting values of the parameters; engine speed, mass flow rate, pressure ratio and ambient conditions which can lead to stall and recommends that the user must keep the operation well away from these limits. The aircraft manufacturer Boeing, programmes the FADEC (Full-Authority Digital Engine Control), to keep the operating point of the compressor well away from the so-called stall or surge lines. Too much far away will increase operational cost, too less far away will enhance the operational risk and how much far away is matter of operational philosophy. These aircrafts are equipped with two Enhanced Airborne Flight Recorders (EAFR). The EAFRs are fitted at two locations, one in the tail section and other in the forward section. The two EAFRs are similar in construction and record a combined data stream of digital flight data and cockpit voice information, with both stored on the same device. The aft EAFR receives electrical power from the aircraft’s main electrical system. The forward EAFR contains an additional power source from the Recorder Independent Power Supply (RIPS), a system that provides electrical power to the forward EAFR in the event of a power or bus loss on the aircraft. This allows the forward EAFR to continue to record available digital flight data, and voice data from the Cockpit Area Microphone (CAM), even after power is lost to other aircraft systems. The aircraft is also fitted with RAT (Ram Air Turbine) for emergency power supply. In case of failure of both engines, RAT is deployed automatically and runs with the pressure of wind. RAT supplies necessary electric and hydraulic power to navigate the plane till engines are restarted. This information about design, manufacturing and operation of Dreamliner aircrafts is fundamental and needs to be kept in mind while analysing the information, explicit and implicit, provided by the preliminary report. The CPM (Crash Protected Memory) module retrieved from the EAFR was found to be in good condition. The data was downloaded and initial analysis of the recorded audio and flight data has been done. The preliminary report discloses limited information only and at this stage no actions have been recommended for B787-8 and/or GE GEnx-1B engine operators and manufacturers. To complete the picture, missing information between events is presumed logically and is subject to correction as and when detailed information is released to public. Let us now sift through the information provided in the preliminary report. Before requesting pushback and startup (07:43:00 UTC, 13:13:00 IST) the pilots must have fed to the onboard computer values of various parameters like total weight, runway length and ambient conditions to determine V1, Vr, V2 and flap and thrust settings. Pilots must have made final adjustments in input data before confirming requirement of full length of runway (07:49:12 UTC, 13:19:12 IST) and before seeking clearance for taxiing which was granted (07:55:15 UTC, 13:25:15 IST). It is important to know the final temperature and pressure which were fed to the computer to get the operating instructions. Aircraft was cleared for take off (08:07:33 UTC, 13:37:33 IST). The preliminary report shows temperature and atmospheric pressure as per Meteorological report - at 07:30 UTC (13:00 IST) 36oC and 1001 hPa respectively, and at 09:00 UTC (14:30 IST) 38oC and 0900 hPa respectively. A temperature increase of 2oC and pressure decrease of 2 hPa over one and half hours may not be significant under normal conditions of operation but may be critical if the engines were required to operate very close to stall and surge lines. Take-off Weight was 2,13,401 Kgs, 97.81% of max. allowed - 2,18,183 Kgs, safe margin of narrow 2.19%. Many long haul flights which require larger amount of fuel, meaning high takeoff weight, are reported to have delayed takeoff or to have been aborted at 40oC. Higher ambient temperature means higher V2 and longer runway requirement. In the present case the pilots had requested full runway requirement. With available conditions at Take-Off, calculated speeds were V1 - 153 Kts, Vr - 155 Kts, V2 -162 Kts. As per the EAFR data, the aircraft achieved V1 153 kts at 08:08:33 UTC (13:38:33 IST), Vr 155 kts was achieved at 08:08:35 UTC (13:38:35 IST). The report says aircraft air/ground sensors transitioned to air mode at 08:08:39 UTC, which means V2 162 Kts was achieved at 08:08:39 UTC (13:38:39 IST). The report shows that the aircraft achieved the maximum recorded airspeed of 180 Knots at about 08:08:42 UTC (13:38:42 IST). Thus increase of 2 Knots from V1 to Vr took 2 seconds, increase of 7 Knots from Vr to V2 took 4 seconds and increase of 18 Knots from V2 to maximum recorded took 3 seconds. This means both engines were getting continuously increasing fuel supply. According to the report, immediately after achieving speed of 180 Knots the Engine 1 and Engine 2 fuel cutoff switches transitioned from RUN to CUTOFF position one after another with a time gap of 01 sec. In the cockpit voice recording, one of the pilots is heard asking the other why did he cutoff. The other pilot responded that he did not do so. This indicates that cut off switches were transitioned by FADEC within 3 and 4 seconds of take off. The RAT hydraulic pump began supplying hydraulic power at about 08:08:47 UTC, within 8 seconds of take off and 5 seconds of achieving maximum speed of 180 Kts. Thus it is logical to assume that both engines flamed out because of surge caused by sudden increase of load on take off. And also logical to conclude that fuel switches were transitioned by FADEC and not by mechanical switches operated by one of the pilots. As per the EAFR, the Engine 1 fuel cutoff switch transitioned from CUTOFF to RUN at about 08:08:52 UTC. The APU Inlet Door began opening at about 08:08:54 UTC, consistent with the APU Auto Start logic. Thereafter at 08:08:56 UTC the Engine 2 fuel cutoff switch also transitions from CUTOFF to RUN. When fuel control switches are moved from CUTOFF to RUN while the aircraft is in flight, each engines full authority dual engine control (FADEC) automatically manages a relight and thrust recovery sequence of ignition and fuel introduction. The EGT was observed to be rising for both engines indicating relight. Complete sequence of events after the aircraft achieved maximum speed, confirms that FADEC was in full control and was functioning correctly and all electrical and mechanical systems were functioning correctly. Any human error in operation and maintenance, and any malfunction of any system must be ruled out. The EAFR recording stopped at 08:09:11 UTC (13:39:11 IST). The statements of the witnesses and the surviving passenger have been obtained by the Investigators but have not been included in the preliminary report. As per media reports the lone surviving passenger has stated that he heard a loud bang and observed light in the cabin flicker before the crash. Both these events support the assumption of stall, surge and flame out. In his Hindi article dated 7 July 2025 posted on Facebook SFS Group, author expressed his view that dual engine failure of AI 171 flight would have been triggered by compressors going into surge and not because of any electrical failure or deliberate action by one of the pilots. English translation of the final analysis in 7 July 2025 article is given below. “As soon as the take-off process started, because of sudden increase of load, reduction of speed of turbines and with it speed of fans would have reduced due to which the air flow would have also suddenly reduced. Due to the reduced air flow, a surge situation would have also arisen in the compressor and the flame would have died due to lack of air in the ignition chamber. All this would have happened in both the engines simultaneously in a moment because the conditions were the same for both and with this the plane lost power, which is evident from the pilot's May Day call. The RAT turbine deployment also indicates this, but the speed of the plane was so low that it could not generate enough power. The landing gear not being able to close also shows that the plane had no power at all.” Analysis herein of the preliminary report confirms the previous analysis. AI 171 accident would not have occurred if enquiry in ANA NH 985 dual engine failure would have arrived at its logical conclusion. It would have been better if AAIB would have recommended that Boeing and GE increased safety margin in their SOP of Dreamliners till the final report. Suresh Srivastava 30 August, 2025 (Author is a member of Society for SCIENCE)