A pilot who had promised to come back to be with his ailing dad, who was pushing close to 90 and in need of care. A doctor couple who had dreamed of starting their life afresh in London with their three kids, two of them twins. A former Chief Minister of Gujarat who had boarded the plane to bring back his wife.
The tragic crash of Air India Flight AI-171 on the afternoon of June 12, shortly after takeoff from Ahmedabad en route to London, snuffed out valuable lives and has raised urgent questions about what went wrong.
A day after the worst crash involving a single aircraft in India, various theories are circulating about the possible causes, from mechanical failure to environmental conditions and human error.
Drawing on my experience as a retired Air Force Wing Commander and flight engineer with decades of operational experience, I would like to hazard a few guesses on the potential factors behind the crash—particularly aircraft weight, environmental parameters, engine performance, and maintenance practices. While doing so, kindly allow me to emphasise the critical importance of safety protocols and technical discipline in aviation.
The weight factor
One critical factor that might have to be assessed while investigating the crash of Flight AI-171 will be All Up Weight (AUW).
AUW is the total weight of the aircraft, including fuel, cargo and passengers. This is calculated before every takeoff. AUW must remain within the safe limits when each aircraft takes off.
AUW limits vary according to the altitude at which the runway is situated and the outside air temperature (OAT). Rain, to quote an example, can influence the permissible AUW.
Then there are the locations that need to be considered as mentioned above. An aircraft that can take off with an AUW of 100 tons from the Trivandrum airport might only be able to carry 75 tons from Leh, even if the temperature at both places is the same. This is because of the variation in air density due to changes in altitude.
Also, at high atmospheric temperatures, aircraft engines are pushed to their limits. This is because the engines always need high-density air for optimum performance and maximum power. But when the temperatures are high, air becomes less dense, putting limitations on engine performance.
From the available videos of AI-171, it is very clear that the aircraft was able to pick up speed and lift off initially. The flaps and leading edge slats helped with the initial lift-off, further supported by the nose-up attitude.
Thereafter, the aircraft could not sustain height. The temperature in Ahmedabad at 13:18 was 40 degrees Celsius. There is a high probability that during the second leg of the aircraft's take-off, the atmospherics messed up with AUW calculations, and the engines and the low-density air failed to support the overweight aircraft.
Also, please remember, that handbags are rarely weighed. On an average, if each passenger carries 5 kg extra in their handbags, a total of 1 ton is added to the AUW of the aircraft, and those numbers are never going into the calculations made by the crew.
There are hints that the Captain had some clue regarding the aircraft being overweight. Why do I say this? Well, AI-171 departed using the full length of Runway 23 at the Ahmedabad airport.
It must be noted that Runway 23 is 11,499 feet long. The aircraft backtracked to the edge of the runway before beginning its take-off roll. A full-length runway is not usually needed for this class of aircraft. But Captain Sumit Sabharwal wanting to ensure that every inch of the runway was used is telling in retrospect.
Drawing from my experience
On various occasions, as military aviators, we experienced the problem of lack of thrust and lift on military aircraft, especially in high-altitude areas.
On one occasion, we had to fly the aircraft a few meters above the Shyok River in the Ladakh sector for 15-20 minutes to burn the fuel to make the aircraft light.
In a similar incident, we had to orbit in a valley for half an hour as the aircraft was not climbing over the Khardungla pass.
But both of these were calculated risks taken due to commitment compulsions. We knew how the aircraft was going to behave. However, such a risk cannot be taken in the case of a passenger aircraft.
The need to examine the state of the engines
Media sources indicate that the last communication from the Captain was of there being no thrust and lift for the aircraft. This directly points to the performance of the engines.
Scheduled servicing of the engines and their timely overhauling are both paramount when it comes to ensuring safety. It has to be ascertained from the engine logbooks and other records whether any kind of shortcuts or unprofessional methods were adopted in the engine maintenance and servicing activities. Even the most sophisticated engines will not perform as required if not maintained and serviced properly.
Here, the aircraft was to operate close to the threshold of the flight envelope, for which it is designed, due to the very high atmospheric temperature and high AUW. In such circumstances, the engines are also expected to operate close to their maximum ratings. Only a properly maintained engine will perform as required at its maximum ratings.
As a flight engineer, I have seen military aircraft engines performing at their max ratings for longer durations than prescribed by the OEM (original equipment manufacturer). Again, to make an engine operate confidently at max ratings at your will, you must be very confident about the engine maintenance activities.
Examining the sabotage angle and the bird-hit theory
There is a claim that the flaps and leading edge slats got retracted after takeoff without input from the pilot. It is true that if the control surfaces do not respond in time, this can lead to a loss of lift. But the aircraft was in its second leg and the first leg was uneventful.
Also, a non-responsive control surface can happen, only if there is a software issue, hydraulic system failure, or a FOD (foreign object damage—a term used in aviation to represent loose articles getting inside engines or control surface actuating parts, affecting their performance).
And here comes the sabotage angle. But remember, the aircraft was in its second leg after an uneventful first leg. And also, nonresponsive control surface will lead to a loss of lift only, not thrust, as called out by the captain.
Bird hit causing an accident of such a magnitude is also unlikely. Even a big bird, like an eagle, will easily pass through the engines of such an aircraft and come out in ashes, temporarily raising the EJT (Exhaust Jet Temperature) . It will not lead to the stalling of the engine.
In the case of the famous flight accident over Hudson River, it was a huge flock of birds that caused the stalling of both engines.
Both engines failing due to bird hits can also be ruled out. But then the captain never called out an engine flame out. Had there been such a flame out, it would have the first warning seen in the cockpit and the captain would be calling out that. This never happened.
What also needs to be examined is whether there was a wind reversal and a resultant tail wind factor during takeoff. A sudden and strong tailwind can also lead to such an accident.
Importance of CVR, how machines behave and cloud over Air India
Once the Cockpit Voice Recorder (CVR) and Flight Data Recorder (FDR) are analysed, the truth will come out. The last words of the crew, recorded in the CVR, are very important in an air accident investigation.
The fact that the lone survivor was sitting very close to the front emergency exit on the port side of the aircraft reiterates the need to emphasise on the emergency rescue procedure for air passengers. The emergency door might have blasted off because of the heat and pressure, and the survivor was able to escape through the exit.
Having operated military aircraft for many years, I can say one thing very categorically. No machine will give away all of a sudden. If it's a machine failure, it might have given a number of warnings in various forms to the operators and the maintenance staff. It is for the human to understand those warnings in time and take remedial actions to bring the machine back in line. This is the thumb rule for any machine.
AI-171 too would have then given enough warnings if there had been a technical issue. Whether there were such warning signs and whether any remedial measures were taken can be found out only during the investigation. Till then, the question marks on the maintenance culture of Air India, once a government company, will remain.
(Wing Commander A Mahesh (Retd) was a flight engineer with IAF and is a keen follower of the latest developments in the aviation industry.)
