The science of flying and how humidity, heat pose challenges

Hindustan Times

15 hours ago

The moments after Air India flight 171 took off from Ahmedabad airport at 40 seconds past 1.38pm were routine for any flight: A massive Boeing 787 Dreamliner barrels down runway 05/23, its nose is pointed skyward in the classic climbing posture. Soon, its nosewheels raise of the ground, followed by the pair of heavier rear landing gears and the hulking wide body jet is now aloft by the physics of aerodynamics.
But soon, that begins to change: instead of gaining altitude, it begins sinking. For 33 seconds, the aircraft maintains this contradictory orientation — nose up, yet descending — before disappearing behind trees and erupting into flames.
This haunting video captures a fundamental breakdown in the physics that governs all flight: the delicate equilibrium of forces— of thrust, that propels the jet forward to overcome drag so that the giant wings generate lift enough to overcome weight.
The root cause for why this breakdown occurred is now the subject of a multi-agency, multi-country investigation led by India's Aircraft Accident Investigation Bureau (AAIB), with Air India, plane-maker Boeing and engine manufacturer GE Aerospace helping uncover technical clues and with assistance from air crash investigators from the UK and the US.
Experts analysing the video — which will now be a crucial part of the investigation --- were struck by how the plane was doomed at a stage of flight where the laws of physics are simple, as opposed to being done in by a more dramatic outcome, such as being knocked off its path by a strong gust of wind or incurring what is known as a tail strike when pilots take off too aggressively.
Understanding these forces becomes crucial as investigators determine whether engine failure, incorrect wing configuration, miscalculation of weights environmental factors — or, in fact, a combination of any of these played a role.
The fundamental forces of flight
Every aircraft exists in constant balance between four competing forces. As the Smithsonian's National Air and Space Museum put it: 'for take-off, thrust must exceed drag and lift must exceed weight. For level flight, lift must equal weight and thrust must equal drag'.
For the 787, this balance is complex. The aircraft's two engines must generate between 54,000 and 74,000 pounds of thrust each, depending on variant and configuration. During take-off, this thrust must overcome the aircraft's drag and provide enough forward velocity for wings to generate sufficient lift supporting the plane's maximum take-off weight of 502,500 pounds, or 228,000 kg.
The margin for error is narrow. While minimum thrust needed for take-off is about 1/18th of weight, this theoretical minimum assumes perfect conditions and unlimited runway length—luxuries rarely available in real operations.
The lift equation
To understand the conditions, and how minute changes in them have significant implication for calculations such as how much fuel to carry or, in fact, how many to board, it's important to look at the precise mathematics behind lift generation.
Lift is determined by an equation that involves speed, a coefficient of lift determined by factors such as wing design, flap settings and angle of attack), and wing surface area. It is here that one of the much-speculated aspects of flight 171's final moments -- the flap's being potentially retracted -- could be of significance. If the flaps were indeed prematurely retracted, the plan would not have the lift it needed.
Another significant variable in this equation is air density.
And air density is sapped by the hot temperature. On Thursday, the temperature at the Ahmedabad airport at the peak of the day was 42°C. In other words, the air would have been far less dense than usual.
According to the Federal Aviation Administration's Pilot's Handbook of Aeronautical Knowledge, 'the less dense the air, the less lift, the more lacklustre the climb, and the longer the distance needed for take-off and landing.
'The standard temperature at sea level is 15°C, and temperature typically decreases by about 2 degrees for every 1,000 feet of altitude. Given Ahmedabad airport's elevation of 180 feet, the expected ambient temperature should have been around 14.6 degrees. On the day of the crash, the temperature was around 42°C - nearly 28 degrees above the standard atmospheric condition. Though the runway length is about 11,000 feet, under such high-temperature conditions, the effective take-off distance available is reduced to approximately 9,600 feet, leaving far lower margin for safe lift-off,' explained Mohan Ranganathan, an aviation security expert.
And then there is humidity, which further compounds these challenges. Higher humidity means more water vapour in air, which is less dense than dry air. This is because water vapor molecules are lighter than the nitrogen and oxygen molecules that make up dry air.
Such conditions, especially the confluence of so many factors, require careful calibration of how much weight pilots can carry, how long their take-off run needs to be, and the angle at which the lift off.
The engine limitation
Modern jet engines operate within precisely defined limits crucial during investigations like Flight 171. Principal limitations include maximum internal pressure the casing can withstand and maximum allowable operating temperature.
This dual-limitation system creates a performance envelope that changes dramatically with environmental conditions. At low altitudes and cooler temperatures, engine pressure limits performance. In hot conditions, reduced density requires engines to work harder for the same thrust, reaching temperature limits before pressure limits.
The FAA handbook cited above states: 'Fewer air molecules in a given volume of air also result in reduced propeller efficiency and therefore reduced net thrust.'
Aviation has a term for the most challenging take-off conditions: 'hot and high.' While Ahmedabad's 180-foot elevation doesn't qualify as 'high' in aviation terms, the combination of extreme heat and humidity creates similar performance challenges.
The aircraft would have thus performed as if taking off from a much higher airport, with correspondingly reduced engine performance and lift generation.
This creates cascading performance penalties investigators must examine closely. If flight 171 was loaded to normal capacity for the London route—carrying more than 125,000 litres of fuel plus 230 passengers and 12 crew—hot conditions may have left insufficient performance margin to handle any emergency.
With inputs from Neha LM Tripathi