Editor’s Note: Les Abend served as a Boeing 777 captain for American Airlines, retiring after 34 years with the airline. He is a CNN aviation analyst and senior contributor to Flying magazine. The opinions expressed in this commentary are his own. View more opinion on CNN.
The circumstances that led to Nepal’s deadliest plane crash in 30 years are still in the very early stages of investigation. But that hasn’t stopped media focus turning to the mountainous terrain and unique challenges it poses to pilots.
Almost all of the bodies of the 72 people on board, including crew, have now been recovered after the Yeti Airlines ATR-72 aircraft went down Sunday on its approach to the newly opened airport in Pokhara, a tourist destination and gateway to the Himalayas.
Assuming the accuracy of cellphone video taken just before impact, it appears that the airplane began to bank its wings at too steep an angle. In addition, the pitch attitude – the angle of the nose relative to the horizon – seemed to be abnormally high.
In this pilot’s opinion, the aircraft entered an aerodynamic stall, meaning the wings were no longer able to provide lift for sustainable flight. A recovery that close to the ground is virtually impossible. What caused the stall? It will remain a mystery until more details of the investigation are revealed. Weather does not appear to have been a factor.
Much has been said about the dangers of flying in the Himalayas, but it is no more challenging than any other geographic area of the world that involves high terrain. Regardless, the strategy is to mitigate the risk. But how?
Having flown into and over areas that share space with the Andes in South America, the Swiss and French Alps, and the US Rocky Mountains, I see knowledge as the best form of risk mitigation. In the flying business, we refer to this knowledge as situational awareness.
Where is the high terrain relative to the current position of the airplane and its future position? Where is the terrain threat on takeoff or landing? If an engine fails on takeoff, what procedures can we safely follow to return to our departure airport? And on approach, what altitude is safe for a decision to abort the landing or to continue?
My airline had specific procedures in case an emergency diversion was necessary over high terrain at cruise altitude. These involved specific escape routes with precise minimum altitudes that would allow our flight to proceed safely to an alternate destination.
Because some Himalayan airports have terrain threats too high for airplanes to climb over on initial departure, landings are conducted from one direction, with takeoffs in the opposite direction.
This can become problematic if a strong enough tailwind exists. (Airplane performance for takeoff is improved with a headwind and reduced with a tailwind. A tailwind on landing risks a high ground speed that could potentially send an airplane farther down a runway on touchdown and possibly off the end.) That’s why an airline imposes wind limitations for airports and airplanes.
Higher elevations reduce airplane and engine performance. Less dense air is not as conducive to lift for the wing as sea level air. Less dense air reduces engine performance. Less performance translates into the need to reduce airplane weight through passenger and/or cargo loading because more of the runway is required for takeoff. But these factors are mitigated through use of performance graphs and charts, with computer calculations performing most of these tasks.
Another aspect to mitigating risk is understanding the weather caused by mountainous terrain, commonly called orographic lifting, which can be unique to each area. Depending on wind direction, orographic lifting can generate turbulence, which is uncomfortable in cruise flight but challenging during an approach.
Turbulence can require a pilot’s constant attention and skill set to maintain a stabilized flight path, especially for airplanes that don’t operate with an autopilot. Good judgment and proper preparation are the best defense.
Lower cloud ceilings and lower visibilities are challenges in high terrains because pilots can’t see the mountains to avoid the threat. Flight crews have to rely on the procedures involved with an instrument approach, which is what we train and practice to perform.
An instrument approach procedure uses the guidance from our onboard navigation and primary flight display, without visual reference to the outside world. And some airports simply require visual contact with the runway at all times, with no instrument approach procedure available.
Many of the instrument approach procedures in high terrains use step-down altitudes that maintain a required height above the ground. The authorized heights become lower as the airplane progresses toward a runway. The airplane reaches a point in the approach where it reaches a “decision altitude.”
The decision altitude is the point that if the crew does not have visual contact with the runway, it must go around and abandon the approach. A go-around is a procedure with which both air traffic control and the pilot are familiar. The procedure always involves a climb and a routing to a specific navigable point to which the airplane can enter a holding pattern.
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Contingencies for the procedures are always considered. As an example, how is the procedure altered for an engine failure, a circumstance that reduces the airplane’s ability to climb over high terrain? The best risk mitigation is simple. It’s a captain’s prerogative and responsibility not to depart if the weather is marginal.
High terrain operations can be challenging for pilots. But the danger is not in the terrain itself but how this risk is mitigated. The Himalayan mountains are no different.