What are “Hot and High” operations? Why airlines prefer four engined jets.

by Vinay Bhaskara

Image Credit – International Airlines Group (IAG)

One of the most interesting challenges in aviation arises from operations in severe conditions, extreme cold, short runways, and more commonly, operations from so-called “hot and high” airports; airfields with high temperatures that are situated at a high altitude.

This curious phenomenon is actually the impetus behind several seemingly incongruous strategic decisions by airlines. It is the reason that Iberia has not yet abandoned its gas-guzzling fleet of Airbus A340s, and the reason that Aeromexico’s Asian flights originate in Mexico City but make a technical stop along the way in Tijuana.

The problem occurs primarily due to constraints on take-offs and landings, which harken back to the basic physics of aerodynamics. Aircraft generate lift by using power from the engine to flow air over the wings. More specifically on takeoff, the engine burns fuel to heat up the air and flow a large mass of air through the engines; generating thrust, which allows the aircraft to speed up down the runway and climb away from the airfield.

However, hot and high operating conditions change this simple calculus in several ways. Firstly, if the airport is located in a region of high altitude, the air pressure is lower and the air is less dense. This means that, at any given speed (all else being equal), a smaller mass of air is flowing through the engines; so a higher airspeed is required to develop enough thrust to take off with a given payload versus at a sea-level airport. The easiest way for an aircraft to make up for this deficit is to roll further down the runway before taking off; thus high-altitude airports tend to have some of the longest runways in the world (e.g. Denver International Airport, with an altitude of more than 1,655 meters, has a 16,000 foot runway designed to handle larger aircraft with high payloads). Even after taking off, aircraft will struggle to climb away from the airfield due to the lower density of the air (again relating to a dearth of thrust).

A similar problem plagues airports with high temperatures. Once again heating air decreases its density, which causes the same mass of air limitations driven by high altitudes. However, high temperatures present an additional challenge in that jet engines have a maximum temperature that they can heat gas up until (The exhaust gas temperature or EGT). On hotter days, there is less difference between the air temperature and the EGTs, meaning the engine adds less heat through the air than in cooler conditions, once again affecting thrust. Thus, airports in high temperature regions also tend to require longer runways (examples include airports in the Southwestern United States, the middle east, North Africa, and the Indian sub-continent). The current standard estimates that the adverse effects of high temperatures kick in en-mass when temperatures rise above 30 degrees Celsius.

It is when these two conditions are combined, that a particularly dangerous cocktail arises; the hot and high airport. When temperatures are high at high altitudes, engine thrust performance deteriorates heavily because the air density is even lower. It is little wonder that such conditions are amongst the most challenging in the world for airlines to operate to and from, even more so when their central hub is located at such an airport. Perhaps the most famous hot and high airports in the world are Mexico City and Johannesburg, home to Aeromexico and South African Airways respectively, as well as several airports in Africa and South America (the core markets for Iberia’s long haul operations).

Bangalore, the home to this site, is another challenging airport for hot and high operations. Located at an altitude of 3,000 ft and thanks to indiscriminate development which has denuded the green cover, the former temperate paradise, frequently tops 35 degrees Celsius or 95 degrees Fahrenheit. It is not uncommon to see smaller Code-C aircraft (Boeing 737s and Airbus A320s) have take-off runs exceeding 3,000 meters (approx. 10,000 ft) during the peak hot hours from around noon to 4pm.

This explains why Iberia and South African Airways have held onto their fleets of 4-engined Airbus A340 aircraft longer than other airlines; quad-jets perform better in hot and high conditions. The reason is mainly due to a worst case scenario; loss of power in one engine.

It really comes down to the fact that if a quad-jet loses one engine, it still has 75% of its maximum thrust, while twin-jets like the Boeing 777s and A330s in the same situation will have only 50% of their engine power available in a failure scenario. Thus the quad-jets can carry more payload given runway length constraints at many of these airports (this used to be a major problem at Mariscal Sucre Airport in Quito).