Ambient temperature, be it hot or cold, has an effect on aircraft operations irrespective of the airport elevation. Whilst the combination of heat and high altitude, as discussed in the SKYbrary article Hot and High Operations, has a particularly detrimental impact on aviation, heat alone can also have substantial repercussions when considering safe and efficient aircraft operations. Extreme heat, common to many areas in Africa and the Middle East, is becoming increasingly more common, albeit for relatively short periods of time, in other areas of the world, including Europe, Australia and North America.
Virtually all commercial pattern aircraft have a published environmental envelope. This envelope includes the maximum static air temperature, by pressure altitude, at which operations are permissible. In June 2017, a sustained outside air temperature of 120° Fahrenheit (49°C), in Phoenix Arizona, forced the cancellation of a number of Bombardier CRJ flights due to exceedance of the maximum allowable ambient operating temperature for that aircraft type.
Pressure altitude is aerodrome elevation corrected for ambient pressure. Density Altitude is pressure altitude corrected for non-standard temperature (and for humidity). The baseline for the International Standard Atmosphere (ISA) assumes sea level atmospheric pressure and temperature to be 1013.2 millibars and +15°C respectively. Under these conditions, Density Altitude and Pressure Altitude are essentially the same. Maintaining the pressure but increasing the temperature will increase the Density Altitude. For a simplistic calculation, the Density altitude increases by approximately 120 feet per degree above the ISA Standard temperature for the pressure altitude. A sea level aerodrome with a temperature of 45°C would have in an approximate Density altitude of 3600 feet (120 x (45-15)). This value will be even higher under high humidity conditions. As aircraft performance is directly related to Density Altitude, temperatures above ISA can result in a substantial performance penalty.
Weather, Turbulence and Visibility Issues
Hot temperature conditions can often result in convective activity. In very dry areas, this can be manifested in the form of turbulence due to uneven surface heating and the associated rising columns of air. In more humid areas, convective cumulonimbus clouds, or thunderstorms, may form with all of the inherent risks, including heavy precipitation, lightning, turbulence, wind shear, microbursts and even tornados, associated. Air quality is often poor during hot and humid conditions and visibility can be reduced in haze. All of these factors contribute to the challenges of hot weather flight.
Aircraft engines of all types have maximum operating temperatures which might be measured as cylinder head temperature, Turbine Inlet Temperature (TIT), Interstage Turbine Temperature (ITT) or Exhaust Gas Temperature (EGT). The maximum temperature values vary by engine type and phase of flight and some parameters, such as maximum takeoff temperature, normally will have an associated time limit. In hot environments, the maximum engine temperature may be reached well before the engine is producing its maximum rated thrust or torque.
In addition to degraded engine performance, aerofoil performance is also affected by high ambient temperatures. As discussed previously, an increase in temperature results is an increase in density altitude. The higher the density altitude, the fewer molecules there are per volume of air. This results in a decrease in the amount of lift that the wings will generate.
In combination, these factors conspire to reduce overall aircraft performance. Under hot ambient conditions, takeoff distances will be increased and climb rates will be decreased. In many cases, the maximum takeoff weight must be reduced based on runway available or the required climb gradient. This, in turn, results in a decreased maximum payload capability that is directly attributable to the hot conditions.
Aircraft Maintenance Issues
High heat conditions can result in significant aircraft issues. Cooling of the aircraft interior can be difficult or, in some cases, virtually impossible, especially in areas where appropriate ground support equipment is not available. Brake components, bleed air systems and electronic equipment are all subject to overheating. Engine hot starts will potentially occur more often and limiting temperature exceedances can become more commonplace due to the eroded margins in hotter ambient conditions. Prolonged exposure to high temperatures can cause sealants to dry out, leading to potential fluid leaks. A combination of heat and humidity can cause and/or accelerate corrosion.
Impact on Personnel
Working outdoors in a hot environment can take a heavy toll on personnel. Dehydration, sunburn, heat exhaustion, heat stroke (sun stroke), and contact burns from hot metal, are all significant risks in a high heat environment.
Performance issues are best mitigated by planning operations, especially those involving moving heavy payloads over long distances, during the cooler hours of the day. Early morning, late evening and overnight departures should be considered wherever practical. Where this is not possible, reduced payloads, amended routings and substitution of a more capable aircraft type could be considered.
Aircraft cooling issues can be mitigated by maximising the use of ground cooling equipment, ensuring that window shades are closed during ground stops and selective opening of doors or hatches thus venting the aircraft to allow heat to escape. In some aircraft types, bleed air system overheat can be mitigated by leaving the flaps and slats partially extended to allow air circulation near the affected components. Engine start procedures should follow manufacturer guidelines for hot weather operations. These might include manual, vice automatic, start procedures or motoring of the engine prior to start to thermally stabilise the engine core. Minimising brake use to the extent practicable and maximising the use of brake fans, if fitted, and release of the parking brake once the aircraft has been chocked, will all help to prevent brake assembly overheat. In some cases, increased ground time between flights will be required to ensure adequate cooling. Other maintenance issues can be reduced by use of robust hot weather protocols and prevention strategies.
Heat related personnel issues can be reduced by ensuring appropriate hydration measures, use of sunscreen and provision of appropriate protective equipment, such as gloves and headwear. Regular breaks, in a cooling environment, should be provided to all line crew and maintenance personnel.