Hot and high
|This article does not cite any references or sources. (June 2008)|
In aviation, hot and high is a condition of low air density due to high ambient temperature and high airport elevation. Air density decreases with increasing temperature and altitude. At any given true airspeed, lower air density reduces the amount of lift generated by the wings or the rotors of an aircraft, which may hamper an aircraft's performance and hence its ability to operate safely. The reduced density also reduces the performance of the aircraft's engine, compounding the effect. Aviators gauge air density by calculating the density altitude.
"Hot" and "high" do not have to be inclusive of one another, though this tends to be the exception. If an airport is especially hot or high, the other condition need not be present. Temperatures can change from one hour to the next, while the elevation of an airport always remains constant. The fact that temperatures decrease at higher elevations mitigates the "hot and high" effect to a certain extent.
Negative effects of hot and high conditions
- Airplanes require a longer takeoff run, potentially exceeding the amount of available runway.
- Low air density hampers an aircraft's ability to climb. In some cases, an aircraft may be unable to climb rapidly enough to clear terrain surrounding a mountain airport.
- Helicopters may be forced to operate in the shaded portion of the height-velocity diagram in order to become airborne at all. This creates the potential for an uncontrollable descent in the event of an engine failure.
- Some aircraft, particularly light general aviation airplanes and older helicopters, have service ceilings so low that they may stall simply trying to maintain level flight. In some cases, aircraft have landed at high-altitude airports by taking advantage of cold temperatures only to become stranded as temperatures warmed and air density decreased.
- Gliders (sailplanes) are especially vulnerable to hot and high conditions. Of course before being cut loose from a powered aircraft, they must be at a minimum altitude above the earth's surface.
Improving hot and high performance
Some ways to increase aircraft performance in hot and high conditions include:
- Reduce aircraft weight. Weight can be reduced by carrying only enough fuel to reach the (lower-altitude) destination rather than filling the tanks completely. In some cases, unnecessary equipment can be removed from the aircraft. However, in many cases, the only practical way to adequately reduce aircraft weight is to depart with a smaller passenger, cargo, or weapons load. Consequently, hot and high conditions at the originating airport may prevent a commercial aircraft from operating with a load large enough to be profitable, or may constrain the firepower that a combat aircraft can bring to bear when conducting a long-range airstrike.
- Increase engine power. More powerful engines can improve an airplane's acceleration and reduce its takeoff run. However, more powerful engines are generally larger and heavier and use more fuel during cruise, increasing the fuel load needed to reach the same destination. The added weight of the fuel and engines may negate the potential performance gain, and the added cost of the extra fuel may constrain the profitability of a commercial aircraft. On the other hand, replacing an older, less efficient engine with a newer engine of more advanced design can increase both power output and efficiency while sometimes even decreasing weight. In this situation, the only real disadvantage is the cost of the upgrade.
- Increase the size of the wings. However, larger wings weigh more and increase drag, making the aircraft slower and increasing fuel consumption during cruise.
- Add high-lift devices to the wings. However, these devices typically add weight and mechanical complexity, increasing manufacturing and maintenance costs. Furthermore, some high-lift devices increase drag during cruise.
- Utilize assisted take off devices, such as rockets, to increase lift and acceleration.
- Inject distilled water during takeoff in the engine (compressor or combustor). Evaporating water reduces the temperature, so more fuel can be added, increasing power.
Alternately, runway lengths may be increased to make up for reduced aircraft performance.
Jet or rocket assisted take off
Auxiliary rockets and/or jet engines can help a fully loaded aircraft to take off within the length of the runway. The rockets are usually one-time units that are jettisoned after takeoff. This practice was common in the 1950s and 60s, when the lower levels of thrust from military turbojets was inadequate for takeoff from shorter runways or with very heavy payloads. It is now seldom used.
Auxiliary jets and rockets have rarely been used on civil aircraft due to the risk of aircraft damage and loss of control if something were to go wrong during their use. Boeing produced a JATO-powered version of its popular Boeing 727 primarily for "hot and high" operations out of Mexico City Airport (MMMX) and La Paz, Bolivia. The boosters were located adjacent to the main landing gear at the wing root on each side of the aircraft.
Several manufacturers of early jet airliners offered variants optimized for hot and high operations. Such aircraft generally offered the largest wings and/or the most powerful engines in the model lineup coupled with a small fuselage to reduce weight. Some such aircraft include:
- The BAC One-Eleven 475 combined the short body of the series 400 with the more powerful engines and improved wings of the series 500. Also featured stronger landing gear for rough field operations.
- The Boeing 707–220, which was a 707–120 airframe fitted with more powerful Pratt & Whitney JT4A engines, civilian versions of the military J75. The 707-220 had extremely high fuel consumption, and only 5 were built, all for Braniff International Airways. The 707-220 was rendered redundant by the release of the turbofan-powered 707-120B, which had even greater power along with much lower fuel consumption.
- The Convair 880. Although Convair only offered one configuration of this aircraft, it had more power and a smaller fuselage than its competitors from Boeing and Douglas. Convair essentially wagered the success of the entire 880 model line on the appeal of an aircraft optimized for hot and high operations. The wager failed; only sixty-five 880s were sold and Convair's nascent airliner business soon collapsed.
- The Lockheed L-1011-200, which was otherwise an L-1011-100 with more powerful RB.211-524B engines.
- The McDonnell Douglas DC-9-20, which combined the smaller fuselage of the DC-9-10 with the larger wings and more powerful engines of the DC-9-30, and was significantly outsold by both.
- The McDonnell Douglas DC-10-15, which combined the fuselage of the DC-10-10 with the larger engines of the DC-10-30. These were specifically designed for and sold to Aeromexico and Mexicana. Only seven were built.
- The Vickers VC10, which was designed to meet BOAC requirements for a large airliner that could operate medium range flights from short runways in southern Asia and Africa. The resulting high fuel consumption compared to the contemporary Boeing 707 prompted all other major airlines to dismiss the VC10.
The marketing failure of these airplanes demonstrated that airlines were generally unwilling to accept reduced efficiency at cruise and smaller ultimate load-carrying capacity in return for a slight performance gain at particular airports. Rather than accepting these drawbacks, it was easier for airlines to demand the construction of longer runways, operate with smaller loads as conditions dictated, or simply drop the unprofitable destinations.
Furthermore, as the second generation of jet airliners began to appear in the 1970s, some aircraft were designed to eliminate the need for a special "hot and high" variant – for instance, the Airbus A300 can perform a 15/0 takeoff, where the leading edge slats are adjusted to 15 degrees and the flaps kept retracted. This takeoff technique is only used at hot and high airports, for it enables a higher climb limit weight and improves second segment climb performance.
Most jetliner manufacturers have dropped the "hot and high" variants from their model lineups.
Less formally, hot and high can also describe a botched landing approach, where a fixed-wing aircraft is too fast (hot) and too high above the glidepath: since the only way to increase the rate of descent is to increase speed, and the aircraft is already too fast, it is unlikely that the approach can be successfully carried to a landing.
Hot and high airports
Notable examples of hot and high airports include:
- Addis Ababa – Bole Airport
- Brasília – Brasília Airport
- Bogotá – El Dorado Airport
- Calgary – Calgary International Airport, especially from late spring to early autumn
- Canberra – Canberra International Airport
- Denver – Denver International Airport, especially from late spring to early autumn
- Edwards Air Force Base
- Harare – Harare Airport
- Johannesburg – O. R. Tambo International Airport
- Kabul – Kabul Airport
- La Paz – El Alto International Airport (not generally a "hot" airport, as average high temperatures are never more than 15 °C (59 °F) throughout the year, but one of the world's highest commercial airports at 13,325 feet (4,061 m))
- Las Vegas – McCarran International Airport
- Madrid – Madrid-Barajas International Airport
- Mexico City – Mexico City International Airport
- Phoenix – Phoenix Sky Harbor International Airport (altitude of 1,135 feet (346 m) is not extreme, but the area's hot desert climate gives it "hot and high" characteristics for most of the year)
- Quito – Mariscal Sucre Airport
- Salt Lake City – Salt Lake City International Airport, especially from late spring to early autumn
- Yerevan – Zvartnots International Airport