In a standard atmosphere, the outside air temperature decreases as altitude increases (some 2°C per1,000 ft). The engine performance is influenced by various parameters. The outside air pressure (altitude), aircraft speed, outside air temperature and bleed demand. All these factors affect the safety of the flight.
Under normal conditions, an increase in altitude brings a combination of two effects. The decrease in air pressure decreases thrust. The decrease in temperature tends to increase thrust. Combination of both is a net decrease of thrust because the influence of pressure is dominant
However, weather characteristics and geographical environment may affect the lower layer of the atmosphere. The standard atmosphere will not be encountered during each takeoff.
Amongst those cases, an increase in temperature can be met when the altitude increases. That is the temperature inversion.
Under such circumstances, an increase of altitude will bring a decrease of thrust that is substantial than usual, because the effect of pressure and temperature both contribute to the decrease.
Effect on aircraft performance
The certified takeoff performance is based on a constant ΔISA during the climb. In the event of temperature inversion, the climb performance will be affected in the cases where the thrust is affected.
However, to affect the aircraft performance, a temperature inversion must be combined with other factors.
During a normal takeoff with all engines operative, the inversion will have no effect since the actual aircraft performance is already far beyond the minimum required performance.
Then, the actual aircraft performance could be affected only in the event of an engine failure at takeoff.
However, conservatism in the aircraft certified performance is introduced by the FAR/JAR Part 25 rules, to take account for inaccuracy of the data that are used for performance calculations. Although not specifically mentioned, temperature inversions can be considered as part of this inaccuracy.
Therefore, a temperature inversion could become a concern during the takeoff only in the following worst case with all of these conditions met together:
– The engine failure occurs at V1,and
– Takeoff is performed at maximum takeoff thrust, and
– OAT is close to or above T.REF, and
– The takeoff weight is limited by obstacles, and
– The temperature inversion is such that it results in the regulatory net flight path margin cancellation
and leads to fly below the regulatory net flight path.
In all other cases, even if the performance is affected (inversion above T.REF), the only detrimental effect will be the climb performance to be lower than the nominal one.
The minimum climb gradient required at the point 35 ft above the runway for the second segment oneengine inoperative is:
– 2.4 % for twin engine aircraft.
– 3 % for four engine aircraft.
The margin between the net and the gross flight path is:
– 0.8 % for twin engine aircraft.
– 1 % for four engine aircraft.
Assuming a 10°C temperature inversion (above T.REF) between the ground and 1500 ft, the effect on the aircraft performance will be as described in the following graph.
The first graph applies to an A320 fitted with CFM engines. However, the effect of the temperature inversion on the engine thrust is quite similar whatever the engine type.
(about 10 % thrust loss with a 10 °C inversion). Thus, the effect on the climb performance, in terms of climb gradient, will be similar whatever the twin-engine aircraft model.
With an engine failure at V1, the graph shows the gross trajectory (curve A) limited by the minimum required second segment climb gradient with a normal temperature evolution with the altitude (-3°C between the ground and 1500 ft). The curve B shows the relevant net flight path.
The curve C shows the gross trajectory with a 10°C inversion from the ground to 1500 ft.
The graph shows that for conservative conditions and particularly an engine failure at V1 and a temperature inversion of 10°C, although the gross climb gradient is affected it should not become a concern.
Should the engine failure occur later during the takeoff, it will provide an additional margin since providing more time and more climb capacity with all engines operating.
The graph also show that there is a margin between the gross flight path (with inversion) and the net
light path (computed without inversion) which still remains available for obstacle clearance.
Obviously, the margin reduces with the distance due to the inversion. However, it is more likely that an immediate return to the departure airport will be initiated following an engine failure at V1 while only very remote obstacles would be a concern.
An extrapolation of the above graphs will show that the actual gross climb gradient (with a 10°Ctemperature inversion) will reach the required net gradient (calculated without inversion) at a distance of approximately 42 000 m for twin-engine aircraft or 38 800 m for A340 models.
This situation could become a concern but again, this is still assuming an engine failure at V1, a climb gradient limited by very remote obstacles, no immediate return to the departure airport, takeoff performed on a hot day condition (while inversion should not develop) and a temperature inversion with a great magnitude. This has a very low probability of occurrence.
Expected temperature inversions during takeoff: Human Factor
Some airlines operating in desert regions were subject to frequent temperature inversions have established with their local meteorology agency policy with regard to the temperature inversions.
The inversions being regularly published by the meteorology agency during the day, these operators take them into account in the takeoff performance determination.
Pilot reports can be also used for inversions encounter report.
Although temperature inversions are of a particular concern only when associated with additional conditions such as high OAT, performance and remote obstacles limited takeoff weight and engine failure, large temperature inversions can degrade the takeoff performance.
Therefore, if frequently exposed to large temperature inversions, and when they are reported, it is still advisable (Airbus Performance) to take them into account for performance determination particularly if obstacle limited and OAT at or close to T.REF.
This permits, as an additional measure, to keep the required margin on the takeoff performance in its whole in the event of an engine failure.
Unexpected temperature inversion during takeoff: Human Factor
If not reported, there is obviously no way to account for the effect of a possible temperature inversion.
If an engine fails during the takeoff while an inversion condition is present, there is no requirement for application of any specific procedure.
The low probability of having all the detrimental conditions previously described met together and no possibility of a return to the departure airport reinforces this.
The abnormal procedures for engine failure will have to be followed and we believe that, during this particular and increased workload situation, there is no room for pilots to speculate for a possible temperature inversion and no way to regain a part of the thrust.
This is particularly true for aircraft fitted with a FADEC fully managing the thrust according to the selected trust Lever Angle (TLA).
At the very most, and in accordance with the recommended procedures for an engine failure during takeoff, in the case where flexible takeoff was used, the performance may be improved if required, by setting the operative engine to the full takeoff thrust.
The engine is protected against EGT limits exceedance with some margins and engine deterioration is limited.
The aircraft performance is determined in accordance with the flat rate concept. The takeoff performance is based on a constant reduction of the temperature with the altitude. However, specific weather conditions may lead to temperature inversions.
There is no doubt that temperature inversions have a direct effect on the engine and the aircraft performance during the takeoff climb.
This effect can be completely ignored when all engines are operative. When of great magnitude and when combined with other severe conditions such as an engine failure at V1, high OAT and performance limited by remote obstacles, it may become a concern. But the combination of all these events is unlikely to occur.
Despite that there is no regulation requiring the taking into account of such an effect for takeoff performance determination, temperature inversions with great magnitude when known, should be considered. This is particularly true if you are operating in areas frequently affected by inversions with a great magnitude.