Why pilots must not ignore the effect of temperature inversion

In a standard atmosphere, the outside air temperature (OAT) decreases as altitude increases (some 2°C per 1,000 feet). The engine performance is influenced by various parameters. The outside air pressure, altitude, aircraft speed, OAT and bleed air systems affect the safety of the flight.

Under normal conditions, an increase in altitude brings about a combination of two effects. The decrease in air pressure decreases thrust. The decrease in temperature tends to increase thrust. The combination of both results in a net decrease in thrust because the influence of pressure is dominant. 

However, weather and geography may affect the lower layer of the atmosphere and the standard atmosphere may not be encountered during each takeoff. Indeed, an increase in temperature can be experienced when altitude increases. This is known as temperature inversion.

Under such circumstances, an increase in altitude will bring about a decrease in the 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 (deviation to International Standard Atmosphere) during the climb. Also called the International Civil Aviation Organisation (ICAO) Standard Atmosphere, ISA is the yardstick to compare the actual atmospheric conditions at a given point of time, a skybrary.aero article points out. The ISA is based on a pressure of 1,013.2 millibars, temperature of 15°C and air density of 1,225 gm/m^3 at the mean sea level. Pressure falls at the rate of about 1 millibar per 30 feet in the lower atmosphere (up to about 5,000 feet). Temperature, as pointed out earlier, decreases at about 2°C per 1,000 feet until the tropopause is reached at 36,000 feet. Above the tropopause, the temperature is assumed to be constant at around -57°C.     

In the event of temperature inversion, the climb performance of an aircraft will be affected in the cases where the thrust is affected.

However, to affect 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.

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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 Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) Part 25 rules, to take account of 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 takeoff only in the following worst-case scenarios with all of these conditions occurring together:

1. The engine failure occurs at V1 (V1 is the minimum speed at which a pilot can continue to take off even after an engine failure). 

2. Takeoff is performed at maximum takeoff thrust.

3. OAT is close to or above T.REF.

4. The takeoff weight is limited by obstacles.

5. The temperature inversion is such that it results in regulatory net flight path margin cancellation and leads to flight below the regulatory net flight path.

In all other cases, even if the performance is affected (inversion above T.REF), the only detrimental effect would be that the climb performance would be lower than the nominal one.

The minimum climb gradient required at the point 35 feet above the runway for the second segment with one engine inoperative is:

1. 2.4% for twin-engine aircraft.

2. 3% for four-engine aircraft.

The margin between the net and gross flight path is:

1. 0.8% for twin-engine aircraft.

2. 1% for four-engine aircraft.

Assuming a 10°C temperature inversion (above T.REF) between the ground and 1,500 feet, the effect on aircraft performance will be as described in the following graph.

The first graph applies to an Airbus 320 fitted with CFM engines. However, the effect of the temperature inversion on engine thrust is quite similar, whatever the engine type (about 10 % thrust loss with a 10 °C inversion). Thus, the effect on 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 1,500 feet). Curve B shows the relevant net flight path.

Curve C shows the gross trajectory with a 10°C inversion from the ground to 1,500 feet.

Airbus takeoff performance

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 due to providing more time and more climb capacity with all engines operating.

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The graph also shows that there is a margin between the gross flight path (with inversion) and the net flight path (computed without inversion) which still remains available for obstacle clearance.

Obviously, the margin reduces with distance due to the temperature 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°C temperature 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.

A temperature inversion in which warmer air masses overlay cooler ones influences air pollution phenomena

Expected temperature inversions during takeoff: Human factor

Some airlines operating in desert regions and subject to frequent temperature inversions have established with their local meteorology agencies, policies with regard to the temperature inversions.

With 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 inversion encounter reports.

Although temperature inversions are of particular concern only when associated with additional conditions such as high OAT, with remote obstacles limiting takeoff weight and with 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 takeoff weight and OAT is 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 the 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 Full Authority Digital Engine Control (FADEC) fully managing the thrust according to the selected Thrust 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.

Conclusion

The engine is protected against Exhaust Gas Temperature (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 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 there being 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 of great magnitude.

(This article first appeared in safetymatters.co.in)

(Cover image courtesy Wikimedia Commons/spotters.net.ua/UR-SDV)