# Why couldn't my aircraft depart fully fueled on a 44C day?

My flight from Abudhabi to SFO was postponed. Official reason given to us was outside temperatures were too high - around 44+ degree Celsius and aircraft can not take off with full fuel loads at such high temperatures.

While sitting at terminal I saw other flights taking off. When I asked staff about those flights, answer was they are short haul - around 4-5 hours distance.

How does out side temperature affect these flights? Does it have anything to do with fuel volume changing with higher temperatures and hence wrong fuel consumption calculations?

• I think better suited to Aviation (and perhaps already covered there) but I think the gist is that warm air is less dense than cold, so provides less lift - hence more energy (fuel) required. (Long-haul being near capacity limit even when cool). – pnuts Jul 4 '15 at 19:50
• Warmer air => Less air density => Less available thrust + Less lift => Decreased maximum take off weight. A basic physic fact is that liquids volumes hardly depends on its temperature (it can vary a by a bit, but the difference is usually negligible). The issue is therefore not aboutt fuel volume changing – Antzi Jul 6 '15 at 9:08
• @pnuts It is about fuel and lift but your comment is kind of backwards. The actual reason is that, since warmer air means less lift, the plane can't take off at full weight. Because it can't take off at full weight, it can't have enough fuel on board to make a long-haul flight. The vast majority of the flight will be at air temperatures far below +44C, so having to burn more fuel at take-off to generate lift wouldn't be a huge deal; the problem is that the engines can't burn enough fuel to generate enough lift to take off. – David Richerby Mar 7 '17 at 12:19

There are two problems:

First, at higher temperatures, the air is less dense; therefore there is less oxygen (by mass) in every cubic metre of it; therefore more air must be ingested by the engine (by volume) for the same quantity of fuel to be completely burnt. If the intake flow rate of air is fixed, then less fuel can be burnt and less power developed compared to a colder air temperature.

The second problem is that the lower density of air reduces the lift generated by the aircraft's wings at any speed. To make up for this the aircraft can travel faster.

In practise this means that aircraft operating in "hot and high" conditionsâ€”altitude also affects air densityâ€”will require longer runways to take off.

If it gets too hot then the aircraft might not have enough runway to get airborne.

For safety reasons the runway must be long enough for the aircraft to come to a stop if the commander decides to reject the take off at the last moment. But fast, heavy aircraft take a long time to stop; so this means the runway must be even longer.

If the aircraft is lightly loaded then it is not such a problem; there is less mass to accelerate and a lower speed must be obtained for take off. Thus a short flight simply takes a bit longer but still gets off the ground. But for AUH-SFO, this is a very long flight and will require a great deal of fuel to be taken onboard (at a guess I would imagine something on the very rough order of 80 tons). The specifics will vary by aircraft, and by how much is on them.

https://en.wikipedia.org/wiki/Hot_and_high

• I can definitely confirm this. I live in a climate that can be very hot in the summer and we occasionally get cases of planes having to leave some passengers behind because of "weather" when it's sunny out. We don't get cancellations because there are other airports around, they can always put down for fuel if they have to. For an overwater flight, though, I could easily see a cancellation. – Loren Pechtel Jul 4 '15 at 22:16
• For jet aircraft, the first "problem" is a non-problem. if density altitude were an issue for the turbojets in modern jetliners, then they would not be able to fly at 40,000+ feet. The FADEC and the compressor of the jet engine mix the fuel in the proper proportion for the density altitude. – rbp Jul 6 '15 at 17:53
• @rbp -- the OAT and power demand (i.e. fuel flows needed) are both far lower at altitude than they are during a T/O. – UnrecognizedFallingObject Jul 7 '15 at 2:43
• @rbp: Fact is, trust will be less at higher temperature. Limiting the take-off in most cases is the remaining climb speed with one engine inoperative. Above a certain temperature and aircraft mass it will be impossible to survive an engine failure above v1. Either mass or OAT must be lowered for the flight to take off safely. – Peter Kämpf Mar 7 '17 at 20:11
• @rbp, you are accidentally right that â€śmore air must be ingested â€¦ by volume â€¦ fuel to be completely burntâ€ť is wrong, because turbines work with excess air and this is not the limiting factor. However, lower density still does limit the thrust, due to RPM limit (at lower density, the same RPM gives lower mass flow) and due to temperature limit (you can add less heat to an already hot air before it becomes too hot for the turbine). So hot and high is very much a problem for turbine aircraft. The reason is just somewhat different from spark-ignited engines. – Jan Hudec Mar 7 '17 at 21:37

Does it have anything to do with fuel volume changing with higher temperatures and hence wrong fuel consumption calculations?

I doubt wrong fuel consumption calculations are involved, but fuel density is a limiting factor for some aircraft, both insofar as limiting takeoff weight and possibly prohibiting takeoff. For example, for a 747-400BCF, a minimum fuel density of 6.0 lbs/gal is required up to 820,000 lb. From 820,000 lbs to 870,000 lbs the minimum changes linearly from 6.0 to 6.43. From 820,000 to 870,000 there are also takeoff centre of gravity (CG) restrictions. The CG must be forward of 19.1% MAC1 at 820,000 changing linearly to 19.5% MAC at 850,000, then linearly to 20.0% MAC at 870,000.

If you wish to see this graphically displayed, go to section 1-05-001 of the manual at http://terryliittschwager.com/WB/manuals/Boeing_747-400BCF_GPR1_WBM.pdf, pdf page 69 for lbs, 70 for kgs.

Also some aircraft have a prohibition against operation in ambient temperatures above a certain point. I seem to remember to remember 54 Celsius for 747-100 and -200 aircraft, but don't hold me to that.

1MAC = Mean Aerodynamic Chord of the wing. The position of the CG is usually expressed as a percentage of the MAC where 0% is the leading edge and 100% is the trailing edge.

• What is CG? Centre of gravity? And MAC? Thanks – Calchas Jul 5 '15 at 21:20
• @Calchas You are correct, CG is center of gravity. MAC is Mean Aerodynamic Chord. The longitudinal CG of large aircraft is usually expressed in terms of the percentage of the Mean Aerodynamic Chord. Thus if the CG is 20.0, what they're saying is that it's 20% of the way from the leading edge of the MAC to the trailing edge. The CG enters into many limitations. What the MAC actually is, is considerably more complicated, but think of it as the distance from from the leading edge of the wing to the trailing edge if the wing was of a constant distance from leading edge to trailing edge. – Terry Jul 5 '15 at 21:34
• The way I think of MAC is that it identifies the relative position between the Center of Pressure and the Center of Gravity. For longitudinal stability, the CG has to be further forward than the CP. Both of these can be expressed as %MAC. – rbp Jul 6 '15 at 17:56

There are a lot of complex answers to a rather simple question.

As others have mentioned, the pressure of air decreases as temperature increases. This means the fuel in the tanks is being less compressed by the air pressure, thus the fuel makes up for this by taking up a larger volume for a specific mass.

Jet airliners measure fuel consumption in mass, opposed to volume. So whilst the volume remains constant, environmental conditions can alter the amount of mass it takes to fill that volume. Simply put, a "full" tank (considered by volume) would weigh more on a cold day than on a hot one.

• Mostly true, but it isn't the pressure of air decreasing which makes the fuel less compressed. It is simply the fuel expanding due to its own temperature increase. This happens no matter what the temperature is in the air around the fuel (particularly noticeable when fueling from underground tanks and the fuel is significantly cooler than the ambient temperature). – Lnafziger Mar 7 '17 at 4:43