I have a hard time believing the software would be written such that it fails completely in overtemperature situations. More likely, it would clamp the temperature to the highest rated value, or (slightly better) extrapolate the performance curves outside of the highest point. With, of course, warning lights, and a healthy dose of disclaimers that the airplane is not actually tested/rated to fly outside the given range (extrapolation is better than nothing, but no substitute for testing or engineering).
That said, similar things have brought down vehicles. An early Ariane 5 rocket was destroyed because the software was designed for an earlier version, and a certain value (which had a larger valid range on the new rocket) overflowed. Instead of making a “best effort”, however, the overflow triggered an unhandled exception which took down the entire software stack (and the rocket).
Not really. Some aircraft have electronic versions of the performance charts available in the flight management system/computer so that you can do the performance calculations more quickly and with a lower chance of human error creeping in. They may also be used by the FMC to show you what your optimum cruising altitude and maximum cruising altitude are, but they are not embedded in the aircraft flight systems themselves (as far as I’m aware, each new generation of airliners has more and more integrated into their systems.)
As far as I know the only aircraft system that directly uses temperature information for a control parameter is the engine control system. Because the engine thrust is sensitive to temperature, it needs to know the ambient temperature to know what limits apply to the engine. When working out the take-off performance the pilots will enter the ambient temperature into the engine management system (known as a TRP in the plane I fly) and the FADEC computers that actually control the engine will then know what the maximum engine RPM for take-off is and limit to that value as well as any other limitations that apply. The FADEC also uses the temperature sensed with the aircraft’s temperature probe, I’m not sure what would happen if the sensed temperature was outside the limits.
When I was flying ultralights and very small homebuilt types of airplanes they really didn’t have much excess engine power. I recall a few hot summer days (ground level temps in the triple digits) when some of my heavier friends literally couldn’t get off the ground. It was so hot that the air had thinned enough that no matter how long they ran their little airplanes along the ground they simply never generated sufficient lift to get even an inch above the ground.
One day, when flying a single-engine Cessna in the Appalachians on another very warm summer day I had an earnest conversation with a much more experienced pilot about how I would not be able to generate sufficient lift to clear a nearby ridge on that day so I should plan to turn much earlier than usual when departing from the airport. Also, I might not want more than half fuel to save weight. Actually, I should have listened to her when she recommended staying on the ground but the conversation did enable me to safely complete the flight. Despite easily clearing that ridge before in that exact same airplane on that day I had only half the rate of climb I’d had on prior flights. Why? Hot air is less dense and that means left lift.
I could go on, but won’t. Suffice to say that every airplane I’ve flown has come with a manual that devoted at least a page, and more typically an entire chapter, to calculating how much lift one can achieve at a given temperature and humidity (which also effects things), with which information one calculates necessary runway lengths and how fast you can gain altitude.
Now, for passenger jets - even if it is capable of staying in the air at a particular temperature/humidity combination it may not be able to safely take off from a particular runway as necessary runway length increases along with temperature. There is one airport near Orlando, Florida, for example, that I flew into and out of on Southwest Airlines. I’m sure few others, if any, on the airplane on a particular day I am thinking of noticed it, but as a pilot I was quite aware of the pilot of that passenger jet performing a short-field take off from that runway. Judging by when/where we lifted off that was entirely appropriate and safe, but if the temperature had been much higher then a departure from that particular runway may not have been wise, even if the same plane at the same temperature could have taken off safely from a longer runway.
In 1990 it was more like tables and charts written on paper but yes, basically, they’d never done the calculations for those airplanes under those conditions.
With a couple hundred passengers in the back is not the time to experiment, you know?
Note that clearing an obstacle is not a function of the lift your aircraft is able to generate, but rather of its rate - or angle - of climb. In unaccelerated flight (e.g. straight and level, steady climb, or steady descent) lift is always equal to the plane’s weight.
It is certainly true that when density altitude is high, climb rates and angles suffer.
Don’t forget there are places in the west which tout the fact they are “mile-high” to start with. Ground altitude is an additional handicap over and above air density. Nobody’s likely to cancel flights from NYC or LAX, but in Denver and such places this is a real concern. Rough rule of thumb, air at 18,000 feet is half sea-level atmosphere. Even if you are not as high an altitude on the runway, you will still have to clear mountains not-too-far away.
Piston engines suffer more than turbines because the thinner air coming in still gets mixed with gas at the same ratio and compressed at the same ratio. 10% less air means at least 10% less power. Jets and turbines are generally designed to run at very thin altitudes anyway, so the power loss (IIRC, not a turbine pilot) is less but still significant. I know taking my motorcycle over some of the higher mountain passes, the loss of power at altitude was VERY noticeable. I’m not aware of any production piston planes off-hand that have turbochargers to pre-compress the air; at that level of complexity, get a turbine instead.
Anecdote: When Boeing was trying to sell passenger jets to China just as it was opening up in the 80’s or 90’s, they were asking the Chinese airline how they calculated load, take-off run, etc. for marginal places like Tibet. (Lhasa is just over 10,000 feet). Using the Russian airliners, the Chinese pilots applied this simple test - load up the plane, do a take-off run. If by point X on the runway they had not gotten off the ground, jam on the brakes, return to the terminal and offload a certain number of passengers, try again…
That’s actually not as much of a problem. Remember it is not so much the lack of performance that grounds aeroplanes it is the lack of performance data over a particular temperature. Given that higher areas will normally be cooler, you may be able to operate from them even though the performance is limited.
Basically hot temperatures and/or high altitudes reduces the load you can take-off with, but temperatures hotter than accounted for in the performance manual will ground the aeroplane regardless of whether or not it could physically fly in those conditions.
Well covered. As Richard Pearse just noted, for many turbine-powered airplanes (jets & turboprops), temperature (density altitude) is often not an issue if you toss the book out the window and assume all the engines will continue working normally. These airplanes are extremely powerful, and are easily able to get off of the runway and climb clear of close-in mountainous terrain at high temperatures and high elevations. That is, until you consider what happens when an engine lets go at the worst possible moment. For the corporate jet I fly, there are not many airports in the world where, if we really had to, we couldn’t get out of dodge with both engines turning. If we landed there, for the most part we can get back out in a ‘natives-are-coming-over-the-fence’ type scenario. However, back in the real world, we always calculate and abide by one-engine-inoperative takeoff and climb performance data. It can be severely restrictive when it’s hot and we’re up at a high elevation airport, especially if there’s higher terrain or obstacles nearby. Even though the chances of an engine failing during that particular takeoff are extremely remote, we won’t (and can’t per regulation) take the risk because the consequences of an engine problem would be… undesirable.
IIRC, turbine pilots must calculate take-off and landing distances for each flight. Is that true? (Of course you calculate density altitude in pistons and helicopters, but ISTR that the rules for turbines are much more stringent.)
I don’t know if it was my dad flying, or if dad was telling me about someone else; but someone was flying a Cessna 150 on a hot Summer day at Barstow-Daggett Airport. DAG is in the middle of the Mojave Desert, and ‘hot Summer day’ means fairly toasty. The pilot’s approach was spot on, and the landing looked normal – until he got over the end of the asphalt runway. The hot, black runway created enough rising air that it took a couple of tries for him to get down.
Yes. We have a book of charts for every runway at every airport we operate to and before every take off we calculate a maximum take off weight for the current conditions (wind, temperature, air pressure, engine ant-ice requirements, and wet/dry runway.) we then make sure our actual weight is lower than the maximum. If actual weight is lower than maximum then we usually calculate what ambient temperature would result in the maximum weight equalling our actual weight. We can then enter that temperature in to the thrust rating panel, effectively fooling the engines in to thinking it is a hotter day and forcing them to use less than max take off thrust. This significantly improves engine life.
For landing we are supposed to calculate landing distances but it is a much more simple process and I happen to know that we can land in 1200 metres which is much shorter than any runway we go to so I don’t bother doing those calculations unless something is unusual (shortened runway, high tail wind, wet runway, etc.)
Aerodynamics at a given IAS are pretty much independent of density altitude. High density altitude implies a sizable difference between IAS and TAS, which is at the heart of the problems discussed here.
