Redundant Power
Redundant Power
Do modern airliners have speed limiters?
A plane needs a lot more power to get it off the ground and through the thicker air at low altitude, than it does at high altitude. Presumably they also have redundant power capacity in case of engine failure, meaning that the engines are never run at full power. Therefore, what would happen if the engines were ramped up to 100% of their operational power, either through computer error, or human override?
I'm interested whether the speed alone could cause it to lose integrity, but also if that factor was ignored, what speed could they reach?
That is a rather complex question, in its implications.
First a few facts: engines are effectively run at 100% of their rated power during the takeoff. Losing an engine then means more runway being needed to reach the lift off point, and reduced climb capability once airborne. And the actual takeoff runway length is a function of the distance required to get in the air when losing one engine at the worst possible moment (i.e. when the distance from failure to being airborne is the same as cutting the live engine, applying brakes and coming to a stop) and the demonstrated climb capability, including obstacle avoidance, is also mandated to be satisfied with one engine out.
Note that 100% thrust is a varying condition that changes with temperature and pressure; thus maximum power on a hot day at high elevation airport would be less than on a cold day on an airport located close to sea level.
Also note that maximum thrust is also somewhat vague concept. For instance, there is the takeoff thrust, that can be endured usually only for a limited period of time, to avoid cumulative damage and over stressing engine elements; and maximum continuous rating, which, as the name implies, could be sustained indefinitely.
There is even, in some cases, an advanced power rating, that puts the engine in an emergency high power level, to compensate for another engine that would have failed. That regime makes the engine a prime candidate for extensive post-flight inspection, potential repairs, even scrap status, but if this what is needed to avoid a crash, so be it.
Now, on to your question. Aircraft have a lot of speed limits, and those are a function of the condition. For instance, there is a maximum speed for having flaps deployed. There is a maximum speed for having the landing gear down. And there is a maximum structural speed limit. Note that all of those are expressed in equivalent air speed. When one gains altitude, the air pressure reduces, and the actual speed is higher than the equivalent airspeed, which is the one that determines the level of aerodynamic stress the aircraft is subjected to.
What would happen if a pilot was to crack up the engine to maximum? That depends a lot on the systems. On some aircraft with FADEC (full authority digital engine control) the throttle quadrant is essentially a request to a computer. Put it to max, and a computer will decide how much power to really apply. Therefore the crew would not be allowed to push the aircraft past its limit.
But what happens if there is a fault? Well, perhaps this is what happened to AF447. If the speed sensor returns a speed that is too low, because of a fault, the computer will crank up power to maintain what it thinks is a constant speed. If this fails to produce the desired result, then it could put the aircraft in a dive to avoid what it thinks would be a stall. And then the airframe could be subjected to speeds far greater that it is designed to sustain, with dire consequences.
There have been instances when an airliner reached speeds that were just above the speed of sound, but recovered. There are possibly instances where an airplane reached a speed that was over the speed of sound and broke up as a result. A lot of things happen when reaching the speed of sound, flutter among those, where a surface starts oscillating in a diverging manner, with increasing amplitude until mechanical rupture occurs.
And if the crew is unaware that the aircraft is doing it (after all, they most likely get the same speed indication on their display than the computer gets, it is all coming from the same sensors) then they are not properly equipped to realize what is happening before it is too late.
Evidently, the best approach is to avoid faulty sensors like the plague. Which may not be the easiest thing to do. After all, the sensors were designed by people who really believed that their design was perfect, and managed to convince certification authorities that this was a flawless system. But I am from a different school; I do not like things that cannot be cross checked using a totally different method.
I would envision a system where other sensors, like accelerometer and strain gages, being used to derive a rough estimate of the airspeed. Should there be a miss-match too high for comfort, then the crew would be alerted.
That is a fail safe system, and I, for one, do not understand why it is not implemented.
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