I was reading about flight RWAF9268 earlier today when a questions popped into mind. When a large commercial jet touches down, is it required to try and come to a complete stop? Or is it allowed to take off again if part of the craft fails (like the braking system, for example)? And what factors would make a pilot decide to stay one the ground, or just abort the landing?
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3 Answers
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When a large commercial aircraft touches down, it is not committed to stopping.
However, it is committed to stopping when the thrust reversers are selected. This is because they take different time to stow and adding power while stowing would likely cause significant asymmetric thrust. And even if they don't, they take their time to stow and you are unlikely to have enough speed and runway when they finally do.
When thrust reverses are not selected, the pilots of course have to consider whether it is more likely that they can stop or lift-off again on the remaining runway. And they sometimes do choose to lift off. There are many incidents where planes went around after tail or even wing strikes or bounces as it's often easier to get the plane under control by adding power. But once thrust reversers are selected, stowing them again means loosing a lot of time and is risky, so go-around is considered out of question afterwards.
answered Jun 3, 2015 at 10:18
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$\begingroup$ This is the correct answer to my knowledge, at least for Boeing jets. $\endgroup$– WakedCommented Jun 3, 2015 at 12:12
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2$\begingroup$ +1 for mentioning that they are committed to stop when reverse thrust is selected. This seems to be the relevant factor in the case cited by the OP. Reverse thrust was selected in that incident (twice, actually,) though the reversers didn't actually deploy. It does seem like violating that rule would probably have been the best decision in this case, though, since it seems that the crew was aware that the reversers weren't deploying and they were still near touchdown speed even when they overran the end of the runway. $\endgroup$– reirabCommented Jun 3, 2015 at 16:23
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1$\begingroup$ This answer (and the comment from @reirab) do speak very well to RWAF9268. But really that was just an example of a situation, not the question. (I only mention this to explain why I'm not marking this as the answer, even though I did vote it up because it's useful to know.) $\endgroup$– Jae CarrCommented Jun 3, 2015 at 17:49
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2$\begingroup$ @JayCarr, the risk associated with retracting thrust reversers while adding power for a go-around is deemed unjustifiable high. When they don't retract fully, or not at the same time causing the aircraft to yaw then the effect is likely catastrophic. So the rule is when thrust reversers are selected, you are committed to land. Of course rules can be broken in emergencies and in hindsight any action that turns an emergency into a good outcome is justifiable. But fact is that breaking rules more often leads to catastrophe than it is preventing it. $\endgroup$– DeltaLima ♦Commented Jun 3, 2015 at 21:29
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3$\begingroup$ Technically the elements that you name (rwy length, grip, a/c weight & speed) of course matter. But it is impossible for a pilot to accurately balance those factors while the aircraft is speeding down the runway. The options have to be considered before landing. When the wheels are on the ground the decision is made, stop or go-around. If it is stop, then TRs are selected. If you can't stop in time with TR, then stowing them again and then applying full power (quite a number of seconds get lost in the process) will unlikely bring you back in the air in time. $\endgroup$– DeltaLima ♦Commented Jun 3, 2015 at 21:34
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Boldface emergency procedures are procedures that the aircrew should have committed to memory and are written in bold text. Generally speaking, the boldface for all emergencies involving brake failures, gear failures, control failures, literally anything that involves control of the aircraft after touchdown, starts with:
If fly-away airspeed available:
1. Go-around
If fly-away airspeed unavailable:
2. Do specific airframe related items.
Emergency procedures like these will be committed to memory, and then the only problem for the aircrew to solve will be if they have enough runway remaining to get airborne again. If they don't have enough runway remaining then usually the fix to is apply emergency brakes (if the pedals hit the floor) or use backup nose wheel steering (for a steering failure) and ride it out.
Note: use of these emergency procedures doesn't necessarily indicate a failure of an aircraft system. If a sudden gust of wind at touchdown weathervanes the aircraft 45 degree off centerline, you can be sure the crew is checking for fly-away airspeed.
answered Jun 2, 2015 at 20:40
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$\begingroup$ And if fly-away airspeed is available but the emergency makes going around a bad idea (for instance, a severe control failure)? $\endgroup$– VikkiCommented Jun 22, 2019 at 22:54
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No, when an airliner touches down, it is not committed to stop.
However, a go around after touchdown is not an easy task and sometimes can be very dangerous. A hypothetical situation is discussed here which is similar to Flight 9268 which you mentioned.
Compared to a landing, more runway is required for a takeoff. After the touchdown, pilots have very little time to decide to take off again if they cannot stop. Fortunately1, most of the time, this decision is made very quickly. However, if an airplane is slowed down below the minimum safe go-around speed, there will (or may2) not be enough runway remaining to take off again. Therefore, pilots should accept a runway overrun, as it is probably a lesser evil.
To take off after a touchdown, there are several very important factors to consider. They include:
- Remaining runway
Probably the most important factor. - If airplane is at or above minimum safe go-around speed, or can achieve with enough runway remaining
- Actual touchdown point
- Density altitude
- Wind
- Pilot reaction time (decision time)
- Inertia of the airplane
- Time required to configuration change
1: In cases when the pilots decide to abort the landing, they decide it within the first few seconds of touchdown. I found a video of this (and this one) showing the airplane took off very quickly when aborting the landing (after the wheels touched down). Most of the time, they abort the landing before touchdown, when the gears are just a few feet off the ground (as seen here and here).
2: This depends on runway length.
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4$\begingroup$ I respectfully disagree. Landing at full flaps, your takeoff roll at max power (super light on gas) is going to be substantially less than what was initially required on takeoff. If you had an emergency right after touchdown you could be airborne in just a few thousand feet. $\endgroup$ Commented Jun 2, 2015 at 20:58
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2$\begingroup$ A short field takeoff is much different than a go-around after touchdown. One assumes a starting airspeed of 0 IAS, the assumes a starting airspeed of around 130 IAS. Once the decision is made to go-around on deck it is much the same as a short-field takeoff, but the initial parameters, less fuel weight, much greater airspeed, etc all work in favor of the go-around. The only variable that is hard to judge is spool-up time. $\endgroup$ Commented Jun 2, 2015 at 21:19
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1$\begingroup$ @RhinoDriver I updated my answer. I said no to the question in the title, but I agree that it was not clear. $\endgroup$– FarhanCommented Jun 3, 2015 at 13:51
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2$\begingroup$ @Waked I added some clarification so that we are on the same page. $\endgroup$– FarhanCommented Jun 3, 2015 at 13:52
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1$\begingroup$ I'm wondering if maybe you should be incorporating Jan Hudec's answer regarding engine reversing? It sounds like that's at least on case where it's pretty cut and dried...not including it makes this answer feel incomplete. Not sure if that's kosher though...gonna put a question on Meta: meta.aviation.stackexchange.com/questions/1648/… $\endgroup$– Jae CarrCommented Jun 3, 2015 at 21:13