If pilots have no control over brakes / air brakes / reversers, can they slow down the plane after landing by carefully steering it off the runway into the grass? Would it be better to put only one gear into the grass or all three?
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$\begingroup$ How could that not be true? Whether it's a good idea; how cost- or safety-efficient it might be are separate questions… $\endgroup$– Robbie GoodwinFeb 16, 2022 at 22:57
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3 Answers
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Depends on what the alternative is. Hopefully you're going fairly slow when you get to that point. In any case, if the choice is between grass and a drop off into a ravine, go for the grass. If the choice is between grass and steel lighting structures off the end, grass wins again. If the choice is between grass and special overrun material like sand or EMAS, well in that case grass loses.
How you do on the grass will depend on the surface condition, how dry it is, etc and how fast you are going. If you're going fairly slowly, you won't likely collapse the gear, just make big furrows. I would prefer to get both mains onto the grass to minimize the risk of going sideways and collapsing the gear, which is what you really want to avoid.
But in any case, you will do better on the turf than going over a cliff or into structures that are going to shred your airplane even at low speed. If you can see what is beyond the end of the runway as you get there, the choice will be fairly easy.
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1$\begingroup$ If the choice is between grass and tarmac, grass also loses, IIRC. That's why the stopping distance on a grass runway is longer than the distance on a paved runway. $\endgroup$ Feb 16, 2022 at 11:34
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1$\begingroup$ @nick012000 Is that based on the brakes working? With brakes, one would would expect to get greater traction, and therefore retardation, on tarmac. $\endgroup$– sdenhamFeb 16, 2022 at 13:06
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$\begingroup$ @sdenham "Is that based on the brakes working?" Yes, though I doubt that grass runways would become more effective than paved ones if your brakes aren't working. $\endgroup$ Feb 16, 2022 at 13:11
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$\begingroup$ @nick012000 That is an independent question, to which the performance with brakes has little obvious relevance. I would guess that a flat, grassy surface firm enough to not be deformed by the aircraft's weight would be about the same as tarmac, though when we are talking about airliners (as we are), that firmness criterion might not hold very often. $\endgroup$– sdenhamFeb 16, 2022 at 13:19
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3$\begingroup$ @Jpe61 "humus". Not to be confused with "hummus"..."humus" is not good in a sandwich. $\endgroup$ Feb 16, 2022 at 20:21
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Not a good idea.
Airliners are heavy objects with a lot of weight on just a few wheels. Airport soils are generally pumped for low ground water level, if needed, to make soil firmer. I know Amsterdam airport Schiphol is. Then still, the grass is in most cases too soft to carry the load, making wheels sink in. This will most likely slow down the one side that goes off first so much that further directional control cannot be maintained, and the aircraft crashes. For this reason, even aircraft specially designed to land on grass would do better to stay on either the grass on the runway during a landing.
Other than that, it's like with belly landings. The best place by far to land an aircraft is on a runway.
An increasing number of airports is equipped with a runway extension composed of material specially designed to safely stop an aircraft after overrunning the runway.
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2$\begingroup$ Next question: is a belly landing better than running off the end of the runway? $\endgroup$ Feb 14, 2022 at 0:20
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4$\begingroup$ @RobertDiGiovanni If caught in the situation, I would make that choice depend on how long the runway is and what the end looks like and other circumstances, but. In general, I would prefer a belly landing. Even if it is a decision for damage. $\endgroup$– user55607Feb 14, 2022 at 0:32
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1$\begingroup$ Better to have all wheels up symmetrically and plan for that than have one gear collapsed / sheared off randomly. $\endgroup$ Feb 14, 2022 at 6:57
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26$\begingroup$ RAF Benson (Oxfordshire, UK) maintained a long grass runway, ostensibly for towed glider launches and landings. However, its ulterior purpose was for the diversion of RAF aircraft with undercarriage issues, because a grass landing almost eliminates sparking. Having a live bomb load is a major disincentive to a runway belly landing. (Source: I was there as an ATC cadet.) $\endgroup$ Feb 14, 2022 at 9:46
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1$\begingroup$ @RobertDiGiovanni : that's highly dependent on what is after the runway. Is it a lake? A cliff? Some buildings? Or a corn field? $\endgroup$– vszFeb 14, 2022 at 12:06
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Typically, you have a main braking system, an alternate one and a backup one. Then you have reversers, and runway over run.
The only time i would choose grass (in an airliner) is the one at the end of the runway.
Also, the runway is a better platform for emergency vehicules and passenger evacuation.
