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In the discussion of Why doesn't the A380 use its outboard thrust reversers, one of the reasons given was that engines 1 and 4 would be at great risk of ingesting debris. It was pointed out that the 747 uses reversers on all 4 engines, and it's just as big (overhanging edge of runway). So I asked in a comment:

  1. If there is fear that the outboard engines would ingest crap (FOD) because they overhang the edge of the runway, why wouldn't that also apply to takeoff, when the outboard engines are presumably at high thrust? Additional: I realize that the jet blast is straight back, but suction on the front of the engine is pretty strong, too (enough to inhale a baggage cart).

  2. Does an engine at reverse thrust kick air (and debris) far enough forward that there is a real risk of ingestion and FOD?

  3. Could that be negated by designing the reverse thrusters to go mostly sideways?

I was asked to repost as a separate question, so here it is...

Community
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Phil Perry
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    The A380 outboard engines are considerably further away from the centre line than those of the 747: https://upload.wikimedia.org/wikipedia/commons/5/52/Giant_planes_comparison.svg – Greg Hewgill Jun 19 '14 at 00:01

1 Answers1

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For the numbered items:

(1) and (2) At takeoff there is (typically, but there are birds) nothing solid that's airborne in front of the airplane, whereas thrust reversers can put small but solid airborne material in front of the engines, especially if they're still being used with high power as the airplane slows.

(3) If you're saying that the air is only directed sideways rather that forward, that would cancel the forward thrust from that air, but it wouldn't get you much braking action.

Some other points:

  • The higher the airplane's speed, the more braking action you get from the reversers, so if you're going to use them, get into them as soon as possible.
  • If it's late night or early morning, and the airport has a noise problem, consider going into reverse but leaving the engines at idle. You'll still get significant braking proportional to the airplane's speed.
  • If you've got a long runway, consider using just idle or less than high-power reverse. In other words, use what you need/want, but no more.
  • Make sure you start out of a high power reverse well before slowing to taxi speed. There are speed guidelines for this, but I forget what they are. Actually, it's best in my opinion to avoid high-power reverse unless you really need it.
  • Make sure you're completely out of reverse by the time you reach taxi speed. Again, I've forgotten the guideline.
  • Consider where you are. If you're in, say, Germany, you can expect the runways to be clean, in Spain maybe not as clean, in Zimbabwe definitely not as clean. You get the idea.
  • Be really careful about using reverse on an icy runway. If your nose is not pointed straight down the runway, the reverse thrust will be working to put you off the side of the runway. An acquaintance of mine put a 747 off the runway at KJFK that way (totaled the airplane, a few people hurt, no fatalities).
  • Consider the characters of the engines on the airplane. Perhaps this is no longer a consideration, but back in the 1990s, P&W engines on the 747 were relatively tolerant of compressor stalls brought on by using too much reverse power at too slow a speed. GE engines, however, were not so tolerant.

The second 747 carrier I worked for used to have maintenance done at Santa Barbara, CA. When we would pick up the airplane, they would close the relatively narrow runway after our takeoff and sweep it. Not a reverse thrust situation, but it does illustrate the FOD problem.

Terry
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    the 747 runway departure, was it this? – Federico Jun 19 '14 at 06:02
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    When I say "sideways", I mean not deflected to 90 degrees, but simply not deflecting downwards, where it would send debris airborne. Maybe it's not feasible to build such asymmetric reversers? The old clamshell reversers could do it, but fanjets use a different mechanism. – Phil Perry Jun 19 '14 at 13:31
  • Why would reverse thrust be dependent on forward speed (bullet 1)? Regarding your last bullet, I've heard of pilots trying to back out of the gate using reversers (no tug available), and they stalled or even damaged their engines. – Phil Perry Jun 19 '14 at 13:34
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    @Frederico Yes. A friend of mine was in one of the jump seats and gave me a detailed account of the event as he recalled it. – Terry Jun 19 '14 at 17:08
  • @PhilPerry The way it was explained to me is that in reverse and at high speed, you have a lot of air with a relatively smooth flow coming in the front of the engine. As you slow, the amount of air the engine has to work with decreases, as well as increasing disruption to the smoothness of the flow. Also, the engine is having to do more "sucking." Explaining the physics of all that is beyond my expertise. In the case of the clam shell devices, it's also a matter of drag. Stick your hand out the window at 70 mph and it's harder to hold it there than at 20 mph. – Terry Jun 19 '14 at 17:24
  • @PhilPerry Using reverse thrust to backup (powering back) was done a lot with 727s. I seem to remember that AA did it regularly but then abandoned it because of problems. I backed up a 727 a few times. I never backed up a 747, but I knew a guy who did once. The big risk is FOD. The 727 engines are up high, the 747 down low and more susceptible. Compressor stall is a worry because of no forward speed pushing air into the engine; it's having to get it's air by sucking. Compressor stalls are never good for an engine, and severe stalls can ruin one. Maybe a question on compressor stalls? – Terry Jun 19 '14 at 17:34
  • On one flight, I can remember sitting at a window seat watching the engines as they were run up before brake release. There was a wonderful little "tornado" reaching from the lower lip of the intake cowl straight down to the ground. Re drag from clamshells, the additional drag during deployment might be a structural issue, but I would think the added braking effect would be welcome. Is there that much drag when undeployed? – Phil Perry Jun 19 '14 at 17:35
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    the NTSB report @Federico shared in the above comment has been moved to http://www.ntsb.gov/investigations/AccidentReports/Reports/AAR9604.pdf – Erich Apr 28 '15 at 01:46