Stitch From United States of America, joined Jul 2005, 26682 posts, RR: 83 Reply 1, posted (7 months 1 week 5 days 20 hours ago) and read 2475 times:
Disabling reverse on the outboard issues could reduce the risk of runway foulment when operating out of fields where it could be a risk.
As I understand it from posts here on the matter, the A380 does not have thrust reversers on the #1 and #4 engine because they are far enough outboard on the wing that they could possibly blow debris onto the runway when engaged.
As I understand it from posts here on the matter, the A380 does not have thrust reversers on the #1 and #4 engine because they are far enough outboard on the wing that they could possibly blow debris onto the runway when engaged.
It's more that if installed, the 1 and 4 reversers when utilized would blow FOD forward of the engine and then the engine suck in the FOD.
"A passenger bets his life that his pilot is a worthy heir to an ancient tradition of excellence and professionalism."
KAUSpilot From United States of America, joined Jan 2002, 1955 posts, RR: 37 Reply 3, posted (7 months 1 week 5 days 15 hours ago) and read 2388 times:
Also, if one thrust revers is deferred (not working), the pilot will typically only use the symmetrical reverser on the opposite side (2 total, not 3). This is done to prevent assymetrical reverse from steering the airplane off the runway's center-line. A deferred reverser is a common occurrence, thus the chart. FOD mitigation, as discussed, is also a legitimate reason.
vikkyvik From United States of America, joined Jul 2003, 8199 posts, RR: 28 Reply 4, posted (7 months 1 week 5 days 14 hours ago) and read 2358 times:
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Quoting freeze3192 (Reply 2): It's more that if installed, the 1 and 4 reversers when utilized would blow FOD forward of the engine and then the engine suck in the FOD.
bikerthai From United States of America, joined Apr 2010, 1588 posts, RR: 4 Reply 6, posted (7 months 1 week 5 days 3 hours ago) and read 2213 times:
Quoting KAUSpilot (Reply 3): Also, if one thrust revers is deferred (not working), the pilot will typically only use the symmetrical reverser on the opposite side (2 total, not 3).
Some one can check me on this but don't the 747 wing have more sweep than the A380? This would put the outboard engines farther back with respect to the inboard engines reducing chance of FOD from reverse thrust vs the A380.
Besides, in reverser flow analysis, you always have to consider and unclean runway and potential for any debris. With an unimproved runway, the amount of FOD will be greater, but the analysis would be the same. It only takes one pebble to ruin your whole day.
Of course, on a 5600 foot runway such as that, probably better to use all of them!
Ignoring the fact that's a 747 on it's last flight to a museum so doesn't need to fly again, the photo shows all of the visible dust and debris is behind the engines. This is the case as long as forward speed is significant. That's why reverse is usually cancelled below 60-80 knots. On a 747 is not much use below that speed anyway. So FOD becomes a greater risk with reverse thrust at low forward speed.
On a normal paved runway the 747 outboard engines are over the runway surface and quite a long way from the grass. The same applies to the A380 but with less margin to spare. If operating reverse thrust on outboard engines near the runway edge is deemed a problem, then surely it's also a problem in forward thrust at TO power too? In that condition the engine is sucking in air from a wide area ahead, including the ground surface both on and to the side of the runway.
Quoting Stitch (Reply 1): Disabling reverse on the outboard issues could reduce the risk of runway foulment when operating out of fields where it could be a risk.
Never heard of this being done on the 747. Anyway the runway foulment (!?) that would happen in forward thrust too, only worse as even more dust could get kicked up onto the runway for the following aircraft to possibly ingest.
As KAUSpilot pointed out symmetrical reverse considerations is the reason why two and four engine thrust data is provided. As well as for pre flight dispatch, this also applies for cases where an in flight shutdown has occurred.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
vikkyvik From United States of America, joined Jul 2003, 8199 posts, RR: 28 Reply 8, posted (7 months 1 week 5 days 1 hour ago) and read 2155 times:
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Quoting Jetlagged (Reply 7): Ignoring the fact that's a 747 on it's last flight to a museum so doesn't need to fly again
I know that.
Quoting Jetlagged (Reply 7): This is the case as long as forward speed is significant. That's why reverse is usually cancelled below 60-80 knots. On a 747 is not much use below that speed anyway. So FOD becomes a greater risk with reverse thrust at low forward speed.
Absolutely.
Quoting Jetlagged (Reply 7): On a normal paved runway the 747 outboard engines are over the runway surface and quite a long way from the grass. The same applies to the A380 but with less margin to spare. If operating reverse thrust on outboard engines near the runway edge is deemed a problem, then surely it's also a problem in forward thrust at TO power too? In that condition the engine is sucking in air from a wide area ahead, including the ground surface both on and to the side of the runway.
I suppose it depends on your definition of "quite a long way". Not all runways have significant shoulders, like, off the top of my head:
Between the outboard edges of the outboard engines on a 744, it is about 145 feet. That's 2.5 feet clearance on each side on a typical 150-foot-wide runway. Of course, with runway shoulders, that will add extra clearance.
On the A380, it is about 168 feet between outboard engine centerlines (can't find the cowling diameter). That's well over the edge on a 150-foot runway, and possibly close-to or over the edge of the shoulder too.
With regard to forward thrust, I doubt you have nearly as much force impacting the ground (from the suction into the engine) than you do in reverse (from the exhaust out to side of the engine).
Jetlagged From United Kingdom, joined Jan 2005, 2452 posts, RR: 17 Reply 9, posted (7 months 1 week 4 days 21 hours ago) and read 2086 times:
Quoting vikkyvik (Reply 8): Between the outboard edges of the outboard engines on a 744, it is about 145 feet. That's 2.5 feet clearance on each side on a typical 150-foot-wide runway. Of course, with runway shoulders, that will add extra clearance.
On the A380, it is about 168 feet between outboard engine centerlines (can't find the cowling diameter). That's well over the edge on a 150-foot runway, and possibly close-to or over the edge of the shoulder too.
With regard to forward thrust, I doubt you have nearly as much force impacting the ground (from the suction into the engine) than you do in reverse (from the exhaust out to side of the engine).
I stand corrected, I was going from memory, based on previous discussions there have been when this came up re the A380. I should have checked before posting.
The airflow through the engine at TO thrust is greater than that in reverse (power is higher). The suction in front of a big fan at high power is considerable, and over a wide area. Anything loose lying around will be drawn in, just as easily as if it have been impacted by reversed air flow. Figure 6.3.2 of that Airbus link shows the danger area around the intake has a radius of 29'. That extends all the way down the runway as the aircraft takes off.
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