Recently a Boeing 747 aborted a take off while still on the ground due to the failure of one engine.
Can a Boeing 747 take off fully loaded on 3 engines?
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6$\begingroup$ This is a question that cannot be answered without further information. A short form answer is that if the engine failure occurs before the speed typically labeled as V1, the takeoff must be aborted as the probability of getting off the ground before the end of the runway is reached is small. If the failure occurs after the V1 speed, the takeoff should be continued and will normally succeed. If there is an attempt to abort after V1, the aircraft will typically be unable to stop before the end of the runway. The V1 speed varies by aircraft weight, altitude, and temperature. $\endgroup$– TerryCommented Jan 25, 2016 at 6:53
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1$\begingroup$ @Terry Didn't you just answer the question? "If A then X; else Y" is a perfectly valid answer. $\endgroup$– David RicherbyCommented Jan 25, 2016 at 7:17
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5$\begingroup$ @DavidRicherby Yeah, I guess you're right, but I am often obsessed with more detail than I have energy to properly compose, so I just comment. I'm quite happy to let those that have that energy get the rep points with a detailed answer. $\endgroup$– TerryCommented Jan 25, 2016 at 7:30
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3 Answers
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The short answer as detailed to some degree in previous answers and comments is that a 747 can takeoff if the engine fails after the V1 speed is reached but not if the failure occurs before reaching V1. Unfortunately, in my opinion, the owner of one of the answers has deleted his answer, which while perhaps imperfect did provide useful information, and perhaps as important will remove the comments.
It's best to understand the V1 speed within the context of a balanced field takeoff. See this Wikipedia article for a full explanation. The first two paragraphs are:
A balanced field takeoff is a condition where the accelerate-stop distance required (ASDR) is equal to the takeoff distance required (TODR) for the aircraft weight, engine thrust, aircraft configuration and runway condition.1 For a given aircraft weight, engine thrust, aircraft configuration, and runway condition, the shortest runway length that complies with safety regulations is the balanced field length.
The rejected takeoff initial actions speed V1, or critical engine-failure recognition speed (Vcef),is the fastest speed at which the pilot must take the first actions to reject the takeoff (RTO). At speeds below V1 the aircraft may be brought to a halt before the end of the runway. At V1 the pilot must continue the takeoff even if an emergency is recognized.
Let's translate this into a practical situation using a 747-200 with old steam gauges. Prior to engine start five plastic bugs would be positioned around the rim of the airspeed indicator: the first at V1, the second at VR, the third at V2, and the last two at speeds not relevant to our discussion. We're rushing down the runway at takeoff power and all goes well. When we reach V1, the PNF says "V1" and the PF takes his hand off the thrust levers (he had them there in case of an abort). When we reach VR, the PNF says "rotate" and the PF brings the yoke back briskly but smoothly to a pre-determined nose attitude (usually 12 or 13 degrees). When the landing gear is off the runway, you'll hear the gear down lock retract unlocking the gear up lever. When the PNF sees a positive climb rate on the vertical speed, he'll say "positive rate", the PF will call "gear up", the PNF will move the gear lever to up. When the airplane reaches V2 the PNF will say "V2".
The important thing to remember here is that if there had been an engine failure after V1, the only difference would have been that the first person to know which engine had failed would have called "engine failure #1" say. On the old 3-crew airplanes, that would usually be the f.e. Even if the engine was burning, no action would be taken until after V2, and probably not until after you had a clean airplane.
Just as a matter of information, here are some V speeds for a 747-200 with P&W JT9D-7Q engines for a Flaps 10 takeoff at 840,000 lbs:
- V1 168 kts
- VR 181 kts
- V2 189 kts
Now for an abort. An abort below 80 kts is no big deal. Between there and 100 kts gets more interesting. Above 100 kts it is not to be taken lightly, and an abort at say, 150 kts, is a bonafide emergency. A friend of mine had to do one of those, and he blew all of the 16 tires that had brakes. In my view, if an engine failure occurs within a few knots either way of V1, you're better off flying.
To quote from an accident report in which I was involved in the investigation:
The bang and the loss of power occurred around V1 speed. Two seconds after the bang, all four engines were brought back to idle, and braking action was initiated. The thrust reversers were not deployed. The aircraft came to a stop 300 m after the end of runway 20, above a railroad embankment. The aircraft was severely damaged; it broke in three parts.
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$\begingroup$ Aw, shucks -- I wouldn't have the foggiest clue if the Belgian accident investigators keep their investigation dockets online. I was looking forward to seeing your name in the docket :) $\endgroup$ Commented Jan 26, 2016 at 0:35
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3$\begingroup$ @UnrecognizedFallingObject When a U.S. aircraft crashes, the FAA goes over everything, understandably so. In this instance an FAA investigator looked at the weight and balance report. When he added up the weights of the individual ULDs, his answer differed from report's total by one pound. Since I wrote the W&B software that was used, it fell to me to explain how that could be. I had to produce a paper that showed why, if you entered the ULDs in kilograms, which the program then had to convert to lbs, it is the nature of floating-point arithmetic that this can happen and is correct. $\endgroup$– TerryCommented Jan 26, 2016 at 4:17
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$\begingroup$ ah! :) Understandable, then. $\endgroup$ Commented Jan 26, 2016 at 12:42
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$\begingroup$ Phew!! I thought you were pilot on that flight. $\endgroup$ Commented Jan 27, 2016 at 15:03
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$\begingroup$ For another explanation of BFL, see my answer here on Aviation.SE. $\endgroup$ Commented Jan 27, 2016 at 22:18
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Seems Boeing can even commence takeoff run with three engines: http://www.avherald.com/h?article=492675e0&opt=0 What do regulations say about this?
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$\begingroup$ Yeah. 3 engines and a full load of 1000 lb bombs apparently. Scary. $\endgroup$ Commented Jan 25, 2016 at 9:31
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2$\begingroup$ @curious_cat -- I suspect the airplane was unloaded for the return flight to Minsk -- I doubt you can do a 3 engine ferry with cargo. $\endgroup$ Commented Jan 25, 2016 at 12:42
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$\begingroup$ @UnrecognizedFallingObject True. My bad. I meant to say scary letting an off practice captain land a plane full of bombs and who lands so badly. $\endgroup$ Commented Jan 25, 2016 at 14:02
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You would have to define "fully loaded" but in general, yes you can take on three engines. It is dangerous to fly on three engines because the thrust is imbalanced. Therefore, it is highly preferable to abort, if that is an option.
answered Jan 25, 2016 at 18:13