Why does the 747 start takeoff with partial power and increase to full throttle later down the runway, when the A380 for example starts on full?
This is not the same question as:
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4$\begingroup$ Regarding the video, keep in mind it’s taken with telephoto lens, so distance and speed of sound play a role in judging engine spoolup versus aircraft movement. $\endgroup$– Cpt ReynoldsCommented May 30, 2019 at 21:30
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$\begingroup$ Also, the last major airline I worked with flying both last generation Airbus and Boeing aircraft, the takeoff procedures differed in so far as on the Airbus, brakes were released after thrust was stabilized at takeoff thrust, while on the Boeing throttle levers were advanced after brake release only. I never checked whether that was „our“ or the manufacturer‘s procedure, though. $\endgroup$– Cpt ReynoldsCommented May 30, 2019 at 21:34
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$\begingroup$ When I was flying RJs, we would "stand the levers up", put them more or less vertical, which would give roughly low 70s% N1, wait for rpm to stabilize, then to TOGA. That was the manufacturer's procedure. If the runway was long, we would do it while starting the roll. If the runway was short, we'd hold brakes until going to TOGA. $\endgroup$– John KCommented May 31, 2019 at 1:31
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1$\begingroup$ Just as a matter of information, as best I remember, on 747-200 aircraft, with JT9D-7Q engines and possibly earlier, if you stood on the brakes and brought the thrust levers to max takeoff power, the aircraft may start moving depending on your weight, the runway surface, and tire condition. $\endgroup$– TerryCommented May 31, 2019 at 4:17
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$\begingroup$ Side note: the A380 (or at least the RR Trent 900 equipping half of them) does not "start on full." Setting the throttles at TOGA while stationary only gives you about 80% power, rising to 100% only when speed becomes > 30 kt - I believe that is in order to increase the surge margin at low speed. $\endgroup$– Deeday-UKCommented Jun 4, 2019 at 18:57
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8 Answers
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The other answers are spot on, but I'd like to address one part, which is the "later down the runway." It is by no means "later."
(YouTube)
In the frame grab above of a 747-400 (taken at 1:12), the captain had already called out "takeoff thrust," the TO/GA system had set the pre-programmed thrust setting, and the 747 wasn't even at the piano keys. (Auditory cues can be misleading to passengers or air side spotters.)
Typically, a low thrust setting is set to make sure all the engines are responding in the same way, and more importantly, will have uniform application of thrust thereafter. This can be done with (standing takeoff) or without the brakes on,$^1$ or during a rolling takeoff. Then it is TO/GA time. (In tail-wind conditions, the procedure may vary.)
Here's another video by JustPlanes from inside the cockpit. Again TO/GA was pressed while the plane was at a crawling pace far from being anywhere later down the runway.
$^1$ Boeing 747-400 FCTM § 3.4
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$\begingroup$ I think one more effect is the intake air speed producing a different engine result at TO/GA. TO/GA while completely stationary should produce a different effect than TO/GA near take-off speed due to the extra air intake available to the engines. $\endgroup$– NelsonCommented Jun 1, 2019 at 1:24
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I can't give a 747 specific answer, but generally, on some engines it's desirable to let the engines stabilize at a moderate power setting, with the N1 equalized there, before advancing them to TOGA thrust. It helps to make sure the engines will all get to TO thrust at the same time, which is important to avoid yawing motions early in the roll. The crew may hold the brakes and do it, not releasing brakes until the move to TOGA, or they may do it on the roll, which is probably what you were seeing.
There may also be limitations in the event of strong crosswinds against going to TOGA while stopped with the wind blowing from the side beyond a certain angle and above a certain speed, requiring that kind of rolling takeoff procedure (to minimize flow disruptions to which a particular engine may be sensitive).
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Mentour Pilot talks about this in his video. The answer is Stabilization.
It takes an engine several seconds to spool up from idle to TO/GA. That's long enough that the engines don't get there at the same time. The difference can be enough to make asymmetrical thrust yaw the airplane on a slick runway.
Whereas, spooling up from mid-thrust to TO/GA happens very quickly, and so the time of differential thrust is too short to matter.
The time from idle to mid-thrust is most of the total time to TO/GA. So they command mid-thrust, wait for both engines to catch up and equalize, then punch it.
They can't correct with rudder because it's ineffectual at low airspeed. They can't correct with differential braking because dragging brakes only works if you're moving.
This is a throwback to the old piston engine planes which would run up the engines and wait to see they all stabilize, perform properly and are equal. They find out about carb problems or fouled plugs before they are rolling down the runway.
answered May 31, 2019 at 0:26
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$\begingroup$ +1 for a novice, an excellent break down. $\endgroup$– cmpCommented Jun 7, 2019 at 6:34
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Typically you run the engines to 60% N1 or so, then do a final sweep of the gauges for abnormalities prior to advancing to either maximum rated thrust or a lower pre determined rating. It’s a means to verify everything works correctly one final time before committing to the takeoff roll.
answered May 30, 2019 at 20:09
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Some engine types on the 747 are not capable of running at the maximum N1 (fan) speed with no forward speed to increase the air flow into the engine, especially at low altitudes and low temperatures.
You don't want to push the engine to the limit of the operating envelope and then have a gust of cross-wind that causes a compressor surge by disrupting the inlet air flow, until you have enough forward speed to prevent that sort of event from happening.
In other words, the engine is at maximum power for the operating conditions it is in, but it is not operating at the maximum RPM the fan is designed for.
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4$\begingroup$ Can you add a source for the N1 info? That would make this a really good answer $\endgroup$ Commented May 31, 2019 at 14:49
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Rolling take-off..recommended practice if the runway is long enough as its easier on the airframe and also on the pax.
Standing on the brakes and running up to take-off power is only done on shorter runways. As the 747s mostly operate from bigger airports you would not normally see this.
If you've never experienced it and don't know whats happening it can be a bit disturbing as the noise and vibration increases a lot and there's a lot of initial acceleration when the pilot gets off the brakes. The last few I've been on were on smaller aircraft (a 737 and an ATR 72).. exciting to say the least!
Anilv
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Another reason is foreign object debris (FOD), which is easily picked up by the outboard engines. This can be avoided by delaying the spool up to full thrust.
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It's not an interceptor/air superiority fighter that needs to be up in the air 10 minutes ago nor is it taking off from an aircraft carrier which has a limited amount of runway length so you need a catapult (steam atm mostly but electromagnetic ones are being developed (and I think they are deployed on the USN's newest carriers)).
For other missions taking of with the afterburner (if you have one) and engine(s) at max isn't the most efficient way.
