Takeoff Engines

[airindia16 0]

This is just a random question which porbably has a simple answer. Why, right at the start of the takeoff procedures, is the engine put at a certain percent n1 and then increased after two seconds or so? You can hear it as onlooker too. I suppose that I am asking this as opposed to just increasing it to full throttle at the start.thanks,airindia16

Edited July 19, 2010 by airindia16

[greggerm 312]

It is customary to bring the engines up to a certain point, check to make sure they are stable and both performing equally, then push forward to full throttle. If an engine was having a bad day and didn't match up with it's counterpart, the pilot might encounter some bad yaw effects due to the mismatched thrust. Bringing up the engines a bit and checking them helps reduce any surprises this situation may cause. -Greg

[Chock 17,769]

Typical jet turbines spool up slowly, and no two engines are likely to spool up at exactly the same rate unless everything is identical on them, which is a fairly unlikely occurrence on anything other than a brand new aircraft with brand new engines. So even if an aircraft's engines are working perfectly within certified parameters, they can throttle up unevenly on a multi-engine aircraft, which might instigate a swing if one throttles up before the other, especially on aircraft with wing-mounted engines, where the leverage an engine could exert might be considerable. Even a couple of percent difference in thrust between two engines on an aircraft with them on opposite wings, when we are talking about an engine that might produce 20,000 lbs of thrust, can be enough to set it off on a swing that may be difficult to correct with very little rudder authority available at the start of a take off roll.An engine on the average twin engined jet airliner such as a 737 or A320, will typically go from idle to 100 percent thrust in about six seconds. In actual fact, such engines have to be able to get up to a certain power level in a specified time to be certified for use on many aircraft, but within those limits things can differ from one engine to another for a variety of reasons, such as whether there is a crosswind component, whether one engine is new and another one a bit older etc, etc. All of these factors mean that any lag on an engine spooling up could mean there is asymmetric thrust for as much as perhaps four seconds, which could mean trouble, especially if there is already a crosswind.So as Greg points out, they are spooled up a bit, and then up to where they need to be for take-off thrust. Sometimes this is done manually up to a point - say perhaps 60 percent of N1 with the brakes on - and then the TOGA (Take Off/Go Around) levers are engaged to set the rest of the thrust automatically and the brakes released, other times it will be done manually all the way in a couple of steps.So it is really just a precaution, but a sensible one.Al

[airindia16 0]

okay, that's what I thought!Thank you very much!

[Captain Kevin 586]

It is customary to bring the engines up to a certain point, check to make sure they are stable and both performing equally, then push forward to full throttle.

Both might be the wrong word to use in the case of three or four-engined aircraft. :( :(

[vololiberista 205]

The main reason is actually to ensure the a/c is travelling straight. If you are not precisely lined up or if there is a cross wind and you fire wall the throttle there is a good chance that you come a cropper in the grass (if you're lucky)The same procedure is used in propeller driven a/c. Got to 80% T's and P's in the green a/c running straight then open wide.This is why if you are going to do a short field take-off you really need to line up carefully.

vololiberista

[Guest bratonabike]

The main reason is actually to ensure the a/c is travelling straight. If you are not precisely lined up or if there is a cross wind and you fire wall the throttle there is a good chance that you come a cropper in the grass (if you're lucky)The same procedure is used in propeller driven a/c. Got to 80% T's and P's in the green a/c running straight then open wide.This is why if you are going to do a short field take-off you really need to line up carefully.

vololiberista

But thats what al said :(

[DC_Ed 3]

The main reason is actually to ensure the a/c is travelling straight.

Hmm, nope.Al got it right.It may be a reason, but I don't think that's the main reason...at least, that's not what I've been taught.

[vololiberista 205]

Hmm, nope.Al got it right.It may be a reason, but I don't think that's the main reason...at least, that's not what I've been taught.

Incorrect!!!!!!!!!!!!!!!!!!!!!!vololiberista

[badderjet 181]

Hmm, nope.Al got it right.It may be a reason, but I don't think that's the main reason...at least, that's not what I've been taught.

Gotta second that. Guess that's what I learned too. Anyways, isn't that basically the same thing? You increase thrust slowly to a certain point, let engines spool up to that and stabilize (preventing the mentioned journey into the grass). Then apply TOGA with hopefully sync'ed engines. I believe that's more or less what both of you said. So what should be incorrect there at all.Just my $.02.Etienne