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JET ENGINE SPOOL UP TIME vs APPROACH SPEED

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JET ENGINE SPOOL UP TIME vs APPROACH SPEED

Old 3rd Mar 2013, 15:35
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JET ENGINE SPOOL UP TIME vs APPROACH SPEED

Hello all

During a training session a flight instructor mentioned that if approach speed was 5 kts below the Vapp, engine sppol up time will increase by approx 10 seconds.
This statement came during a discussion of the Turkish airlines Amsterdam accident.
Appreciate your input on the subject
Thanks
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Old 3rd Mar 2013, 17:06
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I confess to not being an expert, but engine spool-up time is specified in the certification rules:

e.g. Airbus states in "Aircraft Energy Management during Approach":

The engine certification (FAR Part 33) ensures a time of 5 seconds or less to accelerate from 15 % to 95 % of the go-around thrust.

The aircraft certification (FAR Part 25) ensures that the thrust achieved after 8 seconds from power application (starting from flight/approach idle) allows a minimum climb gradient of 3.2 % for go-around.
.

Last edited by toffeez; 3rd Mar 2013 at 17:14.
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Old 3rd Mar 2013, 18:10
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thank you but what the instructor was talking about seems to be related to approach speed but thanks for your time
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Old 3rd Mar 2013, 18:19
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IAS has nothing to do with engine spool up times--the certification criteria doesn't specify airspeed.
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Old 3rd Mar 2013, 18:37
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5 kts below bug speed ? Quit trying to calculate spool up time and pay attention to attitude and power. If you leave the speed there , it will only decay more as pitch is maintained / increased to maintain approach profile. Careful with pitch trim as a remedy here too. If you have to calculate spool up time in this scenario , you are reaching for the wrong remedy. Hands on the thrust levers , scan and aim for what is your normal thrust setting for the approach. Getting below bug speed requires significantly more thrust to stop the trend , requiring further pitch and power changes. Best to be anywhere else but below bug speed for the obvious reasons. Pay attention at all times. Finally engine spool up time relates to thrust lever position. If it is at idle it takes time... while you are counting....everything else goes to s**t. Good luck.
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Old 3rd Mar 2013, 19:47
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galaxy thanks a lot.......6000PIC, the last thing on my mind should such a situation ( God forbid ) is to count spool up time, but it's a piece of information that i wanted to check, all theoretical of course, thanks for the input
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Old 3rd Mar 2013, 20:40
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The only sensible explanation of the original statement I can come up with is that this instructor meant something different than what was posted here.

During a training session a flight instructor mentioned that if approach speed was 5 kts below the Vapp, engine sppol up time will increase by approx 10 seconds.
Given that engine spool up time is related to the ability of an aircraft to transition from a 3 degree descent angle to to a specified positive climb gradient in the landing configuration, starting this transition from a lower approach speed would result in a greater time required to achieve the specified positive gradient. The actual time required for the engines to accelerate from flight idle to go-around thrust would not be appreciably affected by the airspeed used, while the time required to reach the required positive climb gradient would.

The above quoted statement would more accurately reflect reality if it were changed to read:

" If approach speed was 5 kts below the scheduled Vapp, the time required for the aircraft to transition from a 3 degree descent angle to a specified climb gradient will increase."
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Old 3rd Mar 2013, 20:40
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IMO the instructor was embellishing the spool up time to try to make a point as elucidated by others already about what happens when below the bug speed.
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Old 3rd Mar 2013, 23:07
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What is rarely discussed with regards to spool up time is engine handling if landing on a slippery runway with a strong crosswind. Read the following extract from the Boeing 737 FCTM carefully. Better still, try it in the simulator because it may give you an unpleasant surprise if you are not aware of the marked delay in reverse thrust spool up times.

Under the heading Reverse thrust and Crosswind (All Engines).
As the airplane starts to weather vane into the wind, the reverse thrust side force component adds to the crosswind component and drifts the airplane to the downwind side of the runway. Main gear tyre cornering forces available to counteract this drift are at a minimum when the anti-skid system is operating at maximum braking effectiveness for the existing conditions.

To correct back to the centreline, reduce reverse thrust to reverse idle and release the brakes. This minimises the reverse thrust side force componenet without the requirement to go through a full reverser cycle, and improve tyre cornering forces for realignment with the runway centreline.....when re-established near the runway centreline, apply maximum braking and symmetrical reverse thrust to stop the airplane.

The surprise factor is how long it takes for the engines to spool down to reverse idle from full reverse. In fact it is about 12 seconds or more. Reverse idle in the CFM 56 is approximately 22% N1 which is same as normal idle in forward thrust. Having got the aircraft aligned once more with the centreline of the runway, and you apply full reverse from idle reverse of 22%N1, well that takes at least 8-10 seconds of spool up time.

The thing to keep in mind is you are not selecting reverse thrust from a normal touch-down speed where flight idle of 32%N1 is still in place. It is from ground idle reverse of 22%N1. That lower figure causes a significant delay in spool up time to full reverse. For that reason, the additional landing roll to get the aircraft stopped on a slippery runway under crosswind conditions can be disturbingly long compared to a normal crosswind landing on a dry surface

Last edited by Centaurus; 3rd Mar 2013 at 23:13.
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Old 4th Mar 2013, 06:19
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Centaurus,

Interesting point.
The necessity to release the braking force in case of weathervane is explained in the fctm.
Sops should and normally provide reduced cross winds 'limitations' for braking action good or less.

The disengagement of AB in that condition and reduction of reversers do increase the landing distance.
However a landing with a touchdown between 1000 and 2000 feet should not end up into an overrun even with temporary release of brakes.

In any case,crews should be familiar with theiraircraft landing distance along with safety buffets(dispatch 1.67 rule),and the effect of crosswind,speed at threshold,possibility of weathervane.....
There should be a MAX touchdown point in every captain head before any landing,if one passes that point,go around should be initiated.
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Old 4th Mar 2013, 13:49
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Given that engine spool up time is related to the ability of an aircraft to transition from a 3 degree descent angle to to a specified positive climb gradient in the landing configuration, starting this transition from a lower approach speed would result in a greater time required to achieve the specified positive gradient. The actual time required for the engines to accelerate from flight idle to go-around thrust would not be appreciably affected by the airspeed used, while the time required to reach the required positive climb gradient would.
I believe the instructor might have meant acceleration from stable approach RPMs, not idle. The engine would have lower RPM on lower speed, and as acceleration rate is slower on lower RPM, every % less RPM will mean more and more time to get through it.

Is this not the reason, btw., that F40 Vref is 1,25*Vs while F30 Vref is 1,30*Vs? (talking about 737) That higher drag in flaps 40 makes for higher N1 on Vref, faster engine acceleration would then allow for a speed closer to actuall stall.
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Old 4th Mar 2013, 18:29
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Are you sure the engine would be at a lower RPM when the aircraft is flying a fixed descent angle at a lower speed?

I think it may be quite opposite.
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Old 4th Mar 2013, 19:01
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I think the instructor was exaggerating the consequences of getting slow on the approach. I think what the instructor should have said is what westhawk said - "If approach speed was 5 kts below the scheduled Vapp, the time required for the aircraft to transition from a 3 degree descent angle to a specified climb gradient will increase."

FE Hoppy,

Are you sure the engine would be at a lower RPM when the aircraft is flying a fixed descent angle at a lower speed?
I don't think being in a descent has much to do with it. It has mostly to do with airspeed/AoA. Being at a lower airspeed should produce a slightly lower engine speed. However, there is such a thing as FADEC and I'm not sure how that manages the effect of airspeed on engine speed.

Last edited by italia458; 4th Mar 2013 at 19:02.
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Old 4th Mar 2013, 19:14
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Forget FADEC it's got nothing to do with it.

Just think of the drag curve and what happens at the back of it.
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Old 4th Mar 2013, 19:31
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FE Hoppy,

Drag goes up, assuming they're on the backside of the power curve. But how does drag going up increase the kinetic energy of the air entering the engine? The engines breath clean, undisturbed air if they're hanging below the wings - which means the speed of the air is the main factor.

How do you know FADEC doesn't have anything to do with this? The thrust lever isn't connected to a manual fuel valve, it goes through the FADEC computer. I don't think you can rule out the FADEC computer from this issue.

Last edited by italia458; 4th Mar 2013 at 19:32.
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Old 4th Mar 2013, 19:57
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So on the back of the drag curve the lower the speed the more drag and therefore more thrust required to maintain it. How the engine is controlled is irrelevant. It's thrust v drag. The flight path remains constant on an ILS. So thrust controls rate of descent. at a lower speed you need a lower rate of descent. This required more thrust.
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Old 4th Mar 2013, 19:58
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http://www.pprune.org/tech-log/35069...ml#post4566320
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Old 4th Mar 2013, 20:47
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FE,

Ahh! Yes that makes sense. I just assumed that the power remained at the setting for Vapp and then the airplane slowed down. I see what you mean now.
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