Spin Up

[PAXboy]

General question: How long does it take a jet engine to spin up?

Obviously that is a long piece of string! So, let us say:

1) Departure: From idle to take-off RPMs on a 73-400 or similar?

2) Missed Approach: At decision height, you have to go around - how long does it take the same engine (1) to spin/spool up?

2a) Am I right in thinking that, due to the time needed for the engine to spin up, on final approach, it is practice to keep the revs above idle, so that it will take less time to reach working revs, should you need them? Then, when the decision is to Land, the throttles are closed to idle?

3) In points above, is there a significant difference between the MD80/737 size and the 340/747 size of engine in the times that they take to spool up?

Many thanks and if the questions are not clear, please shout.

[Mad (Flt) Scientist]

FAR 25:
Sec. 25.119 Landing climb: All-engine-operating.
In the landing configuration, the steady gradient of climb may not be less than 3.2 percent, with--
(a) The engines at the power or thrust that is available eight seconds after initiation of movement of the power or thrust controls from the minimum flight idle to the go-around power or thrust setting; and
(b) A climb speed of not more than 1.3 VS.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-84, 60 FR 30749, June 9, 1995]

As a consequence of that, credit cannot be taken for any thrust level above that achieved after 8 seconds. It is therefore a well balanced design if flight idle is set such that TOGA is achieved within the 8 second window. Since the rule specifies the minimum thrust, where the pilot actually has the throttle doesn't matter.

From this rule you could deduce that all engines likely have spool-up times from idle to max power of approximately 7 seconds, for the go-around case.

[mustafagander]

WRT question 2a) Most big jets use an approach idle setting, generally selected automatically when a landing flap setting is selected.

Most big jets require considerable thrust on approach, so spool up to G/A is almost instant.

[777AV8R]

Ummm..but does the 737-400 have approach idle...I don't believe it does.
An idle turbine engine on approach is a certain prescription for disaster....don't do it!

The old adage.."Ran out of altitiude, airspeed and ideas, all at the same time".

[QAVION]

PAXboy.... Could you perhaps tell us more about why you need to know these things. Is it from a technical point of view or a flying technique point of view?

E.g, regarding your first question:

"1) Departure: From idle to take-off RPMs on a 73-400 or similar?"

Are you aware that most procedures call for the engines to be stabilised at a power setting lower than takeoff power before full thrust is selected? E.g. 1.1 EPR or perhaps 70% N1. If you want to know how fast it takes from Ground Idle directly to TO thrust, it might be better to ask this question in the Engineers forum (Engineers do slam acceleration tests as part of their engine tests).

Yes, most, if not all jet aircraft have an Approach (or "High" or "Flight") idle, because of the time it takes for a jet engine to spool up to full thrust for a Go Around. This is normally inbuilt into the engine control system logic (Unless the Approach/Ground idle system is faulty, you wouldn't need to worry about setting the idle thrust higher than normal... The thrust lever position Vs engine rpm settings are adjusted automatically). But, as 777AV8R says, your engines shouldn't be at idle during the approach anyway. It's only in the last 25 feet or so (varies with aircraft type) you would pull the thust levers smoothly back to the idle position for touchdown*. The Approach Idle setting is cancelled a few seconds after landing (unless the reversers are being used).

*It's important to have your levers at idle at touchdown as your stopping aids will not work with the engines out of idle (speedbrakes, thrust reversers, autobrakes).

Hope this helps.

Rgds.
Q.

[Genghis the Engineer]

Worst I've ever seen was 8 seconds from idle to MTOP, which was on an Adour engine fitted to a Hawk T1. No significant experience of airliner engines, so can't help there although more modern fast jets tend to be shorter.

The term "Spin up" incidentally is actually used for the stress case in undercarriages when it first impacts the ground on landing. "Spool-up" is the correct term.

G

[Young Paul]

Both 737's and A320's have an airborne idle, which is higher than ground idle. However, you don't make your final approach with idle thrust. High idle on the 737 is about 30% N1, and a typical approach power setting is 55-62%. Ground idle is about 20%. Comparable for A320. In fact, if you haven't stabilised at your approach power setting by 1000', company rules say that you go around.

Thus, response to go around power is only about three seconds - less than the length of time it takes to pitch to go around attitude.

[PAXboy]

Many thanks to all.

QAVION: My question is no more than idle curiosity as a semi-frequent paxer.

I was aware that engines are stabilised before t/o thrust is applied. Mainly due to listening to engines outside the window over 37 years as pax! My question was more of the time lag in the approach for go round. That has been more than satisfactorily answered.

On the occasions that I have been in the jump seat for landings, one has naturally been focussing on the view out of the window and observing the manipulation of the yoke - than thinking to ask about throttle levels. Of course, next time I am in the jump seat ... I shall probably need a jump wheelchair, so much time will have had to go by!

Thanks also Genghis for elucidating Spinning and Spooling.

[afrikanablue]

737ng
for departure, thrust is set at 40% N1 before pushing TOGA, an overshoot of about 4% N1 occurs and finally stabilizes within 2% of the T/O thrust (reduced or full) by 80kts.

There are three phases with different minimum idle settings; ground, flight, and approach. Approach idle has the highest RPM setting, and is maintained as the minimum till touchdown. The approach is flown at an average thrust setting of about 53% N1 for the -700 and 62% N1 for the -800 (moderate landing wts & flaps 30/40). The initial application of TOGA sets a reduced GA thrust with ROC upto 2000 fpm, the second push gives you full GA thrust.

[Dan Winterland]

The GE CF6 on a 747-400 has a flight idle N2 of about 74%, some 12% higher than the ground idle setting.

Older engines tend to be longer. About 11 seconds on the Viper in the Jet Provost I seem to remember.

[411A]

The RB.211 series spool-up very quickly, due to their three shaft design.
But NOT as quickly as the Allison 501 series on the Electra...in this case, instant power is the name of the game.
Ahhhh, Lockheed!:D :D :D

[Kotare]

I thought those Electra engines operated at 100% all the time - apart from a switched ground idle setting for taxy. No need for spool-up, just pump gas and change blade angle - instant grunt.

[john_tullamarine]

.... and didn't the old L188 lift up its skirts and go when it was empty .... the ASI was hard wired to the throttles ...

[777AV8R]

Well..geee...what about my Herkie Bird? Same thing..Those Allison 501-D22As...instant torque!

But, you Electra gang were a bit different...you had your engines hung upside down! lol

Yes, the Electra danced! We used to use them on a short leg at times alongside a 737...almost the same flight times. She was a great airplane.

[Flight Detent]

Hi guys,
Of course....the Allison CONSTANT SPEED engine revolved at 13,820 rpm ALL the time.....therefore, instant power!!

Also, I thought the Airbus A320, especially that real early example, I think doing a little demo, for a small crowd, somewhere in France, took a REAL long time to wind-up, I think I could just hear the sound of those engines winding up as it was decending into the treeline!!
That WAS an example of how unexpectedly long it takes to wind up from ground idle (in his case!), to full power, too long!!

That's why Boeings have both a ground and flight idle setting, as mentioned prior, to flight idle when either 25 or 30 of flap are set, at least on the B747 Classic!

Cheers

[PAXboy]

Thanks again folks. I had not realised that the N1% was so high on approach.

I know that when jets were first being introduced into RAF service (very late 40s early to mud 50s) there were many, many crew lost as they learnt how to handle them.

The spool up time was very long and, on approach in particular, the problem was that folks converting from prop would leave it too late to apply power. With the time lag, nothing would appear to happen, so they applied more throttle. Then the power arrived all at once and they were too close to the ground to recover it.

Quite apart from training, the modern turbine moves to a very different beat than the jets of those days.