Throttle use on approach
 

Evening to you all.

Ive just watched one of those "in the cockpit" videos of a 757-200 from Manchester to Dalaman. Im curious about one aspect and its not a 752 thing only im sure.....

The approach turn inbound was at 8DME and the flaps and gear taken in the turn for a fully visual man land at Dalaman, Turkey.

All the way down the approach the P1 made constant adjustments to the thrust levers, I presume to control the descent rate. In PA28 or a 152 etc, the response to this action is immediate and as a result gives precise and responsive control over the entire approach. My question is that how effective is this method when flying a fully manual approach in a Jet? I imagined that the spool up/down time would be such that the ability to control the descent rate on approach would be rather to little, too late!?

Are these Power Plants twitchy enough to do this!?!?

You guys and gals make it look so easy........

Kind Regards to you all..

The thrust levers are used to control speed in a jet.

We set the body angle at 2.5 degs nose up in the landing config. Then select around 1.2 EPR as a rough power setting at normal landing wt.

Small adjustments have to be made, but thats about the gist of it.

We don't use the rudders to stay on the loc in a swept wing jet either as it causes more problems than it solves. (as a generalisation) I read a post by someone who was finding out the hard way.

Small A/C flying, has significant differences in technique when compared to large swept wing A/C.

All in all it's something you learn very quickly.

Fly Safe.


Please God grant this Forum a spell check!!


[This message has been edited by nohat (edited 29 April 2001).]

There's a thing called 'slam check' which is done on air tests. This is where you literally slam the throttle from the idle position to full power, at the same time starting a stopwatch. This is to check that the engine fuel control system is working as it should. Times vary - the older design of engines can take as long as 10-12 seconds to reach full power from idle, the more modern engines a lot less. The acceleration is not linear - that is, to begin with the engine responds very slowly. However, as the %rpm increases beyong a certain figure, say 80% (it varies from engine type to engine type) the acceleration becomes very rapid indeed.

An approach is always flown with flaps and gear down, and to overcome the drag, the engines are set at a fairly high rpm. This means that adjustments in power are felt quite quickly. The engine response is not, therefore the problem.

The problem is with the inertia. A big aircraft is slow to respond to small power changes, but as Nohat says, it is something you learn about in training and adapt to very quickly.

Hope the simple explanation helps clarify - it's not magic, it's experience!

Yes, I agree with the above posts. Typically, it will take about 6 to 8 seconds for an RB211 to spool up from idle to full power....or something like that.

In our SOP,s we have to be fully stabilised in the chosen configuration at the VERY LATEST 500 feet above ground.

That means: Flap and gear etc. AND thrust selected and in the range of 1.2 EPR, so in case of wind shear or such, you can click the go around switch and receive pretty much instantaneous power to climb away with.

As someone once said to me: Fully stabilised is not 4 whites, no power, nose down and good speed. It is just as equally not 2 reds and 2 whites, good speed and power off......

But it is not exactly rocket science .

So you're a rocket scientist .... that don't impress me much :)

Sq

Interesting that you guys use EPR as a thrust ref when manually flying (are you all on the 757?).

Airbus main parameter is EPR (on the v2500s anyway), but I find it a lot easier to use N1 for power setting when using manual thrust - it's probably just a personal thing but I find that a larger change in the numbers for the same thrust change makes it easier to assess the accuracy of my thr setting. Also, is it me or does N1 respond to a Thrust lever position change quicker than EPR? Feedback welcome!

45% N1 for finals fully configured in a A320, give or take a bit for weight.

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...proceeding below Decision Height with CAUTION...

Speedbird,

When you fly your PA28 or C152 I would argue that you should use POWER (throttle) for airspeed control and ATTITUDE (pitch) for aim point. The only time this won't work is when you are at full throttle or idle power and then you must use the secondary effect of pitch attitude to control the airspeed.

If you think about the old wives tale of attitude for airspeed and powere for rate of descent, then to commence the take off roll, you should line up on the runway and then lower the nose to accelerate.

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ChickenTrainer, on PDL

AB.

In answer to your questions, I am dual rated and fly 767-200/300 and 757's. You are correct in that EPR is a rough way to set power when fine adjustments need to be made.

I only use 1.2 EPR as a starting point and then bring N1 into the scan for fine tuning.

As a matter of interest the N1 setting is about the same on both 767/757 on approach.
That is around 60-65% N1 on both GE-C2 and RB211 engines.

Makes the whole thing a lot easier.

Chickentrainer, I have spent many many hours "bashing" the circuit with my students before I became a Boeing driver.

You use power to control ROD and pitch to control IAS in the landing config in a light A/C.

I have no idea if you are an instructor or if you have ever been one, but I suspect not.

Unless aerodynamics have changed since I used to be an instructor, I think you will find that PPL's are still trained this way.

I have not instructed for some while now and techniques could have changed.

Maybe a current instructor would like to step in and tell me IF this is so.

Duh, still no spell check!!!!!

[This message has been edited by nohat (edited 30 April 2001).]

[This message has been edited by nohat (edited 30 April 2001).]

Chickentrainer

If you are too high on an approach, try this: Dive the nose to correct your altitude. Then slam the throttles back to correct the building speed. What happens? At the end you'll find that you may end up at the correct altitude but too fast and unable to slow down. This is what you suggests to do?

The try the following instead: Slam the throttles back and only adjust the nose to maintain the correct speed. You'll find that you'll get a much steeper decend.

It's been tested :)

I might not be able to flare correctly (see my previous posts!), but I've not had any problems with my approaches. I do it like this: downwind, I get established at 90 IAS with flaps 10.
base, reduce speed to 80 and set flaps 20. This is where the speed reduction process starts. I pull back on the throttle quite significantly at the same time as i raise the nose to bleed off the airspeed. When I have 80 IAS, I let the nose drop to the correct glideslope and increase throttle so as to maintain 80. The same thing goes for finals, where I reduce power and wait for airspeed to bleed off to 70 and then let the nose drop to glideslope.
I find that on a good approach, I should be able to correct the variables with both pitch and power. If not, I'm either too high or too low. All this is based on my training on the C172 so I'm not shure of how it works for the big birds...
cheers

Chickenman....think of your statement if flying a glider on approach...a more simple aircraft. consider your airbrake to be your engine..

( i.e full airbrake is power at idle)....

if undershooting the standard procedure is to close a/b's, hence reducing RoD and extend the approach...
in your version of flying pull back on stick to reduce RoD and you get into the nasty situation if unexperienced of stalling on the approach....not a good move in any aircraft type

perhaps you should fly a bit more...even if it is just in a glider...the principles are roughly the same.....

Excellent point, glider insider, never though about explaining it that way.

low n' slow, if you're going to fly an ILS that way, you'll have some serious problems maintaining the glideslope.

Flying the fan driven Sunday craft or the mini 4 holer during the week, I just fly the thing and don't think about what I'm doing...
Guess it's called instinctive piloting :)

Joke apart, setting about 62% N1 (according weight and ATC) on the ARJ85/100 seem to do the trick. Then just extend/lower the bits, and there you are, a grease...
(P.S. the ARJ is of course THE most forgiving aircraft in the world ;) )

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... cut my wings and I'll die ...

nohat

Can you please explain to me why in a jet you use thrust levers to control speed but in a light aircraft you use pitch attitude?

You give me a precis of your instructional background and I'll give you one of mine.

Tor

If I was too high on approach, commensurate with lowering the nose to regain the desired approach path, I'd reduce power, because I know the speed will increase as a secondary effect of lowering the nose.

If I was too low, I'd raise the nose and increase the power.

I don't know about you, but I'd never slam the throttles either way. If my attitude/pitch corrections didn't look like working, I'd go around and try again.

Glider Insider

I don't think the comparrison is correct. With powered flight, the aim is not to fly a glide approach but a powered approach.

I bet none of the powered flyers when they commence descent from S&L in the circiut reduce power first to start descent. I;ll bet they lower the nose perhaps simultaneously reducing power to control the speed.

For some obscure reason, the old wives tale of power for airspeed and pitch for RoD, (but only in a lightie!) has krept in over the years.

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ChickenTrainer, on PDL

Thanx Tor!
I haven't started with my IR yet so I guess I'll have to get rid of that nasty habbit. Works for VFR though!
cheers

Tor,

Not sure I would agree with you. Any aircraft has a best rate of climb speed. That is the speed at which you will also descend at your slowest rate. If you accelerate to a much higher speed you will create more drag and will descend much faster.

Hence on a jet the speedbrake is much more effective at high speed than at low speed. It is also well known on a jet that they do not go down and slow down at the same time very well at all. So if you want to go down you increase speed and only slow down when you level off.

So you go around a lot? ( :))

Seriously, you should try setting up for a very steep approach, it makes good practice.

It's basic aerodynamics. You would want the most induced drag if you want to get down quickly. The best way of getting that is not to lower the nose and control RoD with power.

On another node, don't you remember what makes an aircraft decend? Deficit of thrust!

Who says it only goes for light aircrafts?

Yes but were talking about approach and not decend. Try bleeding of the excess speed in the flare and you would almost certainly make an overrun

[This message has been edited by Tor (edited 01 May 2001).]

One thing chicken missed is that you should always try to be within gliding range anyway.

As far as his setting up a descent goes, he is dead wrong about dropping the nose and reducing power to slow down. Actually, even in a 172, you have to drop power, typically to 1500ish RPM, ***wait for it to slow down*** ***in level flight***, then allow the nose to drop to maintain 75, 80 kts in the glide.

At this point, pitch controls airspeed, and you can add or reduce power to change your landing aim point.

Also, contrary to what chicken says, to come down faster, you can raise the nose a little, to slow down and use a less efficient glide. I believe that 75 kts is best glide for distance in a 172, and 65 kts is best for time, within reason you can use the slower speed to come down flatter but steeper, though you may need to add power near the flare to land smoothly...

I think using power to control RoD and pitch to control speed in a light a/c must have something to do with response times. Think about it:

Pitch responds quickly to stick input

Altitude & airspeed both respond quickly to pitch

Alt & airspeed respond more slowly to power setting, but:

Airspeed can drift to an unacceptable (esp. low) value very quickly, so:

Control airspeed with pitch (quick response) and altitude with power (it can wait), otherwise, you end up in the familiar situation of being in the right place at the wrong speed.

I would have thought jets would have had a somewhat more sluggish pitch response and also a more sluggish response in altitude in response to pitch, due to their inertia, hence the reversal of logic.

Out of interest, autopilots control alt with pitch and speed with power, but they also have feedback from power => pitch and vice versa, so they respond with both at the same time, just like an instinctive pilot.

Pie.