Piston vs Jet

[canta]

I read recently (from a 737 website) that as the 73 makes an approach speed is contolled through use of power and descent rate through elevator.

This is the opposite for as we teach in GA where ROD is controlled through power and speed through elevator. I realise that they help one another here also though.

Is there a point at which the changeover occurs between the two ?

Idea appreciated.

Cheers

[411A]

As a general rule yes, jet transport aircraft use thrust to control indicated airspeed on approach, with the stabilizer/elevator used for rate of descent.
Oddly enough, there is only one widebody jet transport that had another control besides the tailplane in this respect...the Lockheed TriStar, with its direct lift control system...ie: spoilers, when landing flap is selected. And...it worked good. :ok: :ok: :ok:
Ahhhh, Lockheed!

Of course, the guys from Boeing equipment didn't like DLC at all. The 'Boeing push' didn't work so good on the TriStar.:uhoh: :uhoh:

[Global Pilot]

I wouldn't say this technique applies to all GA training. In my early training (Grob115s) whilst on finals I was taught to chase speed with power and to maintian the aimpoint with pitch. It works pretty good for pistons too!

Possible reason for this beng that the flight school's main business was grooming cadets for Qantas, Cathy amongst others.

[Genghis the Engineer]

If you go back 6 months or so, John Farley wrote an article in Flyer looking at the differences between the two methods, and discussing which was most appropriate.

The general conclusion seemed to be that either would work, but that JF favoured power for speed, and elevator for RoD on any aircraft, disagreeing with much of the GA teaching community.

G

[Crossunder]

...and this topic has been discussed time and again in pprune...
BTW; does anyone have an electronic copy of the Farley article?

[NigelOnDraft]

Solve the problem by doing both...

Jet Provost Finals turn is Power v RoD / Attitude v IAS. Once you roll out at 300' becomes Stick v Touchdown Point, and Power to control IAS bleed off to Threshold speed...

NoD

[Genghis the Engineer]

BTW; does anyone have an electronic copy of the Farley article?

There's a regular on the "private flying", "tech log" and "flight test" forums, called John Farley who talks about Harriers a lot, you could ask him ?:cool:

G

[scroggs]

NoD, surely as the JP (at least the Mk3) was constant-thrust/variable-noise, the approach technique was largely more luck than judgement (as I remember it, anyway!) ;)

[ChrisVJ]

I learned to fly 30 years ago and the reasons for using throttle for height and stick for speed were very clearly demonstrated to me then.

Having set up a steady long final my instructor told me that I looked a little short (the threshold was creeping up the screen,) so to try lifting the nose, of course the speed dropped immediately and I spent the next twenty seconds chasing the ASI up and down the dial with the throttle.

Next time round we set up the plane again trimmed out real steady and as we we coming in a little high he said to ease back the throttle. Of course the speed came back a couple of knots but the nose dropped and the speed came back pretty well immediately. We played with the throttle all the waydown and barely touched the stick at all till rounding out. That demonstration is just about as clear in my mind today as it was then.

If a GA plane has good manners and trims out properly once you are set up on final you should be able to fly it all the way down to flare just on the throttle

Never been there but I know Jets are different.

It frequently seems these days , not just in aviation, that some of the classical tenets get left out of basic training and as an old fogey I am left wondering whether it was just my memory or are these young people really missing something?

I

[ITCZ]

Both methods work 'okay' in a GA type.

The 'stick for aimpont, thrust for speed' is almost exclusively the recommended method in a jet transport flying a final approach.

The jet has very STRONG primary effects of control, and WEAKER secondary effects of control.

If you see your aimpoint 'rising' in your windscreen, then you pull back on the stick. STRONG primary effect of increasing angle of attack is MORE LIFT, which will put your aimpoint back where you want it quickly.

The weaker secondary effect of increasing angle of attack is to wash off speed. So you add thrust. Strong primary effect, increase speed. Very weak to negligible secondary effect is to increase lift (the 'vee' in the 'half rho vee squared' part of the lift equation).

So flying finals with 'stick for aimpoint, thrust for speed' is far better in effecting the changes you want to your glidepath and speed, when the aircraft you are flying has strong primary effects of those controls with weak secondary effects.

(With 3,000hr in bush pistons in my dim dark past, i agree with the GA lads/lasses who advocate the reverse for light pistons. Very effective for short fielders. But most people who are being ab initio'ed these days are given one method that works reasonably well for pistons and best for the larger types they aspire to.)

[Old Smokey]

This thread is getting old (like me) but having converted more pilots from props to jets than I can remember -

During approach, prompt attention to Glide Slope / Descent Rate and Airspeed are required (along with many other things), therefore the most appropriate input for quickly correcting these is required, in short, what are the primary aircraft responses to either an elevator, or a Power / Thrust adjustment?

Consider a Below Glide Slope / Below Profile situation -

For the propeller aircraft, an increase in Power will deliver instant gratification to the pilot by way of a lift increase, reducing rate of descent. A thrust increase also takes place, with the secondary effect that, after a period of time speed will increase.

For the jet aircraft, an increase in Thrust has no effect upon lift, and would accelerate the aircraft even further downwards, i.e. an INCREASED Rate of Descent. The only 'instant gratification' available is through the elevators, to increase angle of attack to produce more lift to regain profile. Follow-up increased thrust will be required to compensate for the speed loss.

This latter situation is very evident on jet aircraft with mid-line thrust, e.g. DC9, F28, Learjet, but not nearly so evident on aircraft with low slung engines, e.g. B747, A300, B737 etc. where a thrust increase causes a pitch-up. thus increasing angle of attack, and therefore lift, a response similar to the propeller aircraft but for different reasons. In later generation aircraft (such as the B777) the AFCS removes this pitch-up, reverting the result to the more traditional mid-line thrust type.

Either type of input will work, but the PRIMARY and SECONDARY responses are opposite when comparing prop to jet.

Safety, and almost all (if not all) flying procedures dictates that the pilot uses the most appropriate primary response to controls in a dynamic situation.

[Wino]

6 of one half dozen of the other.

It doesn't matter what you fly, the equation you are looking for is
ATTITUDE + POWER = Performance.

was true in a DC-3, 737 A320 A300 and a 727s *(Though rather difficult to do in a A320 because the flight controlls are constantly interpreting what you think you meant, so you can't set an attitude and leave it.....)


Andy

[Old Smokey]

The problem with the formula is that Jet engines produce thrust, not power - and therein lies the difference.

[ShyTorque]

The RAF used to be strong on teaching "point and squirt" on piston engined aircraft, i.e. Fly the aircraft to the place you want it to touch down on elevator and control the airspeed with power. It does work for light aircraft, I used to teach it as required by the syllabus for ab-initios and very few studes had any problems with it.

It is more likely to result in an accurate landing point, obviously critical on a jet with a relatively high landing speed but equally as important on a light aircraft landing on a short field. The downside is that the student must be able to recognise the limits of the safe approach angle "band", i.e. not too steep and not to shallow.

It is obvious at the average flying club which folks were taught which technique. The ones floating well down the runway before touching down are genarally using the other method.

[Old Smokey]

Not surprising that the RAF favoured the "Jet" method (to call it that), most of the RAF trainees were probably destined for jets.

A personal opinion is that the "Jet" method (Power / Thrust for speed, elevators for flight path angle) would work equally well of prop or jet aircraft. For centre-line thrust jet aircraft, the "Prop" method (Power / Thrust for flight path angle, elevators for speed) could produce some alarming results. For the 'under-slung' jet engined aircraft, you would probably get away with the the "Prop" method because of the pitch-up caused by thrust increase......until the AFCS came along and removed it.

[MLS-12D]

JF favoured power for speed, and elevator for RoD on any aircraftI guess he doesn't fly sailplanes.

[Paul Wilson]

Although I read the John Farley article, I can't remember if he mentioned it (I think he did), but you can take the speedbrake to be the throttle in a sailplane, and I believe he mentioned that the lever even works in the same sense i.e. forward for more power (less drag) back for less power(more drag). So either technique can be used for sailplanes.

[Capt Claret]

If one holds with the premise that Attitude controls airspeed and power controls rate of descent, then to take off, one should lower the nose to accelerate down the runway and at an appropriate speed apply power to commence climbing.

As this procedure is unlikely to result in a satisfactory take off, one can assume that it is safe to say that power primarily controls airspeed, and (pitch) attitude primarily controls aim point. Both have an affect on the other, so it is not uncommon to have to adjust both to achieve the desired flight path and airspeed.

To teach in GA the opposite, whist it can be argured to be true, is simply trying to complicate what should be straight forward, IMHO.

[flyboy007]

Autothrottles+Autopilot=Performance.

[411A]

Sadly, the BY 757 accident at Girona proved otherwise.
Wonder just when some will wake up and smell the coffee?:uhoh:

[flyboy007]

I wouldn't have thought that any automation was at fault in that one!! All the problems occurred after AP and ATS were disconnected.