Power or pitch?

[Final 3 Greens]

I know that there has been much discussion in the past about whether stick or throttle controls speed/sink.

Without re-running the old argument, I wonder if there is an an case for using different techniques in an SEP during different flight phases?

e.g. during final approach, use stick for speed, throttle for sink recognising that the a/c is at the back of the power/drag curve and likely needs pitching down (as well as power)to accelerate; but

During a descent at altitude where the configuration is lessy draggy, manage the required sink with the stick and the airspeed with the throttle, recognising that this phase is very much about energy management and that once trimmed correctly the addition/reduction of power is the critical input?

Of course Pitch and Power = Performance, so the argument could be circular, so I guess I am thinking about the way we chunk the overall picture into tasks that we can manage.

I would be very interested in what the guys with the experience think.


[This message has been edited by Final 3 Greens (edited 05 July 2001).]

[212man]

So what happens when ATC ask you to maintain a set IAS in the pattern for spacing, eg " maintain 150 kts to the outer marker"?

You cannot go wrong with the basic premise that pitch controls speed, and power controls height. Any combination of the two to achieve an aim is fine, but it is a COMBINATION and you should appreciate that. If you wish to maintain a datum, you should know what controls what.

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Another day in paradise

[grade_3]

Hi,

I'm not one of the experienced ones but I'll give you my thoughts anyhow.

Firstly, I think that either technique works acceptably well in your C172/PA28 class of aircraft that a lot of PPL's fly. It's mostly when you get into bigger, heavier aircraft (even light twins) with more inertia that using the correct techniques matter.

You're also correct in saying that either technique <i>could</i> be used, but there is an easiest one to use at different times.


Generally, final approach is most easily flown controlling speed with power, glidepath with attitude and alignment with bank. This works better on heavier aircraft as their greater inertia means that power changes will take much longer to affect the glidepath, whilst pitch changes will be much for effective. Obviously if you change the attitude you may need to change the power setting, but as you say it's a circular argument.

The exception to the technique above is during descent on a base leg or on an ILS. For these situations the easiest technique is to set the power as required for the rate of descent and adjust the attitude until you get the speed that you desire. Then adjust the power as req'd for the RoD and attitude controls speed. (e.g. Base in a C172 is flown at approx. 1500rpm, setting the attitude as req'd to acheive the required speed and flightpath).

I've found that those techniques seem to work best, as a lot of people I've seen who have been taught to fly a light plane with the opposite techniques have a lot of difficulty adapting when learning to fly a bigger aircraft or flying ILS's. They either (a) have dreadful speed control during final approach or (b) keep poling the attitude back and forth to maintain an ILS glidepath.

So yes, there are different situations where different techniques work best, and they don't necessarily have to be at the ends of the flight envelope.


Just my opinions anyhow, be glad to hear some others.

Cheers,


Grade 3

[Final 3 Greens]

212man

I don't understand your point about why there would be trouble maintaining a set airspeed - I am asking a question about flying in a controlled manner, not out of control!

Also I quite agree that the inputs are a combination.

grade_3

Many thanks. I have just done some time in a jet sim (proper instruction, not a jolly) and was taught the techniques that you describe.

I fly a Beagle Pup (100hp) which is why I made the comment about a forward pitch input being required at lower speeds/power settings (no redundant power, especially with landing flap extended), but the sink with stick, speed with power approach seems to be much easier to use at height, as there is less tendency to "chase" the ASI.

Many thanks for your thoughts.

[Kermit 180]

With light singles, theres no substitute for "Attitude + Power = Performance".

Attitude in the pitching plane is controlled by the elevators. With a given power setting an attitude will give you an airspeed. Power is therefore left to control ROD. Too high, reduce power, lower nose to maintain the desired airspeed, and vice-versa.

KISS - 'Keep It Simple Stupid'

Kermie

[I Fly]

Final 3 greens, how have you been taught in your first descending lesson? If you are a Instructor, how do you teach descending? Attitide controls the speed. For the early student the approach should be the same. If later on you fly a little rocket and want to controll speed with power that's fine.

[4dogs]

Folks,

Yes, it is an old argument.

A comment first about helicopters. The great thing about helicopters is that they enjoy direct thrust control: the cyclic points the thrust vector and the collective controls its magnitude. The relevance: attitude is the primary airspeed control and power is the primary rate of climb/descent control. For all intents and purposes, this primary allocation of control does not change. Forward speed is relatively slow so most corrections are small and there is time to get things sorted out.

I use the word primary because there is always secondary cross-coupling in that a change in one control generally creates a need for a secondary correction in the other control.

In aeroplanes, the primary control for airspeed is attitude and that for rate of climb/descent is thrust for most of the time when manoeuvring (power is just a proxy for thrust). The only time that we reverse the primary allocation of controls in most common aeroplanes (ie less control power and not so flash thrust values) is when the flight path is constrained and airspeed is less critical: ie, we are either maintaining level flight or flying to a specific point on the ground.

I would never die in a ditch over the level flight case and most Naval aviators will argue that the primary allocation of controls does not change - however, they are usually given highly responsive (pitch and thrust) machinery that allows them to maintain speeds to within a couple of knots while flying down the ball to their wet postage stamp landing point.

In the most discussed case of flight path constraint, final approach to landing, the argument goes as follows. I fly to my final entry gate for either a visual straight-in or an ILS using attitude for airspeed and thrust for rate of descent. When I get there, I effectively change the plot by fixing the flight path angle and planned point of impact with attitude and I let thrust become the variable to control the speed. The interaction between controls means that a change in one will effect the other, however the precision of attitude control over that of setting thrust suggests that attitude be used to control the most critical variable.

The other (important but not controlling) issue that comes into play is that of Flight Directors. They are demanders of rates of change and do so by demanding pitch or roll attitudes. In particular, in ILS mode, they can demand only pitch changes for glidepath control. Most heavy metal instructors have little tolerance for poor F/D discipline and will expect that the flightpath is maintained by following the pitch demands.

My experience is that folks flying down final in fast machinery trying to control their flight path with power while using attitude for airspeed have the most trouble and get overloaded very quickly.

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Stay Alive,

[email protected]

[Charlie Foxtrot India]

IMHO in the early stages of training it is important to be consistant. If you tell the stude at straight and level, climbing and descending stage that they control the airspeed with attitude, and control altitude or rate of change of altitude with power, and that the stall recovery is to increase airspeed by lowering the nose; it then becomes contradictory at circuit stage to change all that and tell them that power now controls airspeed. Understandable if they then have trouble maintaining correct glide speed in glide approaches and forced landing practice.

So I reckon attitude = airspeed, and power is the only fundamental difference between us and gliders and gives us the luxury of choosing our altitude and graeter control of our rate of descent on the approach. (And the luxury of being able to go around!)

The technique for doing an ILS in a Baron doesn't really work in a Tomahawk...

[grade_3]

4dogs: That's the best description of the different techniques I've yet come across. Thanks. :cool:

CFI: I have a few questions and comments about some things said in your post. ;)

I think that whilst it is important to keep things consistent for a student, teaching them the methods you described above leads to problems when they do step into a Baron to fly an ILS.

Attitude doesn't = Airspeed, Attitude = AoA, which, whilst related to airspeed does not solely control it.

An example: If you wish to maintain level flight but decelerate from 100 - 70 KIAS, what do you do? I teach to set the power as req'd for the speed, select, hold and trim for the new attitude / AoA required to maintain level flight and adjust if/when required. I also teach exactly the same in instrument flying (as also shown in the old pub. 45) -
Set the power for the speed, Select the Attitude, Check, Hold, Adjust, Trim.

With the example of a glide, I explain this as selecting the attitude for the best L/D AoA. At MTOW the speed will be spot on the published speed, but may vary depending on a/c weight (as it should!) This also gets the students head out of the cockpit, not fixated on the ASI. Same goes for climbing, set the Attitude for the AoA, not the speed.

The stall recovery is another point: you don't recover from a stall by increasing airspeed as you pitch the nose down, you decrease the AoA of the wing. If you teach anything else then it starts to get contradictory when you do accelerated stalls and aerobatics. It is easy to demonstrate how IAS has very little to do with the stall (except as am incipient symptom when S/L) by simply performing an accelerated stall. That darn horn and the buffeting doesn't stop until you reduce the AoA!

Finally, if someone is taught the techniques described by above 4dogs they will fly a Baron down an ILS with no problems. They are also able to fly a Tomahawk with that technique equally well, but they just have much better speed control in the Tommy than they otherwise would have !

Welcome all comments as always.... :)

(edited for typos :rolleyes: )

[ 08 July 2001: Message edited by: grade_3 ]

[I Fly]

grade_3, would you please tell us how you teach descending. That is the 3 versions, glide, approach and cruise, and how you then translate that into approaching to land? I am always willing to learn.
I agree with your stalling and straight and level.
I thought the (Australian) Day VFR Syllabus under 5.3 and 18.3 spells it out.
As you said 4 dogs explanation is exellent, how you then can contradict it, I don't know.

[212man]

F3G,
I would have thought my comment was self explanatory, and did not mention "being out of control".

If you are maintaining a glide slope using pitch attitude, you will be varying the airspeed, ergo if you have been asked to maintain an airspeed by ATC you are not meeting their request.

I cannot comment on flying large FW, but would tend to agree with the remarks about flight directors given the response time of turbofans and the subsequent a/c response. When I was first taught to fly FW it was in the Bulldog and there was never any question of using the controls in their primary role. As I recall, maintaining accurate datums (such as airspeed) was pretty high on the instructors' requirements.

Ironically, the a/c I fly now has a 4 axis autopilot that uses different control laws depending if it in 3 or 4 axis mode. In 3 axis it uses pitch to control Alt or V/S or IAS, in 4 axis it uses pitch to control IAS and power to control Alt or V/S (or glideslope)

[Final 3 Greens]

212Man


Ah - I understand your point! I was talking about flying attitude (sink) with stick, and controlling speed with power.

I am familiar with the relationship power & attitude = performance, I was more interested in how we should "chunk" the process into control inputs.

Thanks for your further posting.

Cheers, F3G :cool:

[grade_3]

I Fly: Firstly, I can't see how my post contradicted what 4dogs posted, but then if I could see that I obviously wouldn't have said what I did, would I ? ;)

My main point with CFI's post was that whilst consistency is important, it must also be a correct explanation of what happens. i.e. the technique remains correct under all circumstances (or at least the vast majority of them). This should be a prime requirement for all instruction, given the nature of people that we tend to remember best that which we learnt first.

Anyhow, I teach descending in the following manners:

Cruise Descent: Reduce the power to the cruise descent setting (lead with power), select the cruise descent attitude, hold (or check) everything to let the aircraft stabilise and then trim. Adjust the power to control the RoD, attitude to control the IAS.

Glide: Close the throttle (rebalancing with rudder as you do so). Use the elevators to select the best glide attitude (usually very close to the straight and level attitude at normal cruise speeds), hold and trim. The speed should settle fairly close to the published best glide speed, but will automatically compensate for the effects of a reduced gross weight or configuration changes. It's an AoA after all.... and a knot or so of inaccuracy doesn't matter a lot in a typical lighty. If the speed is waaaay off, check and adjust the attitude if required, and confirm the config. is correct.

Approach: Select an aiming point on the runway. Concentrate on keeping the aircraft on a constant approach down do the aiming point. If you are tracking directly for the aiming point then it shall remain constant and everything else shall appear to "radiate" out away from it (A bit like in the Windows "flying through space" screen save - the center dot remains steady and the others rush away from the middle). Control your flightpath with attitude (pitch for glideslope, bank for alignment) and control your speed with power. I explain it as the "Point and Power" technique - keep the aircraft pointing at the aiming point and control speed with power. Obviously you still need to refer to the runway aspect for approach angle, but I find the students pick it up quickly with no confusion and can use it with authority to control their approach angle.

The reason I switch to Att=Flight path, Pwr=IAS for the final appraoch is that it is (IMO) a more accurate way of flying finals, it works equally well in all aeroplanes and because your flightpath along finals is to a definate aiming point, not to acheive a given set of numbers on the instruments (this was described by 4 dogs as a constrained flight path).

I'd like to make the following observations also. The Day VFR Syllabus (http://www.casa.gov.au/avreg/fcl_lic/download/vfra_22.pdf) in sections 5.3 and 5.6 describes basically what I have outlined above for descending and approaches. Note however that it says:

Nose Attitude and Power is selected to maintain cruise descent IAS (+/- 10 KIAS +/- 150 fpm of nominated RoD)

and...

Coordinated use of power and nose attitude are applied to control approach path angle and speed

It doesn't say specifically how to do it, just what should be done. Really this discussion is about the best way of describing to a student how we perform the manoeuvers, whereas we all use a combination of the 2 techniques to acheive the desired flightpath.

I hope the explanations above are sufficient, and if I am misinterpreting or contradicting anything 4dogs posted please let me know, willing to learn also :)

[BEagle]

Very simply, the control column controls IAS and power controls RoD until the aircraft is aimed at a fixed touchdown point. As the attitude is now fixed, small power corrections are made to correct speed errors whilst small control column movements are made to keep the touchdown point fixed in the windscreen. Students find it far, far easier to roll out of the final turn, lower full flap, adjust attitude to obtain correct IAS and then use 'point and power' on the approach, constantly adjusting the attitude to keep the touchdown point fixed with small control movements as small power adjustments are made to maintain approach IAS.

[ 09 July 2001: Message edited by: BEagle ]

[Tinstaafl]

I've taught using both methods. Different schools use different methods. Can't say I've ever noticed a difference to the time students take to achieve the various milestones.

It is an artificial distinction anyway. Together pitch/AoA & power/thrust gives some combination of IAS & ROC.

Like CFI, I think consistancy of method is more important in initial task learning. With experience we become more familiar with inertia effects, control effectiveness & coordination and then we all mix and match depending on circumstance.

My personal preference for ab-initio is pitch/IAS & power/ROC coupling as I believe I have a well integrated combination of instructional method/explanations/lectures/briefs etc.

When I have to teach the alternative view then it takes me a little more thought, and some more care, to make sure I remain consistan,t otherwise I have no problem switching hats.

[CAT MAN]

What about a speed unstable swept wing jet, what then???

[Mr moto]

ALL aircraft fly the same way and this is essential for a newcomer to understand. Its whats gonna keep him alive! Speed control=pitch attitude.

However, One may find that a student gets better results if they are having problems landing by just pointing the nose at the threshold and controlling the speed with the throttle.

Otherwize, make them glide approaches every time cos the above technique does not work when the engine has failed.

[kabz]

Here's a good article by someone who thinks you should trim for speed, then reduce/increase power to descend/climb...
http://www.avweb.com/articles/eyeofexp/eoe0031.html

[Arm out the window]

Most of us probably lean towards the method we were ourselves taught.
I came through the Aust military system, which for a long time has been using this technique for ab initio training:

"Base" technique - set power and attitude for descent, balance and trim, then adjust airspeed with attitude and ROD with power. This also applies for cruise descents.

"Finals" technique - select aim point, maintain the aircraft's flight path to that spot with attitude, and maintain IAS with power.

The transition point between the two occurs as you roll out on finals and pick your landing spot (numbers or whatever).

One thing that I have found instructors and students alike to be sometimes confused by is expecting the attitude picture to remain constant down finals once you're there.
That works (until the late stages of finals) if your airspeed remains constant, but if you are flying an approach that requires airspeed to bleed back from some start point to a minimum threshold speed, then the attitude required to maintain a constant glidepath will need to be raised as the speed reduces.

An analogy I used to use was to get yourself at the right starting point on finals (a "gate" of height AGL, speed and distance out from the threshold), then imagine a set of railway tracks running from your eyes to the desired landing spot.
Attitude would be used to maintain the aircraft "on the rails", and power to either maintain a desired airspeed, or allow it to bleed back towards threshold speed as the case might be.

I like the concept, but then again that might just be because it's what I'm used to.

[dragchute]

Both concepts are valid.

A change of attitude in a performance aircraft with low total drag will have little or no effect on speed. Therefore power is best used to control speed and attitude to control the aiming point (ballistic approach) – whether it be derived from a glide-slope indication, PAPI or simply the runway markings. The technique will also overcome the adverse effects caused by wind shear.

However the use of power to control rate of descent and attitude to control airspeed (conventional approach) is essential when juggling an approach path for a short field landing. The aircraft is normally trimmed to a speed equal to or better than 1.3VS in the landing configuration. At fifty feet power is reduced and round-out commenced for a touchdown at minimum flying speed. If a higher than normal ROD is encountered the application of power is the only available option to recover from a high sink rate as any further back-pressure will probably lead to a stall!

For this reason students should be taught the conventional approach technique. Furthermore it is in line with standard descent techniques taught in every text-book I have read on the subject. Leave the ballistic approach for graduates to performance aircraft.