Base Leg Flaps

[The Wicker Man]

Just wondering what flap settings most instructors use on base leg. Some I speak to insist on using full flap on base, others on using one stage or 10 degree on a c172, for example. There seems to be a lot of argument on this at the school where I work, some insist that full flap is selected on base to minimise work load, trimming etc, on final, and others point out the danger of a stall/spin accident turning base to final with full flap. And of course glide distances,. if the donkey quits on approach. Any views appreciated Thanks.

[the dean]

hi wicker,

matter of personal choice really, but we always teach 10-12 degrees ( ours does'nt have the detent type lever ) , and as required for practice or necessity on finals.

in fact unless we are practising full flap approaches we generally use the take off setting ( we use 10-12 degrees for take off )for all touch and go's...reduces the workload.

i would prefer to leave it until then, so as you say, you get a better view of the runway and appreciation of glide profile on finals rather than on base, and this does'nt just apply to 172s.

its just like...some like a power reduction of downwind before the turn..thats not what we teach...for the reason you state ( possibility of stall/spin ), but also its when you turn onto base that you get a better appreciation of the runway and you aiming point..you might have a headwind on base or you might have a tailwind so the power reduction might have to be done right away after the turn or left a bit longer due to a headwind on the day....

flaps likewise are a matter of choice but i would certainly not be looking for anything more than 20 degrees on base.

gear up.....

the dean.

[modelman]

Flap selection

On 150/152/172 always been taught on base,throttle back,into white arc,20 deg,trim for 65kts(150/152) or 70kts(172) 30 deg on final (unless a strong x-wind when stay at 20 deg).
Have never used 40 deg (yet)

Seem to work ok for me.

MM

[Duchess_Driver]

We teach 20 on base, then as required on final.

What surprises me is your

"There seems to be a lot of argument on this at the school where I work, some insist ..."

statement. There should be a standard which the school has adopted and every instructor from CFI down should be working to that standard.

Poor Johnny is in for a shock when someone else gets in if that's not the case.


Happy flying.

[Nathan Parker]

Just wondering what flap settings most instructors use on base leg.

Ideally, 20 degrees. If high or tight, then go ahead and put in the remaining flaps. Better to be stabilized as soon as possible on final.

danger of a stall/spin accident turning base to final with full flap.

I see no connection whatsoever.

course glide distances

If you lose the engine, raise the flaps. No big deal.

[P.Pilcher]

"Why put the brakes on, then increase the power to compensate?" Said a very experienced instructor in days of yore when I learned (he was also a test pilot):
"Yes" I said, "But is it not fair to say that the use of 10 -20 degrees of flap at low approach speeds lowers the nose to give a better view."
"I would not argue with that," said 20,000 hours experience against my 8!

P.P.

[homeguard]

In a flap

Sometimes I have to admit to being flappergasted.
Consider what the purpose of flaps are for.
1) to provide the pilot with good forward vision during slow flight (such as the approach).
2) to allow a higher power to be maintained; piston engine temperature, torque with a turbine. if a piston, reduced likely hood of plugs oiling and caburettor icing.
3) the benefits of propellor slipstream effect
4) Reduction in the stall speed
Flaps should not be used to get out of trouble such as when high and fast - power and attitude does that better. Raising flaps could lead to a high rate of sink just at the wrong time.
The pilot should know the reduction in airspeed that each stage of flap will generate and plan for that accordingly. Dragging the aeroplane in all the way from base leg seems futile to me.
With regard to the C172. If on the initial approach (base leg) the aircraft is configured with 10 degrees of flap and trimmed 80kts IAS then following with flap 20degrees will achieve 70-75kts, 30degrees 65-70kts and finally 40degrees if you have it will result in 55-60kts. Knowing that you then must decide what you want given the conditions. The student must understand that the greater the deflection of flap the less will power exceed drag during the approach and also during a go-around.
The PA28 first trimmed at 80kts with flap 10 degrees will thereafter give at flap 25 degrees 70-75kts IAS and with 40 degrees 60-65kts.
Whatever the aircraft the student should know the figures for their aeroplane and use them accordingly with purpose. The student then can be taught to use them intelligently to suit the prevailing conditions.
The scrapyards are full of bits of aeroplanes because the pilot tried to arrest speed and lose height on approach without a thought to reduce power. Unable to arrest descent in windshear or perhaps unable to make a succesful go around.

[Nathan Parker]

Consider what the purpose of flaps are for.

Flaps were invented to lower the stall speed in order to use less runway in takeoff and landings, yet still allow for the increasingly high cruise speeds. They are best understood in that context. However, the higher flap settings won't decrease the stall speed much; in a C172, the 40 degree setting will give you maybe a 1/2 knot reduction. This setting is purely designed to steepen the descent angle.

Removing flaps doesn't necessarily result in a descent rate at all. I have removed all of them 1 foot above the runway without any loss of altitude. Works fine as long as you increase the AOA to compensate. Since the 40 degree setting results in only a small increase in lift coefficient, you need to make almost no adjustment when you reduce the setting from 40 to 30.

The student must understand that the greater the deflection of flap the less will power exceed drag during the approach and also during a go-around.

Power doesn't exceed drag ever, since they measure different quantities. Thrust may or may not exceed drag, but on a landing approach, drag exceeds thrust, or the airplane would not be descending. The larger the flap deflection, the greater drag exceeds thrust, which steepens your approach.

[homeguard]

Power/drag

Nathan Parker
A correction to my contribution which should be defined better. i.e 'power available over drag' which I wrongly took for granted was obvious.
Flap may indeed reduce the take-run but often as not will degrade the take-off distance. Where you get your idea of the sole reasons for flap I don't know. However, please open your mind to the many reasons for the development of flap, they are numerous.
If you insist on continueing with your current practice, as described by you, I hope that you continue to do so no more than 1ft above the runway.
The degree to which flap will generate additional lift and/or reduce the stall speed is type dependant. Best Rate climb is achieved with maximum excess power over drag.
Your definition of the forces in a descent are wrong. Albeit that power is not always considered but thrust is.

[Nathan Parker]

A correction to my contribution which should be defined better. i.e 'power available over drag'

That's still wrong. You must compare power with power required, and compare thrust with drag (or thrust required). You can't compare power to drag because they're measured in different units.

Flap may indeed reduce the take-run but often as not will degrade the take-off distance.

Depends on where the takeoff distance is measured to. For a close in point, the more thrust the airplane has, the more beneficial the effect of flaps. For the Part 23 50 foot obstacle or the Part 25 35 ft obstacle, flaps will always decrease the takeoff distance for other than the most underpowered aircraft.

Where you get your idea of the sole reasons for flap I don't know.

Didn't say "sole", but implied "primary", and every aerodynamics book printed describes flaps in this context.

If you insist on continueing with your current practice, as described by you, I hope that you continue to do so no more than 1ft above the runway.

You imply that it's dangerous, which is absurd.

The degree to which flap will generate additional lift and/or reduce the stall speed is type dependant.

Well, of course, but the goal of flaps is to increase the maximum lift coefficient. There is really no such thing as "additional lift", since lift = weight in unaccelerated flight.

Best Rate climb is achieved with maximum excess power over drag.

Sorry, wrong again. ROC = (Power Available - Power Required)/Weight.

Your definition of the forces in a descent are wrong. Albeit that power is not always considered but thrust is.

Nope, it's flawless. Your angle of descent has the equation

Sin(descent angle) = (Thrust - Drag)/Weight

When thrust is zero, you have

Sin(Descent Angle) = -Drag/Weight

Vast numbers of references available upon request.

[VFE]

I have removed all of them 1 foot above the runway without any loss of altitude.

Not dangerous at 1 foot but quite possibly a short cut to misery for family members when their budding pilot tries that at say, 500' on the approach after experiencing an engine failure... raising flap like that causes the aircraft to sink. Do you raise flap all in one go on a go-around? No. Or... at least I hope not!

Far better to lower full flap when you know yer can make the runway. Common sense ennit? Train your pilots to adjust the trim and maintain descent path from the start of Ex.13 - what do you want them to be? Pilots or robots?

VFE.

[Nathan Parker]

t but quite possibly a short cut to misery for family members when their budding pilot tries that at say, 500' on the approach after experiencing an engine failure

My point is that if you don't sink when one foot above the runway, you won't sink at 500 feet, as long as you increase the AOA. Even if you fail to do so, you won't sink 500 feet. The sink is brief, until the aircraft's natural stability will ensure that lift = weight. You're probably most vulnerable at 20 feet, more than 500.

Do you raise flap all in one go on a go-around? No. Or... at least I hope not!

Not intentionally, but I have had students do that inadvertantly. You get a bit of a sink, but it's not a huge problem, as long as the go-around is otherwise properly performed. (Most pilots fail to lift the nose during a go-around.)

Far better to lower full flap when you know yer can make the runway.

No, I disagree. Doing so tends to destabilize the approach at the very end, just when the greatest amount of precision is needed. Besides, that last notch of flaps is the easiest to remove.

Pilots or robots?

I'm teaching them understanding, rather than the rote memorization of "Never raise the flaps to extend the glide", which is wrong and dangerous, and demonstrates a misunderstanding of what flaps do.

That said, robotic behavior is a positive in many ways, as demonstrated by the safety of the airline training programs. Teaching what many call "piloting" is the absence of rules and structure, allowing the pilot to make decisions in real time when he doesn't really have any understanding of the physics involved. Hence, I teach full flaps for all approaches, removing the distraction of that decision-making from the equation.

[VFE]

If you retract flap all in one go, ok your can stop the sink by increasing angle of attack, yeah? Well if you are demonstrating that to a student then there's really not that much more workload involved in getting them to adjust trim on short final after lowering flap when within easy gliding distance of the runway is there? Or am I being daft here?

Do you teach big airliner style circuits too perchance?

VFE.

[Nathan Parker]

Well if you are demonstrating that to a student then there's really not that much more workload involved in getting them to adjust trim on short final after lowering flap when within easy gliding distance of the runway is there?

Just unnecessary. The late addition of flap produces a large change in the drag characteristics of the aircraft. If there aircraft was already stabilized, the pilot just ruined it. Stabilized is more than trim, it's also glidepath control. The last notch of flaps mainly added drag, so there will be a steepening of the glidepath at the very end of the approach. Not good, IMO.

The attention is better spent focusing on runway alignment, keeping the touchdown point in sight, and a butter smooth roundout

Do you teach big airliner style circuits too perchance?

Actually, my circuits are so tight that in a C152, I can overtake much faster airplanes. I also teach full stall landings and exiting the runway at the first taxiway. And I harp continuously on rudder coordinaton.

So there.

[VFE]

I'd love to anally disect your post Nathan but as long as yer students are safe I guess that's all that matters mate.

Beware of sending them to short field strips with obstacles on approach and climb out. Argh! I sed I wouldn't comment dint I!

Would be interested in BEagle's comments on all this...

VFE.

[shaun ryder]

Have to agree with you there about disecting the infallible ones methods.

I have removed all of them 1 foot above the runway without any loss of altitude.

Ever heard of ground effect? Do that in a PA28 at 500'... well not a good idea for most.

but on a landing approach, drag exceeds thrust, or the airplane would not be descending. The larger the flap deflection, the greater drag exceeds thrust, which steepens your approach.

Thrust? In a propeller aircraft only during a Vx climb I think, when the difference between thrust required and thrust available is greatest. Its called maximum excess thrust.

Drag exceeds thrust? We are talking about coming down here, thrust does not come into it, we are not flying jets now. Try and explain it better. Descending with power and flap for a given airspeed reduces the descent gradient. This is because adding power reduces the need for so much forward component of weight in order to balance the drag. Therefore a reduced descent gradient.

I find some of your explanations rather dubious Nathan, you even have me confused, never mind your students.

[Paper Planes]

The main reason instructors teach students to lower the flaps in stages is to make the ride smoother for passengers. You can certainly go for full flaps when in the white zone and you as a pilot would not really notice the effects. A passenger that does not fly in small planes often will most likely feel uncomfortable.

[Nathan Parker]

Ever heard of ground effect?

GE is irrelevant. Merely a reduction in induced drag when closed to the ground. No, it's not an increase in lift, because lift = weight in unaccelerated flight. If lift were greater than weight, the aircraft would be accelerating vertically. (Load factor > 1)

Do that in a PA28 at 500'... well not a good idea for most.

Everyone is quick to say "Oh, that may work in your airplane, but it won't work in an X". Very seldom the case.

Thrust? In a propeller aircraft only during a Vx climb I think, when the difference between thrust required and thrust available is greatest. Its called maximum excess thrust.

Yes. Your point?

Drag exceeds thrust? We are talking about coming down here, thrust does not come into it, we are not flying jets now.

Whether it's jets or props is irrelevent. Your descent angle is controlled via the thrust deficit. Power vs. Power Required controls descent rate.

I find some of your explanations rather dubious Nathan, you even have me confused, never mind your students.

No doubt, but most pilots have a very limited understanding of aerodynamic theory. What I've related is freshman material in any aeronautical engineering course. If you own an aerodynamics text, I'll supply page references.

[DFC]

Why not get the student to look at the requirement that never changes i.e. the requirement to cross the threshold in the landing configuration (the one prescribed by the flight manual) and the correct speed from a stabilised approach towards the correct aiming point.

If that is acheived then there is a good chance that the performance fingures published by the manufacturer will give atleast a clue as to the landing distance required. If one does not do the above then no point in even looing at the performance figures - they are meaningless.

There are 101 ways of acheiving that correct threshold crossing.

However, if we add in some workload management and the requirement for lots of quality lookout at all times then the configuration changes will naturally fall into a patern that has nothing to do with square, oval, bomber or microlight circuits but with time to the threshold.

Having got away from the pattern then the pilot can simply configure the aircraft in stages in good time for a safe landing...be it from an approach with power, a practice glide or even a forced landing.

Smooth unrushed configuration changes during the approach to land need to happen in sufiient time to have the aircraft stabilised by one's minimum height to be stabilised by....most light aircraft operators use 200ft unless it is an emergency. i.e. if not fully configured and on target speed at 200ft then go arround.

The whole idea of do this at that position in the circuit will only really work fully if there is no other way of making an approach which avoids that place in the circuit.

Related example - where do you put the gear down? How many would say at the downwind position? but what if you are on a straight-in? where is the trigger there?

How about you put the gear down at 4nm from touchdown. Now there is a fixed point that no matter how you approach the circuit you simply have to pass. Of course the 4nm from touchdown point in the standard circuit is somewhere about the downwind position and of course this leads nicely into the CAA guidance of aircraft in the circuit at downwind (about 4nm from touchdown) compared to a flight on a straight-in approach calling long final - more than 4nm from touchdown.

Thus when it comes to flap then there is simply the requirement to have landing flap selected, the aircraft trimmed and the approach stable by the decision height.

You must put the flaps out on base works great until.....slightly tighter circuit requested by ATC, tailwind on base, chop the power.....DOH Carbheat (too little time for the period of heat before changing power), must start down, shove the nose down, DOH, need to slow for flaps, where is the runway DOH we are on the centerline, bank and pull it round, DOH we are high, DOH DOH DOH........Did I mention lookout up final? NO!!!

Perhaps better to think OK we are going to be a little tighter in this circuit so we are closer to crossing the threshold than expected so we need to start the configuration changes (and perhaps descent) here at 2nm to go (on late downwind in this case).

How many instructors get their VFR students to relate their position in the circuit to distance from touchdown? One can talk all night about lift drag ratios and excess thrust but if it is a bomber circuit, a tight slipping oval spitfire arrival or a forced landing into some farmer's field.....it allcomes down to judgement as to how far to go to the "threshold".

Regards,

DFC

[The Otter's Pocket]

Yer a cut and paste pedant me lad.

I am right la la lar