Watch the speed!

[Mike Cross]

While SSD is right we don't have an AoA indicator in most light aircraft.

The mechanics of the killer spin off the final turn are quite simple.

Assume a LH circuit. Pilot is on base, approaching the extended centreline. He rolls left into the turn and realises he's going to overshoot the centreline so he tightens the turn a bit (maybe he misjudged it, maybe he has a slight tailwind component). So he's now in a steep turn. At 60 deg bank the stall speed (alright the speed at which we reach critical AoA) is getting on for 1.5 times what it is in level flight (if you want the maths behind that it's here)

Because of the turn the left wing is going slower than the right wing so that will stall first. The result is a sudden roll to the left and a near vertical arrival.

As an example an Arrow III has a stall speed in the landing config of around 55kt. Heel it into a 60 deg bank and it will become 77.8kt.

Food for thought.

Mike

edited for incorrect numbers

[Genghis the Engineer]

Originally Posted by mazzy1026

SSD - in a regular training aircraft (i.e one not capable of aero's) how can you stall with a high airspeed? (Not arguing this - just want to know how).

Perhaps a demo in the chippie at Liverpool this weekend

In a turn.

Stall speed in a turn = level stall speed / SQRT [cos (bank angle).]

So if, say, an aeroplane stalls level at 55 knots, then with 60° of bank it'll stall at 78 knots in a balanced turn.

SSD's rather more extreme approach works too!

G

[robin]

Of course there is the other classic stall/spin scenario glider pilots are taught to avoid

Low on the base leg and turning towards finals. Wanting to stretch the glide the nose is held up and speed decays. Left wing is lowered a bit, but not too much, and a lot of left rudder is put in to bring the nose round in a skidding turn. Right wing goes forward generating more lift and left wing goes backwards and stalls

Result a stall/spin at low level and no hope of recovery. It is one reason why glider pilots are/were taught to make a well-banked turn onto finals

It can also happen to power pilots, but from my experience, most power pilots use a gentle turn onto finals and it can catch them out, too

[littco]

Originally Posted by Genghis the Engineer

In a turn.
Stall speed in a turn = level stall speed / SQRT [cos (bank angle).]
G

Not sure if i've read it wrong but i thought the stall speed is the SQRT of the Load factor * the level flight stall speed.

Load factor = 1/cos bank angle

Therefore stall speed = level flight stall speed * (SQRT of 1/cos bank angle)

In other words

55 * SQRT 1/cos 60

=
55 * SQRT 1/0.5
=
55* SQRT 2
=
55* 1.41
=
78Knts

Which admittely is the same answer as you, sorry to be particular. No offense just looking to clarify

[Saab Dastard]

A couple of posts earlier on placed an emphasis on "attitude" -

'fly the attitude, not the instruments'

Good advice, but one caveat: beware of runways with a pronounced upslope when on final approach - the "correct attitude" (judged wrt. the runway) could cause you to fly too slowly. The opposite occurs with a downslope - flying the "correct attitude" could cause an overly-steep approach, resulting in higher than ideal threshold speed.

SD

[Genghis the Engineer]

littco, you've just done the same maths in a different order.

G

[DenhamPPL]

Must thank Lister Noble for starting this thread. Some very interesting replies after sorting the wheat from the chaff

I'm heading towards first solo (next week or the following once I've completed the Air Law exam) and noticed on Tuesday while doing circuits with my instructor how quickly the airspeed can bleed off in the approach configuration especially when turning final if you don't keep the nose down in the turn (I was unconsciously pulling back on the yoke slightly as I banked to line up with the centreline).

I bled off 10kts in the turn (from 70 to 60) but corrected the error before getting too close to the stalling speed. I didn't repeat it on my final circuit and performed the best landing so far in my training apparently

I didn't consider the increased stall speed at that angle of attack at the time (15-20 degrees). I will do more in future after reading this thread.

Denham has quite a tight LH circuit on 06 so there isn't much time to do everything - then again I chose Denham for that reason - its quite a challenging circuit for students with a very "short" final.

Thanks again for all your posts. Reading and remembering.

Andy

[Lister Noble]

Genghis we did get a POH to read,we are also taught 80mph up to flare,then reduce power,then throttle off at hold off before landing.
I tend to use slightly lower speeds,ie around 75mph and was told it was Ok if it felt right for me.
And I reckon I had some of the best instructors going!

Saab Dastard,it was an uphill runway,07 at Old Buck,the slope does play tricks on the eye but I never thought of that implication before.
Lister

[MichaelJP59]

What I don't quite understand is that everyone's done basic handling - i.e. taking the aircraft up to a safe height and playing with speed, angles of bank, different configs, finding what it takes to stall the aircraft and so on. In my experience in the trainers I've flown I've found I have to be really and deliberately unfeeling to provoke anything like a stall - such as slowing down until the controls are *really* sloppy then pulling right back ignoring all stall warners.

So it worries me with the above dire warnings of the fatal turn to final and so on - am I missing something that could catch me out big style or is there not enough "flying by feel" taught?

[bmoorhouse]

It was mentioned earlier in the thread and then not mentioned again so i'll put my 5 pence worth in on TRIM.

I fly an ageing PA280-140 with a long approach speed of 85 mph (inconsistent units in aviation really wind me up - but the ASI is in mph). It is the short wing version of the PA28 and is very reactive to flap in both attitude and drag.

If you get well trimmed on base so she'll fly hands-off at 85mph with 2 stages of flap it takes only minor yoke and power inputs to follow a perfect glidepath. Even slightly out of trim and it is a struggle right down to the threshold, even more so when there is gusting.

I was lucky with my instructor who explained trimming very comprehensively and after every power change would shout TRIM - it is now second nature to trim to zero input pressure for the power & attitude. It makes flying so much easier because you are flying the plane and not fighting it.

[172driver]

I am definitely with the various posters on the increased stall speed in a bank. Hence my instructor drilled into my head only shallow turns in the pattern (circuit), i.e. max 30 degs bank.

However, I don't believe flying an airplane by feel/seat of pants is a good idea. Read the POH, fly the arplane (ANY airplane!) by the numbers and you'll be fine. Many a pair of pants was lost trying to fly by their seat...

[IO540]

I am sure somebody is going to have a go at me for saying this, and I don't wish to start yet another "why do some people fly such big circuits" thread, but isn't there a slight contradiction between safety (careful and correct airspeed maintenance around the circuit) and flying the tightest possible circuit?

I fly a TB20, so by the end of the downwind leg I want to be trimmed at 90kt. Sometimes I have arrived quicker but I make sure I am trimmed to 90kt on the base leg. I verify this trim position with hands off the yoke.

The gear (and 10deg flaps) goes down on the DW leg, and unless descent is desired the MP goes up to 20" to maintain the power over the extra drag.

So the base to final turn is done at 90kt. The stall speed in this config is 70kt (at MTOW, 30deg bank) and 1.3 times that is 91kt which makes 90kt quite reasonable (Vref = Vs * 1.3) for a safe turn. Even safer if the turn is descending, with a slight vertical acceleration during the turn (unloads the wing).

Then landing flap, and speed drops to 80kt. No more 30deg turns now (one hopes) so we have 59x1.3=76.7 which is fine for a Vref of 80kt.

I do the base turn at the proper place for the published circuit, only to find some other pilot cut me up on the inside, having flown a much tighter circuit. OK, in a C150 you can fly a tighter circuit, but not that much tighter. Usually they mis-judge it and I have to go around. Sometimes several times (e.g. Sunday, Stapleford).

Occasionally, it's obvious that there is an instructor on board, so the student is doing this on his orders.

To me, it's patently obvious that at least some poor sods are not taught proper speed management. They are taught to fly a "proper airmanship, young man" tight circuit, which the instructor obviously knows how to do, having been doing it for the last few years. But the student won't know the effect of G (i.e. the combination of the effect of bank angle and vertical acceleration, etc) on the stall speed. Most likely, he won't have a clue what the stall speed is for the various configurations - I never knew this in my PPL training. One should not be flying max-performance tight circuits, say 10kt above level-flight Vs, at this level of training.

I could fly DW a bit slower, with full landing flap which gives a safe speed of 63 x 1.3 = 82kt (or so, the power setting will have quite an effect too) but why should I fly around with a plane which now has a sluggish control response, just to skim off 8kt? The margin for turbulence is now pretty thin and I still have two turns to make, or one longer one if flying an oval circuit which a lot of people don't like.

One can make the same case for any combination of the many types found in GA. Pick any two with a 20-30% difference in Vs, and the slower one will be tempted to cut up on the inside.

People should be taught to fly the proper speeds, and they should be taught to follow the person in front, not to try to save about 50p in fuel by cutting up the person in front.

[stiknruda]

However, I don't believe flying an airplane by feel/seat of pants is a good idea

then

Read the POH, fly the arplane (ANY airplane!) by the numbers and you'll be fine.

You are of course quite right, but I would suggest that you are not flying - you are merely operating an aerial conveyance to a set of prescribed parameters and IF anything were to unbalance that equilibrium, you would be in a whole heap of hurt.

The best ever single lesson I learnt was how to tell that the aeroplane is out of balance just by what it is telling your bum - seat of the pants, flying. The balance ball is handy but I can generally react to an out of balance situation by my subconscious translating the message from my ass and redirecting it to my feet before my eyes notice that the balance ball is out.

The second lesson is that IF THE AIRCRAFT IS ALWAYS FLOWN IN BALANCE IT CAN NOT SPIN.

And the third lesson, far easier in a stick aeroplane than a yolk type but the same principle applies:- the aeroplane will only stall with the stick in two positions regardless of the speed. So in effect, the stick gives you the same information as an AoA indicator but in a less graphic form.

Lister - I made good my promise of last night and have offered to take R spinning after a three axis lecture!


Stik

[S-Works]

Well said Stik....

[Genghis the Engineer]

Originally Posted by Lister Noble

Genghis we did get a POH to read,we are also taught 80mph up to flare,then reduce power,then throttle off at hold off before landing.
I tend to use slightly lower speeds,ie around 75mph and was told it was Ok if it felt right for me.
And I reckon I had some of the best instructors going!
Saab Dastard,it was an uphill runway,07 at Old Buck,the slope does play tricks on the eye but I never thought of that implication before.
Lister

75mph = 65 knots (the value in the -180 POH ).

So why were you worried at seeing 65 knots on short finals then?

(And incidentally, the POH says 75kn/86mph on initials, which is actually slightly faster than your instructors were suggesting, but knowing the type pretty well, I agree with your instructors that it's safe to come down a few knots on that, particularly in a lighter aeroplane, and especially into a relatively short runway since that wing floats forever if you try and land too fast).

G

[SAR Bloke]

I disagree with the 'seat of the pants' brigade. As Lister said in his original post, it was the sloppy controls that alerted him to check the airspeed. This is where low speed/stall/spin awareness training comes in.

However, the way to stop yourself getting into trouble on finals in a light aircraft is to scan the speed. Forget AoA as most light aircraft do not have an AoA gauge fitted.

There is one other important point of accurate airspeed on finals that has not been mentioned yet which I believe is particularly important for low hours pilots. A known speed will have a known control response. Allow the speed to decay and the 'sloppy' elevator response during the flare could cause a heavy landing. Too fast and the 'firm' response could cause a balloon followed by a heavy landing. The old saying of 'a good landing comes from a good approach' is true and speed is one of the important factors.

[Rod1]

Stik, good one.

Rod1

[littco]

Remember also we are talking ISA here and as such a sudden drop in wind speed reduces ISA without you even doing anything, possible windsheer or gusty wind can all contribute to this. A 10Knt drop in ISA due to drop in wind speed isn't that impossible, especially if there are buildings or trees in the local area .

[whiskeytangofoxtrot]

There's this flight instructor podcast around called the Finer Points, the guy posts a short lesson every week in mp3 audio. Quite nice listening for those commuting-to-work trips imho.

Anyway, he gave a quite interesting commentary on how the "usual" case of "turning to final" stal&spin might develop, was very good at least in my opinion. Listen to that episode here. And the whole site is www.thefinerpoints.net.

I'm not in any way connected with the site, apart from finding it a while ago and enjoying it on the way to work and back

[Whirlybird]

So it worries me with the above dire warnings of the fatal turn to final and so on - am I missing something that could catch me out big style or is there not enough "flying by feel" taught?

What you're missing is the difference between recognising something when you're concentrating on it 100%, and doing so when a lot else is happening. For a low hours pilot, there's a lot to do in the circuit, and particularly on the turn to final. You're working out where you are, what to do next, what to say on the radio. And if it becomes a bit too much, and you're overloaded, something has to go, and that something could well be your awareness of the speed bleeding off.....just for a few seconds, but that's all it takes. Believe me, it's very, very, very easy to get caught out big time in such a situation. The only way to prevent it is constant vigilance, constant practice, and not pushing yourself beyond your limits to a point where you will get overloaded.