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Side-slip without stalling

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Side-slip without stalling

Old 20th Apr 2015, 12:04
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Question Side-slip without stalling

OK, I have read this carefully.

Being a simple person, I need some simple guidelines for EFATO - where is my plan right and where is it wrong (or at least unduly risky)?
Assume a) a PA28 / similar or b) a 172 / similar

I take off, and at 450 ft there's a metallic death rattle and the engine stops.
Assume best glide speed (Mayday if poss)
Turn into wind and I see the only suitable field is more or less in front, but I am already nearly above to the downwind fence, and it's a short-ish field.
Check Vfe; select full flap; set initial approach speed; fuel off; mags off; electrics off; door unlatch.
Now I need to lose a lot of height pronto, so I start significant side-slips to left and to right, ensuring I am maintaining the initial approach speed each time I go through 'wings level'.
Low enough; wings level; final approach speed; touch down; brakes; steer round obstacles; ground loop as the far fence approaches.

I'd rather like to avoid stalling into the ground while I'm doing all this,
  • So what's the max slip I should apply with full flap?
  • Is full flap appropriate?
  • Am I right to stick with the initial approach speed while slipping?
  • Should I be using an entirely different process?

It would be nice to have this in my head before I need it.

SD
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Old 20th Apr 2015, 12:19
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First a word of caution, Side slipping WITH stalling is called spinning !

Most light aircraft will side slip without problems but you MUST maintain a greater margin above the stall so look for about 10 KTS above the flight manual approach speed ( get the number from the flight manual and NOT any place else).

Practise this at a height that a recovery from an inccipent spin can be recovered from.

Check the flight manual for side slipping prohibition, some aircraft with full flap selected can while being side slipped can have poor elevator pitch control.

Side slipping is a very valuable technique regularly use by glider pilots who don't have the luxury of putting the power on and trying one more time, I can't understand why more power pilots don't use it.
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Old 20th Apr 2015, 12:20
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1. If my memory is correct it is prohibited to slip a C-172 at full flap.
2. It is prohibited to slip an older C-150 (with 40 flap).

That being said, you want to loose heigt FAST?
Get as far away from best glide speed as possible.

Options:

a) GO full flap ASAP, and DIVE at max flap speed, aim about 2OO yard in front of the touch down point, to get as much resistance as possible, then flare just above ground. A full flap C-172 is draggy.

b) GO full flap ASAP, and slow down to just above stall speed streight ahead, DO NOT BANK or you WILL STALL/SPIN in, and let the airplane sink. The lower speed will cover less ground while sinking fast. At 50ft a gentle forward nudge to get some speed to flare properly.)

c) GO FULL rudder slip without flaps, at well above stall speed. (the higher the speed, the more drag you create.) Aim 300 yard in front of touch down point
But then it gets complicated. Level out, drop full flap, flare, a lot to do in little time, and judgement will jhave to be perfect.

These are things to try out at altitude like 3000-4000 ft or so.
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Old 20th Apr 2015, 12:40
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1. If my memory is correct it is prohibited to slip a C-172 at full flap.
2. It is prohibited to slip an older C-150 (with 40 flap




Memory incorrect.
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Old 20th Apr 2015, 13:23
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1. If my memory is correct it is prohibited to slip a C-172 at full flap.
2. It is prohibited to slip an older C-150 (with 40 flap).
Half correct.

Some models of C172 had an 'Avoid slips with flaps extended' warning on the panel, and in their POH.

I don't recall a similar warning on any model of C150 I have flown.

No explanation was offered in the 172 POH for the warning, but there are two possible explanations.

1. Disruption of the airflow over the tail, causing pitch control difficulty, as suggested by A and C.

2. On poorly maintained/rigged aircraft, it is possible for wear in the flap multi-track system to allow sufficient sideways movement of the flap, during a side-slip, to jam, or restrict the aileron movement.

I have experienced both of the above on both types, and so now I don't side-slip with flaps in either.

I would suggest that Vilters' option a) Is the safest way to loose height steeply in a C172.

As to the questions posed by the OP, I can't improve on the advice given by A and C, except to add that the higher the speed in the side-slip, the more effective it will be.


MJ

Last edited by Mach Jump; 20th Apr 2015 at 13:47.
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Old 20th Apr 2015, 13:34
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MJ is mostly correct. All Cessnas may be full flap slipped. Some 172's have an "avoid" placard, because the wake vortex from the extended flap just impinges on the stabilizer, and causes the risk of instability (at about 50 kts, for my experience) This is not harmful, it just demands a bit of attention in pitch.

Do not stall while slipping and you will not spin.

If you have a genuine forced approach, do what you need to to get safely on the ground, flap limiting speeds are secondary, exceed by a little, if it will make the landing safe.

Drag increases as a square of the speed, so you get more drag from the flaps if you fly faster. If you need drag, extend all the flaps, and fly faster.

Be very sure to allow yourself additional speed for the flare, to you have some inertia to arrest your rate of descent. Arriving to the top of the flare at minimum flying speed will result in an unarrested hard landing.

You would rather force land 5 knots too fast than too slow. If you have to crash into an object, better the far fence at 10 knots, than the near one at 50 knots, because you ran out of speed, and could not make it over.

A slip is better than flaps for glide path control higher up the approach, as you can apply and then remove it, without a lift penalty. Once flaps are selected, they should not be thereafter raised during that approach - you're committed.
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Old 20th Apr 2015, 13:40
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Once flaps are selected, they should not be thereafter raised during that approach - you're committed.
Unless your a 777 on final Heathrow, both engines out.

I do wonder at the numerous suggestions that during an EFATO that a lot of consideration should be given to strictly adhering to the POH, like flap limiting speeds "No slipping with flaps" etc. The priority is the pilot gets down and walks away unharmed and insurance sorts out the possible wreck. I know some will say that going outside the POH adds to the problem, but I'm not convinced.
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Old 20th Apr 2015, 14:04
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Quote:
As to the questions posed by the OP, I can't improve on the advice given by A and C, except to add that the higher the speed in the side-slip, the more effective it will be.

MJ
Can you elaborate on that?

Are you saying that a higher speed will produce a higher rate of descent? In which case will the increased ground covered at the higher speed not simply negate any advantage from the higher descent rate?
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Old 20th Apr 2015, 14:18
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I don't know that you need to consider increasing the speed in a side slip to increase it's effectiveness. As far as my experience goes, you can't drop much faster than a rock, and that's what every aeroplane I've side slipped seems like - a rock. (C150, C152, C172, C172RG, C182, Maule M5-260, PA28-140, PA28-150, PA28-160, PA28-161, PA28-180, PA28-181, PA28-200R, M20C)
And watching some "old dogs" landing short at some back-country strips, it's quite amazing to watch them land almost vertical in a side slip.

I think one should get an instructor that's comfortable with side slips and full flap landings, and try both the side slip and the full flap "dive" method, and see the difference. Both are effective, but I believe that a side slip give you that little bit more control of the decent. (my opinion is mine)
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Old 20th Apr 2015, 14:27
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I've been learning to fly the wonderful Extra 300 with a very experienced instructor these past few weeks.

Something we've looked at in the extra is the dreaded base to final stall (in a skidding turn) & stalling in the slip on final - which is something you need to be aware of in the Extra, as you usually HAVE to slip on final, else you can't see the runway!

As said by these guys, as long as you keep a sensible attitude, not too high an angle of attack, this will give you some margin over the stall & you're fine. Keep the nose nice and low.

We demonstrated what happens when you put in too much backpressure & rudder in the turn... you get a very, very nasty surprise. For me this involved flipping over instantly, then subsequently recovering from an inverted dive 60 degrees or so nose down. Obviously we practiced this at some height. I can assure you, if you do it in the circuit or on final you are dead.
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Old 20th Apr 2015, 14:39
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The only thing to add to A&C and the OP is that side slipping in both directions will slow the overall rate of descent. A decent sideslip can let you keep the field in sight - choose the direction depending upon which side you are sitting and then hold it in as long as necessary - remember that you will be carrying a little more speed as you come out of the sideslip.
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Old 20th Apr 2015, 14:46
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Can you elaborate on that?
Yes.

The drag produced by the side-slip increases as the square of the airspeed, so the increase in drag should more than compensate for the increase in groundspeed, which may be less than you at first imagine, due to the angle of descent. To illustrate this, think about a vertical descent. You can increase the airspeed as much as you like without increasing the groundspeed at all.

MJ
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Old 20th Apr 2015, 15:19
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I can't understand why more power pilots don't use it.
Me neither, makes putting it right where you want it a piece of cake.

Re the 172's, I haven't got the POH in front of me but one of our fleet is the 172K with 40 degrees of flap available. I'm pretty sure it says something like don't slip with full flap for the reason mentioned above, you get a bit of a nodding dog effect as it disrupts the flow over the elevators. Incidentally it's amazing the effect those last ten degrees have over the 30 degree flap 172's.
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Old 20th Apr 2015, 15:48
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...you get a bit of a nodding dog effect as it disrupts the flow over the elevators.
In my younger, and more adventurous days I found the effect varied from one aircraft/model to another. Some didn't show any unusual effects, some just did the 'nodding dog' thing to a varying degree, but the one that 'impressed' me most, suddenly pitched down and took 200-300 feet to recover.

Although the 40 flap setting had it's uses, I was, on balance, glad when they reduced it to 30.


MJ

Last edited by Mach Jump; 20th Apr 2015 at 22:42. Reason: Punctuation
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Old 20th Apr 2015, 18:05
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First a word of caution, Side slipping WITH stalling is called spinning !

Most light aircraft will side slip without problems but you MUST maintain a greater margin above the stall so look for about 10 KTS above the flight manual approach speed ( get the number from the flight manual and NOT any place else).
I rarely disagree with what A&C posts, but do on this occasion. "Most" aircraft won't spin from a gentle stall when slipping as the outer wing is likely to stall first and yaw the aircraft away from a spin. A skid will very much induce a spin as it will turn you into it. Probably not well explained and there are always exceptions and also very ham fisted pilots who don't "feel" what the aircraft is doing. Also, by adding speed you are cancelling out any gain you are trying to get from slipping in the first place.

The only thing to add to A&C and the OP is that side slipping in both directions will slow the overall rate of descent. A decent sideslip can let you keep the field in sight - choose the direction depending upon which side you are sitting and then hold it in as long as necessary - remember that you will be carrying a little more speed as you come out of the sideslip.
??? - No it won't ... Slipping will increase the rate of descent and if you slip correctly and hold attitude you shouldn't have any more speed as you come out of it.

SS
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Old 20th Apr 2015, 18:06
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Thing is, you can slip by over-ruddering, or under-ruddering compared to balanced flight. If you over-rudder you are set up for a spin if you approach stalling angle (in-spin rudder, high AoA). So always under-rudder.

In other words if you are turning left use right rudder to slip, if right, use left rudder. That way you are under-ruddering. So slipping turns are good!
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Old 20th Apr 2015, 20:08
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In other words if you are turning left use right rudder to slip, if right, use left rudder.
They wouldn't be slipping turns if you didn't...
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Old 20th Apr 2015, 21:40
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The OP also mentioned a turn.

Assume best glide speed (Mayday if poss)
Turn into wind .....

How much of a turn is being considered? If at 450 ft, the aircraft is probably still climbing straight ahead and should be pretty much into wind.

If taking off with a few knots downwind a 180 degree turn won't work. If up to 90 degrees crosswind, would a 90 degree turn be considered?

Forget about turns into wind. As a low time pilot, when the engine dies at 450 ft, you will struggle to remember your EFATO checks let alone which way the wind is blowing. Add to that the reactions of any passengers. Land straight ahead with maybe a small turn either way to pick a suitable spot.
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Old 20th Apr 2015, 22:22
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I've often done full-flap side-slips in 172s, 177s and 182s without any effect on the elevator/stabilizer. However there was an effect on my non-pilot passengers on one flight and I now have a policy of side slips, only when solo or with a pilot passenger!

Some aircraft, particularly gliders, have unreliable airspeed indicators when slipping. The key is to ignore this and just maintain the pitch attitude, or slightly drop the nose a little, for an added safety factor.
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Old 20th Apr 2015, 22:30
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I did lots of slipping with 172 at full flap there were no problem.Important think is don't reduce your speed from normal approach speed.

In emergency situation most precious think is your life.Put full flap do slipping you will decent more than 1000 fpm easily.More speed more equals more drag.
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