Slip approach to cross-control stall

[Zonkor]

Did it ever happen to anybody? How large do you consider the risk? How about during a side-slip base-to-final?

I love side and forward slips, especially on final, but the lack of airspeed reading at close ground is a bit unnerving. :)

How do you keep track of airspeed? Just play it by ear?

[AdamFrisch]

I actually do it on my twin all the time, which is not normally what one does with twins, but she's a bit of a bush plane so I feel it's appropriate...;)

I just leave a lot of margin speed-wise and never had any troubles. Even when I've gone full deflection on rudder. But I generally keep the speed up to 80kts on final, even though she stalls at 56kts full flaps, so I have a lot of margin. Sometimes on shorter fields I'll let it get slower on final to reduce rollout and then I should remind myself not to do in such a scenario, at least not a severe one. Good you reminded me, actually.:ok:

[Tarq57]

Never had a need to do it at low speed - that's what flaps are for.

I've utilised slips a lot for losing excess speed toward the end of a short approach without cooling the engine too fast, just until slow enough that the flaps/gear can be lowered.

[Prop swinger]

Set the aircraft up on a stable approach, note the attitude & then maintain that attitude during the slip. You should be able to come out of the slip reliably at the same indicated airspeed as before.

Practise at height - set up the approach, slip the aircraft & then raise the nose towards the stall. The types that I've tried this in have all run out of rudder authority & started to come out of the slip before stalling.

[Heston]

Please sir, what's a forward slip?

[ArcticChiller]

No need to look at the airspeed indicator, as long as you don't increase angle of attack. If you start a sideslip and leave the elevator alone the speed shouldn't dissipate. In this case you just add a lot of drag but your angle of attack (speed) remains the same. If you start a sideslip and pull back on the yoke you would loose airspeed - as in every other condition of flight. Am I correct with this explanation?

Heston, a forward slip is a crosswind correction on final to align the aircraft with the runway. Which is in aerodynamic terms the same as a sideslip.

[Shaggy Sheep Driver]

A forward slip is one done while not turning. All slips are 'opposite rudder and aileron'. The main thing when doing turning slips is ALWAYS USE RUDDER OPPOSITE TO THE DIRECTION OF TURN. In other words, if doing a slipping turn from left base to final, use left aileron with right rudder. Never over-rudder INTO the turn - that way lies the spin!

As has been said, the ASI will probaly be innacurate in a slip so judge speed using attitude. Don't let the speed build up during the slip as that negates the purpose of doing it (Steeper rate of descent without speed building up).

Some aeroplane come down like an upside-down Harrier when you slip - Cubs, for instance, their slab-sided fuselage providing lots of drag. Others, like the Chippy with its slender and cylindrical rear fuselage, benefit less (it's the drag generated by pushing the fuselage sideways through the air that increases the rate of descent for a given speed in a slip).

[Agaricus bisporus]

Well, to back up Heston it's an expression I've never heard in 30 years and nearly 20,000 hrs flying...
How can "slip" be anything except lateral?

[BackPacker]

Aerodynamically speaking a sideslip is the same as a forward slip. You fly through the air sideways, creating drag.

The different names are just there to distinguish a different purpose.

With a sideslip you are typically on the centerline, aligned with the centerline, but want to offset a crosswind. So you bank into the crosswind first, then use rudder to keep the fuselage aligned with the centerline.

With a forward slip you are again on the centerline (and there is no crosswind), but want to create more drag without drifting off the centerline. So you lead with the rudder, and then use the ailerons to keep the aircraft on the centerline.

In both cases you are slipping: not flying in the direction the nose is pointing, but partly sideways. The difference is where your nose is pointing. With a sideslip your nose is pointing to the runway, and the sideways component of the slip is used to offset the crosswind. With a forward slip your nose is pointing maybe 30 degrees to the side of the runway, but you are using the sideways component of the slip to stay on the centerline.

Never over-rudder INTO the turn - that way lies the spin!

Just to add that it is perfectly possible to spin if you rudder the other way too. Again, aerodynamically speaking there's no difference, but psychologically you are more likely to pull, increasing AoA and eventually stalling into the spin, if you rudder into the turn. Furthermore, if you yaw against the turn the view of the final approach typically gets better as the nose of the aircraft is no longer in the way.

[john_tullamarine]

Well, to back up Heston it's an expression I've never heard in 30 years and nearly 20,000 hrs flying...

A peculiarly American distinction. Referred to as such in various FAA publications.

The rest of us just do a slip to suit whatever the need may be at the time without giving this and that purpose different and fancy names.

[djpil]

In general, there are more pro-spin forces and moments in a skid than a slip.

Most of my students get to experience both. In the airplanes I use, a stall in a skidding turn results in an aggressive entry to a spin cf to a sideslip which is spin-resistant.

[Silvaire1]

I think the practical difference between a skid and a slip on approach is that if you stall out of a slip the aircraft will more typically roll 'over the top' (reducing bank angle) which allows the pilot time to reduce angle of attack. Conversely, stalling out of a skidding turn rapidly increases bank angle in the same direction. There is no aerodynamic difference, but gravity and the horizon aren't aerodynamic factors.

In the Cessna Aerobat flight manual there is an exercise in which the aircraft is stalled out a slip, the pilot reduces angle of attack as the aircraft rolls through level, and the plane ends up banked equally in the opposite direction. Repeating causes the plane to fly an S-turn.

[Agaricus bisporus]

The definitions of (side) slip and skid have nothing whatsoever to do with centrelines, runways, or anything else of a geographical nature.

They describe only regimes of unbalanced flight.

[Silvaire1]

The definitions of (side) slip and skid have nothing whatsoever to do with centrelines, runways, or anything else of a geographical nature.They describe only regimes of unbalanced flight.

The direction of gravity plays a role in the differentiation, and the horizon is typically normal to the direction of gravity.

[Shaggy Sheep Driver]

the horizon is typically normal to the direction of gravity.

You'll have to come and do some aeros with me. The direction of gravity can be anywhere (down, up, sideways or anything in between), as can be the horizon. The two are not neccessarily co-ordinated!

Yes, you can spin off a sideslip with out-turn rudder, but it's much less likely than during one with in-turn rudder. The latter is really asking for the nose and inside wing to suddenly drop in the direction turn and at circuit height what follows will be a brief death spiral of half a turn or less.

[Crash one]

"Forward slip". Although there is tecnically no such thing & geographics don't come into it. If you apply, frinstance, right rudder & left aileron on (geographic) final with a stiff left hand crosswind, the a/c will point down the runway, track down the runway, with left wing down a bit. Geographically forward but side slipping with respect to wind.
My silly opinion.

[djpil]

In the Cessna Aerobat flight manual there is an exercise ... they call it a vertical reversement if my memory is correct. Needs quite aggressive control inputs.
Compare with the aggressive spin entry from a skidded turn to a stall.

[Pilot DAR]

How large do you consider the risk? How about during a side-slip base-to-final?

The risk is very manageable, with adequate skill and attention. You can slip right to the point of touchdown, and some of the rollout. Once on the ground, it will depend upon being able to slide the tire though, so don't try it on dry pavement!

http://i381.photobucket.com/albums/oo252/PilotDAR/Aircraft/IMG_2465.jpg

[Big Pistons Forever]

As was pointed out in an earlier post the ASI is essentially irrelevant to safely perform a slip. As long as the correct gliding pitch attitude is maintained you can't stall.

Your average Piper/Cessna is rudder limited so you will reach maximum rudder travel at quite modest bank angles. However a slipping turn will allow you to increase the bank angle and therefore increase the sink rate. Therefore the maximum possible sink rate will be a power off full flap glide in a slipping turn, something that could be a lifesaver in the event of a forced approach.

Personally, for low time pilots, I discourage slipping on final in Cessna's. They have effective flaps so if you are so high that full flaps and idle power is not enough to make your selected touchdown point, I think the approach is so far from the ideal flight path you should just go around rather than pushing a bad approach by slipping.

[bubbers44]

I agree, if for some reason you feel the need to slip always use the aileron to turn and opposite rudder to prevent a cross control stall. Once on final forward slip all you want. I have done it hundreds of times with no problem. Skidding on to final with rudder and opposite aileron will get you. Art Scholl taught me that in the 60's and he was one of our finest aerobatic pilots. He gave me the honor of teaching his aerobatics course at Flabob airport in Riverside, Ca. I was the guy that flew his clipped wing cub to all of his air shows.

[bubbers44]

Please realize I am only talking about if you need the forward slip to lose altitude before turning final. As always use coordinated controls all the time.

[flyinkiwi]

Generally I only sideslip on final when practicing glide approaches or forced landings. I agree with BPF that if you feel you have to slip when on final in a Cessna single then you should be considering a go around because you should have fixed the problem a long time ago.

[Shaggy Sheep Driver]

I agree with BPF that if you feel you have to slip when on final in a Cessna single then you should be considering a go around because you should have fixed the problem a long time ago.

'Fix the problem'? What about if things change? I remember being close-in base for one airfield in the Chippy when I espied a spam can droning in on long 'bobmer' final. I was just commencing a go-around while cursing pilots who do 3-county circuits when the A/G guy suggested I use the parallel grass strip. Power off again and full flap re-applied I was too high, but a sideslip allowed me to make the grass strip with ease.

You need to be flexible when flying - and the more 'tools' you have in your box the more flexible you can be. Being proficient in sideslipping is a useful tool for anyone to have.

[Steve6443]

Our flying school was adamant that the student should be proficient in what they deemed "landing from unusual positions" - in this case it meant being high. So much emphasis did they put on this that we spent the best part of 2 full lessons first doing a regular approach, touch and go and then approaching, say, 400 feet high and slipping whilst on final in order to lose that excess height without gaining speed - those hours were possibly my most enjoyable lessons.

The reasons they gave for this were - in the case of engine failure where you had chosen your landing field and suddenly found yourself too high to make it in, a go around is not possible so slipping could loose you the excess altitude without gaining speed. IIRC, in the US the student pilot has to show his proficiency in carrying out slips before they can go solo.....

As Shaggy Sheep Driver says, it's all about adding skills to your competencies, the more you have, the more challenging conditions you can master without being overly concerned.

[Silvaire1]

Slipping was taught to me as a basic part of standard landing procedure when I learned how to fly in a non-flap aircraft 10 years ago. Its safer in that type of aircraft to approach higher over the houses, then slip down to a normal profile on short final. Quite satisfying too.

Instruction that slipping was OK on final, but that skidding from base to final was very bad (standard instruction from say 50 years ago) encouraged me to learn why there is a difference. Then, having figured out something theoretically reasonable (its not something that is widely written about) I asked for a high altitude demonstration from my instructor, and got one.

Now I more often fly an aircraft that really doesn't really like to slip much and as mentioned above, sometimes I feel like there is one less tool in the tool box. Luckily the thing can be made to sink like a rock by other methods.

[Big Pistons Forever]

'Fix the problem'? What about if things change? I remember being close-in base for one airfield in the Chippy when I espied a spam can droning in on long 'bobmer' final. I was just commencing a go-around while cursing pilots who do 3-county circuits when the A/G guy suggested I use the parallel grass strip. Power off again and full flap re-applied I was too high, but a sideslip allowed me to make the grass strip with ease.

You need to be flexible when flying - and the more 'tools' you have in your box the more flexible you can be. Being proficient in sideslipping is a useful tool for anyone to have.

You have to learn to walk before you can run. So for new pilots working at getting the aircraft on speed and on the correct flight path when flying the final approach should be the first priority. When you can do that consistently then of course you should build your skills by practicing more demanding profiles.

However on a pretty regular basis I see too high, too fast approaches followed by flat or even nose wheel first landings. IMO the solution for these pilots is not working on slipping, it is working on the fundamentals of aircraft control and going around when they don't get it right, not pushing a bad approach and then landing on the nosewheel.

[The500man]

Another benefit to slipping on final, is that you can avoid shock cooling. A power off glide may not be good for your engine or more specifically your exhaust but with a bit of power on you will descend at a slower rate if you fly the same approach speed. Using sideslip with power however you can match the power off rate of descent and save your self the extra maintenance cost!

[Shaggy Sheep Driver]

However on a pretty regular basis I see too high, too fast approaches followed by flat or even nose wheel first landings. IMO the solution for these pilots is not working on slipping, it is working on the fundamentals of aircraft control and going around when they don't get it right, not pushing a bad approach and then landing on the nosewheel.

Too true! These folk havn't learned to land properly. IMO ab initio training should be in tail wheel aeroplanes, which won't allow such abuse as that and will demand correct technique.

If someone can't land properly, it's unlikely they'll have the skill to learn correct slip techniques anyway.

[India Four Two]

Yes, learning to slip as a way of increasing rate of descent on final is an important skill. It's taught to glider pilots all the time. In gliders with a nose pitot, the ASI reading often becomes meaningless, but as others have said just maintain the correct attitude.

I used to do it all the time in towplanes. A steep full-flap approach, aiming to cross the airfield boundary high, to avoid dragging the tow rope across some unsuspecting motorist and then a maximum slip (as much bank as possible while keeping straight with full opposite rudder) down to the flare and straighten up and land. A very satisfying exercise that becomes second nature after a while.

I try to avoid slips when carrying passengers though after having scared some, when I did a slip without pre-briefing them. :=

[Gertrude the Wombat]

I try to avoid slips when carrying passengers though after having scared some
Went for a joyride in Austria - the local flying club was having an open day.

On approach I wondered "how TF are we going to get over those trees and still have enough of that short grass runway left?"

The answer was to fall out of the sky sideways having passed over the trees, and then land on the numbers. The passengers weren't warned!

[IFMU]

When I used to tow at Harris Hill, sometimes the tow reel would break and we would have a conventional fixed rope instead. If we were landing over the scenic overlook I would stay high till well past them then slip it hard. We only had 1100' of pavement. Slips are good.
Bryan

[phiggsbroadband]

Hi... I just read the POH for the C172N and it says 'Avoid Slips with Flaps Extended'. So in some planes you do not have that option. However 40 degree Flaps are very efficient.

[India Four Two]

phiggsbroadband,

Note that it says "avoid", not "prohibited". I've done slips with full-flaps for years in 172s, 177s and 182s. I've never run into the elevator vibration that is reputedly the reason for the "avoid" comment in the POH.

[Flyingmac]

Note that it says "avoid", not "prohibited". I've done slips with full-flaps for years in 172s, 177s and 182s. I've never run into the elevator vibration that is reputedly the reason for the "avoid" comment in the POH

Ditto. I'll add that in all those years I've never come across a 'Forward slip'.

[India Four Two]

I've never come across a 'Forward slip'

flyingmac,

You have to cross the pond to hear it. Here's a good review of the US terminology:

Air Safety Institute Instructor Reports (http://www.aopa.org/asf/publications/inst_reports2.cfm?article=6156)

Interestingly, although I've flown in Canada for many years, I hadn't noticed (or had forgotten) that TC also uses 'forward slip'.

[Flyingmac]

I've read it. I haven't been able to make sense of it.



The forward slip is performed just like the turning slip, but the heading remains constant. Its purpose is to lose altitude without a corresponding increase in airspeed, which would be the case if you simply pushed forward on the yoke. The sideslip is commonly used in crosswind landings. Here, the rudder is used to maintain centerline heading while the ailerons are used to maintain the centerline track and keep the upwind wing low. Larger aircraft require earlier transitions because of their higher approach speed and greater mass. In some cases, a slight power adjustment is needed in a sideslip, to compensate for the increase in drag. Again, sideslips take practice, but mastering them will ensure consistent crosswind landings.


I still have no idea what a 'Forward slip' is.

[India Four Two]

Flyingmac,

You are on a nice straight final approach, but are high and want to increase your rate of descent. Apply aileron to start a roll but use opposite rudder to prevent a turn and to keep on the approach track. So you are now "slipping", but still flying "forward".

[Flyingmac]

Flyingmac,

You are on a nice straight final approach, but are high and want to increase your rate of descent. Apply aileron to start a roll but use opposite rudder to prevent a turn and to keep on the approach track. So you are now "slipping", but still flying "forward


Seems to me that you've just described a wing down, crosswind landing technique. Not something I would use to boot off some height after clearing the trees.

[john_tullamarine]

I've never run into the elevator vibration that is reputedly the reason for the "avoid" comment in the POH.

I suspect that the reason for avoiding pronounced slip in high wing aircraft is the risk of significant flow separation from the downwind side of the fuselage and loss of lift on the best part of half of the horizontal stabiliser.

So what you might say ?

This picture (http://www.dhc4and5.org/Caribou_Ha_Thahn.jpg) of a Caribou shortly after having its tail shot off by friendly fire on short final in Vietnam shows what might (will ?) happen on short final with no lift from the stabiliser. Also a good advert for getting the comms protocols organised ahead of time.

Losing half or thereabouts of the lift might still give you a big fright or worse ....

If the OEM puts a caution in the POH, it's an overconfident pilot who disregards it without knowing the ins and outs of the what led to the caution ...

[OpenCirrus619]

As far as I can see a valid translation is:
ENGLISH---------------AMERICAN
Wing Down Approach---Side Slip
Side Slip---------------Forward Slip

Might help clarify somewhat :D - alternatively it could just add to the confusion :ugh:

OC619

P.S. If someone can explain how to add a table nicely please do
P.P.S. Grrr

[Crash one]

During my limited gliding time there was a side slip final approach which ended in kicking off the drift & rolling level as you touched down.
This final approach would be with the a/c tracking in a straight line on runway heading with the nose pointed sideways & the opposite wing down, Shimples!
The slipping turn was as implied, rudder & aileron reversed but balanced to produce a turn as well as lose height. Calling it a forward slip just confuses the issue. The thing is moving through the air in a straight line, where you draw that straight line through the airframe is anybodys. Also shimples. I can't see the problem:ugh:

[therealdooga]

I guess the reason why we European's aren't getting it is, we're always flying (mostly) foward, even in a slip - otherwise we run out of lift ;)

I never heard about the different terminology (Germany), other than maybe a wing-down landing being mentioned at some point. It's a slip, the plane goes forward and a bit sideways, no matter the intend, position in the circuit or whether it ends up on the runway or not, in my opinion. Makes things simpler for me to only have on term for one thing...

[mmgreve]

So after a completely unrealted discussion of forward and side slips, does anyone have any input on the question initially asked?

If I undetstood him correctly, he is wondering if you're putting yourself in danger of a low level spin when slipping in?

I would think yes and always keep the speed up for the same reason (and because ASI becomes less reliable), but I don't recall seeing any accident reports where a spin happened from a stabilished slip approach (with decaying speed, so not totally stabile)

[therealdooga]

Fair enough, back to topic (sorry).

Yes, of course you are increasing the likelyhood of a spin when crossing controls at lower speeds - the point is, if you're doing it on final because you're too high, that height is what you burn to keep your speed high enough despite the slip (which is the whole point). The one thing to do in the small Cessna's that I'm flying is to keep the nose at least at the same attitude it was at before the slip (when you were going in at around 75-70kts at the beginning/middle of the final approach in my case), or maybe even put it down a bit more, because the slip kills speed rather effectively. I actually only slip on final when I'm really high enough to comfortably put the nose down a bit (only a bit!) while slipping, I do not at all like to do it when approaching the ground.

Exception: recently I came in for a landing shortly before a black rain front passed over the airfield, and it appeared that the wind had already changed direction lower to the ground (according to our tower) while higher up it was still blowing from the rear, in relation to the runway we were advised to take based on the ground wind direction. Also I came in really high on final because someone else in the circuit with me was just ahead and didn't see me, so I circled to give him some room, gaining height in the circle to avoid a hill that comes up below that part or our circuit. When I turned onto final again that excess heights, taken together with about 12-15kts of wind blowing from behind, meant that I almost couldn't get the plane down in time to hit our (very short) 07 runway. I was basically standing fully in the rudder and keeping the plane aligned with the ailerons during the entire final leg, putting the nose down to keep enough speed to deal with the gusts of the approaching front, and only straightened her out a couple of meters above the ground shortly before touchdown because miraculously (and thankfully) the wind almost dropped to zero down low.
That was a case of slipping her down almost to the ground, but with definitely enough speed in the bank to do that safely...