Slipping with flaps

[StinkBug]

I know there's been quite a bit of discussion here, with a lot of people saying that it's fine to slip with the flaps, but I know that in the POH for the PA28 I was trained in it specifically said to avoid slips with full flaps. I think there is even a placard on the dash as well. Seems these discussions are always a bit focused on the particular airplane being discussed, but also often go way off on a tangent about what the particular pilot is doing. So lets open this up as a general discussion about a range of planes, and a general discussion about slipping and try not to get sidetracked on why we are slipping, or other options for bleeding altitude, speed control etc.

I'm curious 1. why slipping with flaps is bad, 2. why people seem to think that it's not bad, and 3. if #1 is true for some planes and not others, which planes is it ok and not ok in, and why?

Obviously there are a crap load of different planes out there with different limitations, so maybe we should limit this to the planes most often used in training and low time Private pilot use. Cherokee, 150, 172, etc.

 

[StinkBug]

Oh, and I'll go ahead and put the "please ignore anything posted by CTLSi" disclaimer right here in reply 1 since that's where most of the tangents seem to begin.

 

[ClimbnSink]

can cause buffeting of the tail. Which amounts to feedback in the yoke and nothing more. Unless said feedback unhinges the pilot. It is a pilot perception problem not an aerodynamic problem.

 

[RoscoeT]

This is one of those few cliche internet discussion topics that has been beaten beyond death. Yes, some POH's recommend against with slipping with flaps. Some people say that in some aircraft this can cause a rudder blanking effect, and for other aircraft types, others say it's hogwash and they've never noticed an issue slipping with up to full flaps.

This is not an important issue. For anyone curious or concerned about whether your airplane can safely be slipped with full flaps, just go up to altitude and try it. Nothing bad will happen. This has already been discussed to death. You can probably google the subject and read more than you ever want to read.

 

[Greg Bockelman]

First off, Avoid is not the same as Prohibited.

I have noticed the slight pitch oscillations that the warning in the Cessnas talks about. To me it isn't a big deal. To others it can be startling. I think that is what the warning is about. It makes some people uncomfortable.

 

[Skip Miller]

I know that in the POH for the PA28 I was trained in it specifically said to avoid slips with full flaps.

Please double check. I have never seen such avoidance language in a PA-28 POH. I am certain that it is not in the Warrior (PA-28-161) or Archer (PA-28-181). Can you tell us which model PA-28 had this language?

Cessna 172s, on the other hand, do absolutely have that language in certain model years.

-Skip

 

[StinkBug]

I suppose that's kinda the response I was looking for, but not entirely. I guess when I read "avoid" or "do not" in the POH I automatically assume it's for a good reason and I'm curious what that reason is. If it's "avoid slips with flaps because the flaps could rip off" I'm gonna avoid them. If it's "avoid slips cause you may get some weird feedback" well then I'm more likely to just do what you said and go learn the plane at a safe altitude. The problem is that there is no reason for the "avoid" instruction given.

 

[nosehair]

Liability. Get trained and become familiar with the peculiarities so that you always feel in control and "know" what to avoid.

 

[zaitcev]

I agree with Skip, it cannot possibly be a POH for a PA-28. It could be placarded by the FBO.

Cessna 172 may oscillate when slipped with large flap deflection. It never occured to me, but I head one had to be really aggressive to start the oscillation. Usually if you're on full flaps in a 172, you should not need slipping, because the flaps are pretty effective. If your approach is such that it can be saved only by slipping while already on flaps, it might be the time to reconsider.

 

[poadeleted20]

I know there's been quite a bit of discussion here, with a lot of people saying that it's fine to slip with the flaps, but I know that in the POH for the PA28 I was trained in it specifically said to avoid slips with full flaps.

I've flown PA28's built as early as at least 1964 and as late as 2000's, and I've never seen that advisory. Can you tell us the year and specific model of your aircraft? But in any event, if that's what it says, its an "avoid", not a "prohibited", and I've been slipping PA28's with full flaps for over 40 years without encountering any problems.

I'm curious 1. why slipping with flaps is bad,

Absent a prohibition in the flight manual or on a placard, it's not bad for losing altitude on final or landing in a crosswind, and in most planes, it's required to land in even the slightest crosswind other than with no flaps.

2. why people seem to think that it's not bad,

I think it's not bad because it provides a simple and easily controlled means of a) landing in a crosswind and b) increasing your descent gradient on final approach.

3. if #1 is true for some planes and not others, which planes is it ok and not ok in, and why?

What would make it not OK in some particular airplane would be if some characteristics were discovered during certification testing that were so adverse that the FAA felt compelled to prohibit it. Other than that, it's perfectly OK because both CAR 3 and Part 23 aircraft certification testing require determination if this would have an adverse result.

 

[CTLSi]

.....

 

[poadeleted20]

I suppose that's kinda the response I was looking for, but not entirely. I guess when I read "avoid" or "do not" in the POH I automatically assume it's for a good reason and I'm curious what that reason is. If it's "avoid slips with flaps because the flaps could rip off" I'm gonna avoid them.

If that were true, it would say "prohibited", not just "avoid", and if it did say that, you can bet I would not be slipping the plane with flaps extended to whatever extent the books said not to do it with.

If it's "avoid slips cause you may get some weird feedback" well then I'm more likely to just do what you said and go learn the plane at a safe altitude.

I think it's prudent to take an instructor along when you try something you've never done before, but other than that, I think that's a good idea.

The problem is that there is no reason for the "avoid" instruction given.

I don't have one of the 172 POH's which have this handy, but IIRC, there was some verbiage in the POH (albeit brief) about why. Can anyone with one of those POH's confirm that?

 

[flyingron]

The POH says that it may result in oscillation not affecting controllability (at least in the 172N book).

The explanation is that turbulent air spills off the full flaps and if you are slipping that air may flow over the horizontal stab causing the oscillation.

 

[StinkBug]

I'll take another look at the PA28 POH next time I'm in the plane. It's a mid 90's Archer III. I could absolutely be wrong, but I'm sure I saw it written somewhere and my CFI has told me that it's not recommended a number of times. He trains in a lot of different planes though, so he may just be translating the "rules" from one plane to another, or going with the most restrictive guidelines.

 

[Clark1961]

I'll take another look at the PA28 POH next time I'm in the plane. It's a mid 90's Archer III. I could absolutely be wrong, but I'm sure I saw it written somewhere and my CFI has told me that it's not recommended a number of times. He trains in a lot of different planes though, so he may just be translating the "rules" from one plane to another, or going with the most restrictive guidelines.

Nothing about "slipping with flaps" in the 'kota's AFM...same TC...

 

[James331]

First off, Avoid is not the same as Prohibited.

I have noticed the slight pitch oscillations that the warning in the Cessnas talks about. To me it isn't a big deal. To others it can be startling. I think that is what the warning is about. It makes some people uncomfortable.

Yep

And if it makes you uncomfortable you just need to go out and do more full flap slips.

 

[poadeleted20]

I'll take another look at the PA28 POH next time I'm in the plane. It's a mid 90's Archer III. I could absolutely be wrong, but I'm sure I saw it written somewhere

If you find it, please share it so we can see the exact language. If it's not there, please share that, too.

and my CFI has told me that it's not recommended a number of times. He trains in a lot of different planes though, so he may just be translating the "rules" from one plane to another, or going with the most restrictive guidelines.

It's called "negative training transfer", and I've seen that before, even among CFI's. Unfortunately, many people don't learn why the 172 has this advisory (or even that it is merely advisory), and then it turns into a game of "Telephone" in which the message becomes increasingly distorted with each retransmission. Once you understand this is an aerodynamic issue specifically related to the relationship of the flaps and horizontal stab in the 172 airframe design and layout (and possibly, although I'm not at all sure, also affecting its aerodynamically very similar predecessor, the C-170), you realize that the issue cannot be expected to affect any other type.

 

[warthog1984]

I'll take another look at the PA28 POH next time I'm in the plane. It's a mid 90's Archer III. I could absolutely be wrong, but I'm sure I saw it written somewhere .

Nothing in the Warrior III POH.

 

[StinkBug]

Well since I fly both Cherokees and 172's knowing this info is still relevant, even if I might have my POH's mixed up I appreciate the info on what is actually going on and what the warnings are about though!

 

[brian]]

Just in case you dropped the previous thread for obvious reasons: the one condition that might be a worry is fuel being ported to the outer wing. I have that warning in my POH - but this has nothing to do with flaps. More in the other thread if interested...

(Good conversation.)

 

[JeffDG]

When I first started flying 172s that had the "avoid" language, I went with a CFI and did some at altitude (on short final is not the time to try stuff out!) just to see what was up. Got a bit mushy and stuff, but nothing too severe.

Now, if I need to do it on final, I know what to expect.

And I learned on the Warrior too...no such prohibition there...I liked to call that "freight elevator" mode for when I was high on final.

 

[TriGear28]

Just in case you dropped the previous thread for obvious reasons: the one condition that might be a worry is fuel being ported to the outer wing. I have that warning in my POH - but this has nothing to do with flaps. More in the other thread if interested...

(Good conversation.)

Yeah, the 'avoid prolonged slips' to avoid fuel porting or throwing the fuel away from the pick-ups is the only one I've seen in a pa-28 manual and that was in a 78 model 161 as I recall.

OTOH perhaps in heavy crosswinds, it would be advisable to shoot a low or no flap landing with a power-on, slip. Cherokees seem to handle slips in a very stable manner if you ask me.

 

[flyingron]

Actually, the 170 has a demonstrated different slip-with-flap problem. While the 170 front half bears a lot of resemblance to the early skyhawks, the empennage is completely different.

The flaps on the 170 allow steep, well controlled approaches making slips unnecessary. Slips with full flaps are to be avoided because if the slip is extreme enough at a relatively high airspeed, the airflow is disrupted over the tail surface resulting in a sudden and deep downward ptiching of the nose.​

The italics in the above is as it appears in the POH.

Some guys in the International 170 club have shown how to demonstrate this. Essentially when you get it into this state the nose pitches down and the elevators are ineffective at doing anything about it. Letting up on the rudder pedals fixes the problem just as in the 172 though.

 

[RoscoeT]

.....

Best post yet.

 

[bobmrg]

I suppose that's kinda the response I was looking for, but not entirely. I guess when I read "avoid" or "do not" in the POH I automatically assume it's for a good reason and I'm curious what that reason is. If it's "avoid slips with flaps because the flaps could rip off" I'm gonna avoid them. If it's "avoid slips cause you may get some weird feedback" well then I'm more likely to just do what you said and go learn the plane at a safe altitude. The problem is that there is no reason for the "avoid" instruction given.

It is not about physical damage to the aircraft, it is about the effect of disturbed air (downwash) on the tail feathers.

Bob Gardner

 

[Pilawt]

Here’s what Bill Thompson, former Manager of Flight Test & Aerodynamics at Cessna, had to say about the issue of slipping with full flaps in the 172 (Cessna — Wings for The World, by William D. Thompson, Maverick Press, 1991, p. 41):

With the advent of the large slotted flaps in the C-170, C-180, and C-172 we encountered a nose down pitch in forward slips with the wing flaps deflected. In some cases it was severe enough to lift the pilot against his seat belt if he was slow in checking the motion. For this reason a caution note was placed in most of the owner’s manuals under “Landings” reading “Slips should be avoided with flap settings greater than 30 deg. due to a downward pitch encountered under certain combinations of airspeed, side-slip angle, and center of gravity loadings”. Since wing-low drift correction in crosswind landings is normally performed with a minimum flap setting (for better rudder control) this limitation did not apply to that maneuver. The cause of the pitching motion is the transition of a strong wing downwash over the tail in straight flight to a lessened downwash angle over part of the horizontal tail caused by the influence of a relative “upwash increment” from the upturned aileron in slipping flight. Although not stated in the owner’s manuals, we privately encouraged flight instructors to explore these effects at high altitude, and to pass on the information to their students. This phenomenon was elusive and sometimes hard to duplicate, but it was thought that a pilot should be aware of its existence and know how to counteract it if it occurs close to the ground.
​

The larger dorsal fin introduced on the 1972 C-172L apparently eliminated the issue.

There is a separate, unrelated phenomenon that Thompson described in newer models in full-flap slips: “a mild pitch ‘pumping’ motion resulting from flap outboard-end vortex impingement on the horizontal tail at some combinations of side-slip angle, power, and airspeed.” This is entirely benign and common with other high-wing airplanes. My Sport Cub did the same thing.

So although the 172L’s larger dorsal solved the pitch-down issue, they kept the cautionary note in the POH because of the latter phenomenon.

Unfortunately Cessna contributed to the “end of the world” fear of slips with flaps, by not explaining the phenomenon in the manuals; and in fact, many earlier C-172 manuals expressly said that slips with full flap were prohibited. I rummaged through my collection of old Cessna owners manuals:

1958 C-172: “prohibited”
1959 C-175: “prohibited”
1966 C-172F: “prohibited”
1972 C-172L (first year of the big dorsal): “should be avoided”
​

The manuals for these older models have been revised since then, and in fact the TCDS no longer carries the prohibition against slips with flaps. The POH for the current C-172S indicates that it's no biggie:

Steep slips with flap settings greater than 20° can cause a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings.

​

But a lot of us old-timers read the scary language in the old manuals back then and that's what we remember.

 

[FastEddieB]

Here’s what Bill Thompson, former Manager of Flight Test & Aerodynamics at Cessna, had to say about the issue of slipping with full flaps in the 172 (...we encountered a nose down pitch in forward slips with the wing flaps deflected. In some cases it was severe enough to lift the pilot against his seat belt if he was slow in checking the motion...)

Must have been anemic!

 

[somorris]

1966 C-172F: “prohibited”
1972 C-172L (first year of the big dorsal): “should be avoided”
The manuals for these older models have been revised since then, and in fact the TCDS no longer carries the prohibition against slips with flaps. The POH for the current C-172S indicates that it's no biggie:

Steep slips with flap settings greater than 20° can cause a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings.

​

But a lot of us old-timers read the scary language in the old manuals back then and that's what we remember.

Thanks Jeff. I thought I had read in our 1966 C-172 they were prohibited. Someone in the other thread (I think is was James3331) asked me to show him. I didn't have the POH anymore, but the POH in the airplane was the original. I appreciate your taking the time to look this up and post it, as well as the info from Bill Thompson. I had been told the warning was because of minor and insignificant oscillations but it looks like from your research it was actually more than that. I never experienced any problems.

 

[Pilawt]

I thought I had read in our 1966 C-172 they were prohibited. Someone in the other thread asked me to show him. I didn't have the POH anymore, but the POH in the airplane was the original.

Here it is, in case anyone asks:

 

[StinkBug]

Yeah, the 'avoid prolonged slips' to avoid fuel porting or throwing the fuel away from the pick-ups is the only one I've seen in a pa-28 manual and that was in a 78 model 161 as I recall.

OTOH perhaps in heavy crosswinds, it would be advisable to shoot a low or no flap landing with a power-on, slip. Cherokees seem to handle slips in a very stable manner if you ask me.

Maybe that's where I was getting the "avoid" language with the PA28. Most of my training was in the Archer III, but occasionally I was flying a Warrior of about the same vintage.

Appreciate all the input guys, it's nice to see a thread pick a subject and actually stay on it every so often

 

[poadeleted20]

Well since I fly both Cherokees and 172's knowing this info is still relevant, even if I might have my POH's mixed up I appreciate the info on what is actually going on and what the warnings are about though!

Technically, that's not a "warning". The aviation community breaks things like this into warnings, cautions, and notes or advisories. In general, a "warning" in a POH is defined as something which if ignored could get you hurt or killed. A "caution" is something which if ignored could result in damage to the aircraft. A "note" or "advisory" is something which if ignored might scare or surprise you but won't hurt anything. I think given what happens and the wording involved, full flap/slip oscillation issue falls in the "note/advisory" category.

OTOH, in the cockpit, a red "warning" light indicates a hazard which may require immediate corrective action, an amber "caution" light indicates the possible need for future corrective action, and a green "advisory" light indicates safe operation.

 

[poadeleted20]

Here it is, in case anyone asks:

I would point out that those old owner's manuals were not FAA-approved documents, and a "prohibition" in them carries no regulatory weight by itself. Only if that prohibition is in the type certificate data or an FAA-approved flight manual (which pre-78 Cessna singles did not have but pre-78 Beeches and Pipers did) would it have FAA authority.

 

[Pilawt]

I would point out that those old owner's manuals were not FAA-approved documents, and a "prohibition" in them carries no regulatory weight by itself. Only if that prohibition is in the type certificate data or an FAA-approved flight manual (which pre-78 Cessna singles did not have but pre-78 Beeches and Pipers did) would it have FAA authority.

Are you saying that in the context of a 91.13 action after an accident, FAA would overlook "prohibited" language published by the manufacturer (though not in the TCDS or an "approved" POH), and in the possession of the pilot, relating to an aerodynamics/handling/performance issue?

 

[yetti]

172SP slips fine with full flaps (even though there is a placard). Did one all the way to the runway on my check ride (someone may have been a bit high on the emergency power off). The other reason about slipping is that the pitot tube is no longer pointed into the wind and the indicated speed may become less that actual. Managing the rudder pressure to go left and right in a slip is probably one of the more fun things you can learn. Kind of like learning to surf a boat on the waves.

 

[Pilawt]

172SP slips fine with full flaps.

So does a 172N with 40° of flap. Just did that yesterday. But the newer models were never the issue. All you get with a 1972 or later 172 (with the big dorsal, which increases lateral stability and reduces how much the airplane can slip) is a little burble from the flap outboard-end vortex hitting the horizontal tail.

The issue was with the earlier models. Once in a while in a slip airflow from the upturned (wing-down) aileron would hit the tail, suddenly and unexpectedly changing the pitch trim. That's what caused the pitch-down events.

 

[TUPilot]

Are you saying that in the context of a 91.13 action after an accident, FAA would overlook "prohibited" language published by the manufacturer (though not in the TCDS or an "approved" POH), and in the possession of the pilot, relating to an aerodynamics/handling/performance issue?

And regardless of the FAA interpretation, it is generally a good idea to adhere to the manufacturers published data unless you have some pretty dramatic reason not to.

As others have mentioned, there have been multiple changes to the tail through the 170 to early 172 to late model 172, and the particular issues are not always evident in every slip, so simply stating that with X number of hours in a 172 it hasn't happened doesn't really make me think its a good idea. I was chatting with someone the other day who decided to see if a warning against spins in a certain airframe was justified. After eight separate multi turn spins he was thinking it was BS... after he almost died on the ninth spin he realized it wasn't.

By the way, the R172E/T-41C Dash 1 prohibits slips with full flaps.

 

[poadeleted20]

Are you saying that in the context of a 91.13 action after an accident, FAA would overlook "prohibited" language published by the manufacturer (though not in the TCDS or an "approved" POH), and in the possession of the pilot, relating to an aerodynamics/handling/performance issue?

No, but it would not be the open-and-shut case of a 91.9 violation for operation contrary to an FAA-endorsed limitation. I can also see the counter argument to such a 91,13 charge that if the FAA knew about agreed with the manufacturer as to the danger of that action, an official limitation should have been promulgated by AD, and absent such an AD, the FAA is taking mutually exclusive positions on the issue.

 

[peerlesscowboy]

................ I was chatting with someone the other day who decided to see if a warning against spins in a certain airframe was justified. After eight separate multi turn spins he was thinking it was BS... after he almost died on the ninth spin he realized it wasn't.

I agree, just because you've kicked the sleeping bear in the butt a few times and he just growled a little dosen't mean he won't wake up and bite you the next time. Was out playing in my T-craft one day, decided I'd see what it'd do if I pulled the elevators up into a stall and just held it there with ailerons and rudder held centered. Nine times in a row it started into an incipient spin and recovered within half a turn, the tenth time it just continued into a full blown spin. What was the difference? I haven't a clue except I guess the bear was getting tired of my provoking?

 

[peerlesscowboy]

The Cessna 170B owners manual states "....Slips with full flaps are to be avoided because if the slip is extreme enough at a relatively high airspeed, the airflow is disrupted over the tail surface resulting in a sudden and steep, downward pitch of the nose."
Note that "full flaps on a 170B is 40*, "extreme enough" slip is not defined nor is "relatively high airspeed".
Also note that the tail surfaces are different on a 170B vs an early 172 vs a late 172 and also note that the later 172s are limited to 30* of flaps.
FWIW I doubt one would encounter the "sudden and steep downward pitch of the nose" in any normal flight maneuvering but be aware it's lurking out there and the Cessna test pilots provoked the bear enuf' to find it.

 

[nosehair]

if the FAA knew about agreed with the manufacturer as to the danger of that action, an official limitation should have been promulgated by AD, and absent such an AD, the FAA is taking mutually exclusive positions on the issue.

Thank you for pointing that out, Ron, and also that these Owner's Manuals are not necessarily a duplication of the original N numbered FAA approved TCDS .