C172 Rate of Descent / Sideslipping

[Irish_Stu]

I'm currently in the latter stages of my PPL training and have been having some thoughts about the rate of descent of a Cessna 172SP in varying configurations. My main question is this - Say I'm on the final leg of a PFL, with 20 degrees of flaps, and I realise I'm too high. Will my rate of descent be higher by keeping 20 degrees of flaps and sideslipping, or should I go to 30 degrees of flaps?

My POH states sideslipping is not permitted at full (30 degree) flaps. I believe this is due to the airflow creating a lack of elevator authority. Is this only really an issue when close to the ground, in the round-out and/or flare? I have no intention of trying sideslipping with full flaps but it would be nice to know if it would be an option if I find myself too high in a real engine failure situation.

Thanks in advance,
Stu

[italianjon]

That's a good one! I think (and this is my opinion) leave 20 degrees of flap and sideslip, as you have the option to come out of the slip and slow the descent again. Once the flaps are down then it is it! You're absolutely committed (I know you're committed anyway - but you take away options)

Once you have corrected the height you can then resume normal service, so to speak... and drop 30 for touchdown.

[Genghis the Engineer]

Stability problems can happen at any height, so the only things special about the approach in terms of not flying a full flap sideslip, are (a) it's the only place you'e likely to, and (b) you have much less height to deal with any resultant loss of control.

Also, if you've not practiced something already, during a genuine emergency isn't really the time or place to experiment!


My advice if you're over-high is either to pick a different field, or to perform an S-turn or two whilst keeping your landing field in sight. (Or turn base later, which would be my general preference, but that to some extent depends upon how you fly your PFLs.)

G

[jamestkirk]

Another way is to increase your ROD by increasing the IAS. Putting the nose down. It is pretty effective.

There is one very important note to make though. You will have to move your aim point back to a place where you can start the round out to bleed off the speed. No stability issues either.

Before anyone pipes up. I AM NOT talking about 150 knots in the descent. An increase of 10-15 knots can have give you that advantage.

Also. It is good fun and a good way of building judgment on things like glide approaches.

If you have never done it before, try it with an instructor as the round out/flare may happen a little quicker. Nose wheel and all that.......

[Shunter]

Either slip it or lower the nose and do a few S-turns, or both. You'd be amazed how quickly you can lose altitude without overshooting. Also, if you dump the drag flap and point it at the floor you may gain airspeed, but you'll lose the height very quickly and all it takes is a good tug on the yoke to bleed the excess speed off again.

[Lurking123]

A reduction in airspeed will also help. The normal glide speed quoted for an aircraft is actually the best glide (ie maximum range) speed. Slow down a bit to Vmp (which conveniently equates to the minimum rate of sink speed) and you will stay in the air longer but cover less ground. Of course, this speed is closer to Vs but in reality for a 172 the difference between the speeds concerned is rather academic.

Having slipped a 172SP with full flap, I would also consider this course of action.

PS. S-turns are very effective but you are bringing in the variable of increased wing loading and therefore increased stall speed. There is a real temptation to 'pull' a bit when aiming for a field. Fancy doing a quick bit of maths at a couple of hundred feet?

[On-MarkBob]

Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails. The best way to loose hight is to slow it down, select full flap then put the nose down while monitoring the flap limit speed. It comes down very nicely and quickly. Sometime a bit of 'faith' is required as you bring the nose up to loose the speed and the runway dissapears from view, but as soon as the flaps are down, push the nose over and you'll be fine.

[bingofuel]

At the risk of being jumped on from a great height by some, there is no reason why having selected full flap (30deg) you cannot raise them again to reduce overall drag if you are getting too low. I do not mean in the last 50ft above the round, but drag flap is exactly that, more drag than lift so the 'overall' result will be less drag albeit with a slight sink initially. Try it at a safe height and see.

[rebra]

Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails.

Do you have a source for this claim? I am genuinely interested, as this is not noted in my POH (Hawk XP).

[RatherBeFlying]

On a PFL to a field, your aim point should be 1/2 to 1/3 into the field. Once you have that absolutely made in a C-172 adding flap past 20 degrees brings you in steeper and gives you more field to land in.

My last C-172 checkout included a number of surprise simulated engine failures from downwind to a short runway. I like to do it short and steep and would end up at 40 flap and pointing the thing at the end of the runway.

The roundout of course has to be done not too low and not too high -- and you do need to carry enough airspeed to have enough elevator control to raise the nose when you need to.

[BackPacker]

IS, Good question and kudos for asking this while you are still learning for your PPL. May I suggest the following things:

1. Stay within the POH. If the POH says don't sideslip with full flaps, then don't. Some test pilot somewhere found a reason for that restriction and that should be good enough for you. And note that not all 172s have this restriction: it really depends on the exact model.

2. Go up with an instructor and simply try out the different techniques. For every trick, write down the vertical speed and the horizontal speed you achieved and do a few calculations afterwards to see which was most effective. For horizontal speed, don't trust the ASI, since it may show serious errors in a sideslip (both due to the pitot tube not being aligned with the airflow, and the static port not being perpendicular to the airflow). Instead, use GPS groundspeed, corrected for wind, or fly directly towards or away from a DME and use that readout, corrected for wind.

Personally I have sideslipped a PA-28, with full flaps, at just below flap limiting speed (top of the white arc) and it's extremely effective - I guess we achieved a glide angle of almost 45 degrees.

The other thing you'll want to find out is if you can place the rudder at the stops, and still have enough aileron authority left to make reasonable turns in both directions.

Oh, and you've got to learn to push the nose down instinctively if you do this sort of thing. Sideslipping just above stall speed (1.3Vs0 or approach speed comes to mind) is a recipe for a perfect spin entry.

[Mark1234]

Personally I'd stick the extra flap in and possibly S turn. Not because I'm afraid of slipping; in a warrior I regularly sideslip - it's effective, and I'm comfortable with it, but the warrior doesn't have a limitation enshrined in the POH.

I have been lead to believe that the restriction noted in the 172 POH is because in the earlier days, a few suffered a full tail stall when so doing, and promptly nosed over completely - i.e. the flow over the tail can be sufficiently disturbed to cause a tail stall, not just a loss of effectiveness. I don't have a verifiable source for that though.

However, the 172 example I fly goes to 40degree flap, whereas yours seems to have 30 (later model). Both contain the same verbage about full flap slips not being advisable/permitted:
If I reduce to 30deg (less than full flap), can I slip where you can't? I doubt that..
Is the warning not necessary for yours? Maybe.
Where's the limit at which you can slip? Dunno...

So that's why I'd not slip - it's quite possibly corporate a$$ covering, but I'm not wagering my life on that - I'll avoid slips with flap extended in the 172 period. That said, every crosswind landing ends with a cross controlled, wing down arrival.. which looks remarkably like a sideslip :E

P.S. S turning shouldn't require much calculation - you're already well above Vs, just let the nose drop a bit in the turn, allow it to gain 5 kts or so, and don't go nuts with 60degree banks..

[Pilot DAR]

Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails.

This is a new one on me, and I've been slipping various Cessnas for 30 years. I used to slip the 185 down from 10500' after dumping a load of jumpers. I have never seen a bent Cessna flap track.

Irish Stu, Though I will suggest caution when slipping fully flapped 172's, to my knowledge (the older ones for sure), doing this is something to "Avoid", it is not prohibited. This stems simply from non ideal handling in pitch at certain speeds. Similarly, a door off on a 172 can induce this at about 65kts, and it just feels a bit spongey in the elevators. It certainly won't hurt you.

During flight test, all of this era of planes had to meet this requirement:


3.118 Directional and lateral stability
(a) Three-control airplanes.
(1) The static directional stability, as shown by the tendency to recover from a skid with rudder free, shall be positive for all flap positions and symmetrical power conditions for all speeds from 1.2 Vs1 up to the maximum permissible speed.
(2) The static lateral stability, as shown by the tendency to raise the low wing in a side-slip, for all flap positions and symmetrical power conditions, shall:
(i) Be positive at the maximum permissible speed.
(ii) Not be negative at a speed equal to 1.2 Vs1.
(3) In straight steady sideslips (unaccelerated forward slips) the aileron and rudder control movements and forces shall increase steadily, but not necessarily in constant proportion, as the angle of sideslip is increased; the rate of increase of the movements and forces shall lie between satisfactory limits up to sideslip angles considered appropriate to the operation of the type. At greater angles, up to that at which the full rudder control is employed or a rudder pedal force of 150 pounds is obtained, the rudder pedal forces shall not reverse and an increased rudder deflection shall produce increased angles of sideslip. Sufficient bank shall accompany sideslipping to indicate adequately any departure from a steady unyawed flight.
(4) Any short period oscillation occurring between stalling speed and maximum permissible speed shall be heavily damped with the primary controls (i) free and (ii) in a fixed position.


The 172 does meet the requirement, but is marginal.

I shall quote William Thompson, former Cessna test pilot, who writes in "Cessna, Wings for the World"...

...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 degrees due to a downward pitch encountered under certain combinations of airspeed, sideslip angle, and center of gravity loadings".

I very highly recommend that any Cessna pilot read that book, it is facinating!

Use caution, discuss with your instructor, and learn. Do not do "prohibited" things in planes. "Avoid" or "Use caution" are just that. Don't worry about flap tracks, just fly well maintained planes, and do walk around inspections as if you're about to hang in the air from the plane.

Be careful, but go and learn, that's what you're up there for.

Pilot DAR

[Fark'n'ell]

Go up with an instructor and simply try out the different techniques

Many instructors coming out of sausage factories these days would not know how to sideslip. They were never taught it.

[S-Works]

Quote:
Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails.
Do you have a source for this claim? I am genuinely interested, as this is not noted in my POH (Hawk XP).

I would also be highly interested in the source of information for that little gem. I have never heard and it is not in my Cessna POH either (Hawk XP!)

[BackPacker]

Many instructors coming out of sausage factories these days would not know how to sideslip. They were never taught it.

Tell me about it.

I was a PPL student with one such an instructor. At that time I regarded him highly, being the lowly student that I was.

Anyway, one day we ended up way too high on approach (my fault). Instructor said we were not going to make it and I told him I agreed, but would he mind if I tried anyway?

So I threw the PA-28 into a full-flaps, full sideslip at almost flap limiting speed and achieved the 45 degree slope I talked about earlier. We made the runway easily, even touching down in the usual spot and vacated halfway down.

Instructor didn't say anything about it afterwards.

[madgav]

Stu

Thanks for asking this question, I'd been wondering about this myself having covered PFLs in the last two lessons. I've not been specifically taught about sideslipping but of course realise that it's an option to steepen the approach. The replies on this thread have been really useful and I'll be sure to discuss with my instructor.
Also I was not aware of the PoH advice on slipping with full flap.

PS I suspect you're training in the same place as I am :ok:

Gav.

My website (http://www.madgav.org/)
My PPL blog (http://www.gmcc0266.plus.com/ppl.htm)

[sycamore]

Don`t forget that 172s usually show oscillations in IAS/ALT/VSI in sideslips due to the position of the static vent and pressure head,You should get a steadier reading if you select`Alt static`,somewhere under the panel ? in both left/right sideslips.(may not apply to all models,but try it and see.
As has been suggested,`S`turns( never turn away from the runway) are probably easiest, keeping the aircraft balanced throughout,or use max flaps to descend to the correct g/path,and then reduce flap setting( at which point the instructor will fail the electrics!),With an `S` turn you can always cut it short if it looks tight.

[Irish_Stu]

Thanks for everyone's interesting replies and rest assured that I won't go trying anything silly on my next solo cross country.

Madgav - we do indeed train in the same place, we've spoke in the past! I'm only down on saturdays now so maybe we don't cross paths as much but I'll give you a shout if I see you.

[Mark1234]

Also interesting - it appears I've been incorrectly informed regarding this..

PilotDAR - is this a repeatable scenario? I'd rather experience the tendancy with some space and get my eye in as it were, than have it just 'happen' one day on approach..

[Pilot DAR]

For me to say go and slip your fully flapped 172 on this forum might take me a tad over the line, but, If we were flying together, and you suggested that you were going to do it ,with some altitude and reasnable caution, I would not be the least bit concerned.

While doing such manuevers in various Cessnas, I have never had anything other than some softness on pitch. The most noticable of this occurred once in a 172, slipping with flap out and a door off. I have not experienced the lightening of the pilot against the seatbelt which Mr. Thompson reports, though I certainly do not doubt him either. The Cessna test pilots really did seem to go to extremes, and for good reason!

While flying a mechanically fit, correctly loaded Cessna (important qualifier to my statement), I have never found any flight characteristic I would report as dangerous or unsafe. This goes from very cross controlled sideslips, right up to required spin entries in C206, C207, and 185 amphibian, among others (do not do without flight test authority). Your challenge will come one day, when you're flying an aircraft with a control or loading problem. That's what has made the hairs on my neck stand up in the past!

Caution and adherence to the procedures of the flight manual are excellent, and to be encouraged, but a pilot who stays "in the middle" of what the plane will safely do, is missing the opportunity to learn more about what it can do. When what it can do, becomes more of what you can safely make it do, you have more skill, and are less likely to be "rattled" by an unexpected event.

I have made many landings, particularly into small bays, where the water was suitably calm, where an aggressive sideslip over the obstacle on final was vital in keeping the landing within the intended area. These are advanced skills, and should be practiced under qualified supervision, but are worth the effort to learn well.

Pilot DAR

[madgav]

Madgav - we do indeed train in the same place, we've spoke in the past! I'm only down on saturdays now so maybe we don't cross paths as much but I'll give you a shout if I see you.

Oops, completely missed that :O. I'm still doing Friday afternoons for the moment but may end up booking Saturdays for the longer nav-ex's (possibly coming soon, depending on weather).

[Mark1234]

Understood - anything I do or do not do will be of my own volition. Simply asking your experiences. Quite fond of getting out of the middle now and then, also in current aeros practice which is something I'd recommend to anyone as an exercise in appreciation of the edges, not to mention good fun..

@Sycamore - indeed.. one type (ASK-13 glider) I've flown will indicate zero airspeed in a full blooded slip - the technique I was taught was to maintain the pitch picture - power/attitude/performance etc.

[Pilot DAR]

I was doing some full on slips today in the C150 (to prevent shock cooling, it was -21C OAT), and with the door off and full flaps, there was a burble, and the elevator did it's own thing for a second at around 60 MPH. absolutely no problem to control though.

[Piper.Classique]

Just out of interest, DAR, does the C150 run out of aileron or rudder first, and it it the same for the 172?
Long time since I have flown either, and I can't remember.....
I do seem to recall the ASI being not a lot of use, despite the two statics.

[BackPacker]

it was -21C OAT), and with the door off

What???:eek:

[Pilot DAR]

My 150 reaches the rudder limit first (even with the taller rudder), which is just fine, as you just hold the rudder hard over, and apply the desired control with the ailerons. My 150, however has aileron gap seals, so they are a little more effective. That said, any single Cessna I have sideslipped, right up to the Caravan, seems to behave similarly.

I was returning from a flight test in a Caravan amphibian into Saskatoon airport with the owner of the operation, who was usually the only company pilot to fly this beautiful brand new plane. The tower, recognizing him (by his voice on the radio, and the registration) gave us a really expedited landing to get in ahead of a 737, who was a few miles out. With the instruction from the tower, which he accepted on my behalf, I carefully entered a very pleasing and effective sideslip from downwind, curving right around to crossing the numbers, with no final approach really ever established. At the peak, I had a 1200 FPM descent, but very stable and controllable. Following a very satisfying touchdown and rollout, I turned to him and asked: "Do all your company pilots get to fly like that?". "No! Never!" he replied, with a grin denoting his special privilege, which I had now shared! Or, we could have gone to the back of the line, and then waited for wake turbulence separation!

The airspeed on the 150 during a sideslip is fairly stable, and the 150 only has one static port. The 172 does have two ports, and seems to behave the same.

The door off flying in the 150 yesterday was necessary for some testing of a project. We only went for about 15 minutes at a time. It was prudent to sideslip down to land, rather than pull the power back, to save the cylinders from shock cooling. During each of my landings, I touched down still at 1800 RPM, then pulled the power off as I slowed down. It keeps the engine happy! It was -20C on the ground during the day, and went down to -32 last night. The 150 has spent the winter in the heated hangar! I'd like to sound like a tough Canadian, but -32 is unusually cold for here, and people here are whining about it - even my passenger - though, it was his door off, not mine!

Pilot DAR

[Gertrude the Wombat]

Dunno about your 172 - ask your instructor - but in the ones I fly it's full flap and push the nose down. I haven't actually tried a vertical descent, but anything much less than that it just hangs on the flaps and doesn't seem to accelerate to beyond the flap limiting speed :)