Slips, sideslips & crosswind landings.
 

I attended an FAA instructor seminar yesterday. One of the FAA guys gave a brief (or lecture, if you like) on slips. I had no issue with most of his talk ie slips/skids/forward/side, however when it came to wing down crosswind landings he stated that the aircraft was in a zero slip condition during this phase of flight, a position with which I disagree. He states that the aircraft is unable to determine crosswind and only experiences wind parallel to the longitudinal axis ie 'on the nose' - which is true in normal balanced flight, however this is not the case here. The aircraft has crossed controls applied.

I think that it is still a sideslip, and only appears not to be slipping because the sideslip speed balances drift speed and the flight path is being referenced to the runway. In other words, I think his error is that he's jumping between frames of reference which is masking the aerodynamics.

Am I wrong? Have I misunderstood something all these years? Anyway, we a have bet over who pays for lunch, to be won (or lost) one day when it's breezy & we have a plane available to which we'll attach a yaw string. I say the string will not be parallel to the longitudinal axis, he says it will...

In my Piper Cub, during a left wing low x-wind landing,with right foot to straighten the nose along the runway, the ball would be on the left. This is a sideslip, also to FAA norms.:rolleyes:

Pick a nice expensive resaurant, Tinstaafl! You've won yourself a free lunch.

Wing down, top rudder ... isn't that a sideslip ? Sure the aircraft knows naught about crosswind on the runway but it sure can see slip when the wind comes in off the nose.

As we would all be aware, not all instructors know a great deal about the detail that which they purport to teach. In respect of a few I have observed, one might go to the extreme of suggesting they know approximately naught about their trade at all. Caveat - the great majority of instructors are very knowledgeable - it's just a matter of sorting the wheat from the chaff.

The US folk further confuse the issue by referring to "forward slips" when sideslipping while keeping aligned with a geographical feature, typical the runway during approach and landing.

So far as the instructor's comment is concerned, it is arrant nonsense, of course. The easiest practical way to demonstrate this is to incorporate a simple yaw string on the cowling or window. The abrupt shift in alignment when slip is introduced cannot pass unobserved. Much easier on a twin, of course. If a single, one would need to keep a constant, low power setting so that propeller flow doesn't mask the slip.

Furthermore, one of the work up exercises when determing aircraft crosswind limits is to check out the aircraft's sideslip capability at altitude.

Are you sure he wasn't just having the group on to see if anyone picked up on the deliberate mistake in the lecture ?

Tinny, do bring us all back a bottle or two from the meal.

He was serious. We even resumed the discussion during the break and he was adamant that it was a zero slip condition. Only one other spoke out in 'my' camp. No one else said a word - and nearly all of them are experienced instructors.

As my B707 flight instructor at DFW said: "if all else fails... RE-train!"

Suggest you tell the FAA guy that; AFTER he's paid the bill.

Chris

PS
Not advocating wing-down technique for the seven-oh, by the way. Can result in expensive noises...

Amazing.

Suggest you go fly the test in a cheap light twin so that the propwash is out of the equation. The yaw string in front of his nose will cause him to go quiet for a little while, I warrant.

Fine theory ... but in reality
 

Yep, that's a slip into wind, but real world technique is something at variance with textbook .. Try that on a low podded item such as a 73 and much crunching could be the result, so.. correct from the book of side slipping. What people tend to do is a fusion of slip to maintain the center line, with a deft transfer to crab & kick at an appropriate height above the tarmac.

I don't know how the subtlety of that technique would transfer to a power-point presentation, my best guess is that it would not.

Try that on a low podded item such as a 73 and much crunching could be the result

I'm a tad out of touch with the 73 Classic these days but it has a decent limit and I don't ever recall any problem in max crosswind landing off a normal sort of approach - balanced tracking with drift on down to, say, 150ft agl or so for max, and then adjust to crossed controls, tracking the centreline with nil drift for the flare, touchdown, and rollout ?

Generally resulted in a routinely much nicer touchdown than in nil wind conditions as well ...

FAA has not impressed me with their aviation knowledge several times before...

It is not the FAA. It is the weekend crash course..

I've worked with the gentleman before and his viewpoint is not something I'd expect from him. I wonder if somehow he's managed to mix 'drift' & 'slip' in his mind?

You might also ask him how he is planning on aligning the longitudinal axis with the base of the wind triangle in a zero slip condition.

Hi John - from the 737NG Boeing Flight Crew Training Manual:

(The classic had a similar paragraph.)

The only time that I am aware of where an aircraft is travelling in a straight line, with one wing low in a straight line (slip ball to one side) and it is not sideslipping is when it has asymmetric thrust.

Yes. And sideslip vs zero sideslip asymmetric flight was covered correctly. It was just this issue of correcting for a crosswind using wing down.

Thanks everyone. I was certainly hoping I'm correct or I'd have some major conceptual adjustments to make. Boeing too. Now we just need a decent x-wind to go play with a yaw string.

Checkers .. then I was probably a naughty boy.

However, I can't recall seeing anyone else at AN impacting with intentional drift. The recommendations will be for heavily compressed oleos which is a very conservative viewpoint. I can't recall any landing with significant crosswind which involved thumping the bird onto the ground - on the contrary, crosswind landings tended to be rather pleasant.

I can't recall what would have been the max crosswind I ever landed the 737 but certainly at or above 25kt ?

A few at 30kt in the Electra but that was a pussycat to put on the ground ... as to Cooma and King Island on the Mouse .. let's not admit to what might have gone on there ...

the FAA instructing a class...

thats your first clue...

Enjoy your free lunch. You are right. The FAA guy needs to get his basic flying manual out again. Any time the ball is not in the center you are in a slip. It is usually caused by rudder but also is caused by differential thrust. Not using enough rudder with an engine failure on a twin puts you in a slip. All slips are the same, just called side if you are using it to drift sideways usually into a crosswind and forward if you are trying to increase your descent angle on landing without increasing your speed because of the fuselage drag.

Crosswind limit for landing is actually 40 kts on dry and wet runways for both 737-800 and -700s. Crosswind limit for takeoff is 25 on wet runways and 34kts with winglets, 36 without on dry runways. Those are valid for narrow runway operation.

It is actually very nice to see an autoland in a 737 at 25kts crosswind where the autopilot establishes a nice and steady bank into the wind and keeps it right until touchdown.

Looking at the original post, it could be interpreted that wing down crosswind flight did not involve crossed controls in this case.

If you are just banking into the crosswind, you can find a point where your gentle banking turn doesn't actually turn, and the resultant vector of flight is along the runway heading.

I think with this interpretation you see an uncoordinated turn, but with little or no forces acting laterally on the airplane as you would experience with crossed controls.

Could this be the point the FAA person was trying to make?