Thursday, January 26, 2006

Slipping and Sliding, Kicking and Screaming

We had a howling crosswind out of the south when we landed at Portland today. As with most southerly wind, mechanical turbulence over the West Hills made for a wild ride on approach. I pulled a decent crosswind landing out of it, but used more rudder than I've used in a while. It reminded me of something that student pilot Aaron DeAngelis wrote in his blog earlier this week:
Also, I didn't use the ailerons as much when lining up on final. I would set my self up, right after turning from base, and use the rudders to keep myself in line. By not using the ailerons, I could keep my right wing down slightly, into the crosswind, so my plane wouldn't be pushed over. I found this much easier than trying to correct with the ailerons, then having to correct my correcions because the wind blew me over.
What Aaron stumbed onto is essentially the proper crosswind technique: use rudder to keep the nose aligned with the runway, and aileron into the wind to compensate for drift. I suspect that his instructor has been trying to get him to do this for some time, but it wasn't until he experimented a little that it clicked; that's certainly the way it was for me. Crosswind technique is one of the harder things for a student to learn because it's something you have to work out in your own head before you can work on the mechanics of it, which are actually quite easy. There are actually quite a few private pilots out there that have a poor understanding of the proper technique, or why they're doing what they're doing. There's a lot of disinformation out there, too. Perhaps I can make things a bit more clear...

In most phases of flight, we correct for wind by crabbing. This means that to hold a course with any crosswind, we intentionally steer a few degrees off course into the wind, so that the wind drifts us onto our desired course. For an excellent graphic demonstrating a crab, look at the beginning of this post by Dave at FL390. See how the airplane is pointed to the left of the green line (the desired course)? There is a crosswind from the left, so Dave crabs his airliner to the left in order to keep on course. From the cockpit, it'd look like the plane is moving slightly sideways.

This works great for crosswinds until you have to land. Moving sideways is a really bad idea at the moment that the wheels touch pavement. Besides the sideways structural loads imposed on the landing gear, you'll find yourself headed for the weeds in a hurry. So we need some way to keep the airplane aligned with the runway without the wind drifting it downwind. That's where the side slip comes in.

If you're old enough, you might remember Galactic Warrior or similar arcade games where you have a spaceship moving forward at a uniform speed, and you'd use a joystick to slide it left or right. Airplanes can actually move in the same way, by banking the wings in the way you want to "slide." Most pilots don't realize this at first because they associate banking with turning. If you use the rudder to keep the airplane from turning, however, you'll simply move sideways in the direction of the bank. When landing in a crosswind, you bank in the direction that the wind is coming from, and "slip" into the wind until it's not pushing you sideways anymore. At the same time, you use the rudder pedals to keep the nose of the airplane on the same heading as the runway.

But wait! demands the grizzled airport bum. What about the "kick method?" Well, it's true that many pilots finding slipping to be an unnatural act of aviation, and prefer to stay crabbed the whole way to the ground, only to "kick" the airplane to runway heading at the last moment before touchdown (using the rudder pedals). Really, though, one of two things is going to happen in that last moment: either the wind will start to drift the airplane sideways (bad!) or you will unconsciously put in some aileron towards the wind...ie, a slip! So really, there is no "side slip method" versus "crab method." It's all the same method, with a difference of when you transition from the crab to the side slip.

So when is the best time to make the transition? To introduce a new student to slipping, I'd have them do it for a good two-mile final. While honing your slipping skills, the last 300-400 feet of altitude should be sufficient. Of course, many weekend warriors make the transition at the last moment, as mentioned above, but more than one has found out too late that they didn't have enough control authority for the crosswind present. About 100 feet above touchdown is a good compromise. Many airline pilots transition to a slip somewhat lower, because prolonged slips feel funny to passengers - but these are guys and gals who fly in some pretty gnarly conditions on a regular basis, and are well attuned to the crosswind characteristics of their aircraft.

One final note: just because you're on the ground doesn't mean you can release that aileron! Keep your correction in, and actually increase the deflection of the wheel or stick as you slow down. You'll notice that some rudder is needed to keep the airplane from weathervaning into the wind. If you find yourself making a crosswind landing on a slippery runway, you'll find that keeping these control inputs in after landing will save you some unwanted excitement.

There actually are two airplanes that I can think of that are designed to be landed in a crabbed condition. One is the classic Ercoupe, which lacked rudder pedals. It incorporated trailing-link landing gear that straightened out the airplane upon touchdown. The other is the Boeing 747. It cannot be banked very much when close to the ground, because of the risk of striking the outboard engines on the runway. Therefore, it is landed crabbed in any strong crosswind; castoring gear trucks protect the landing gear from sideload damage and keep the airplane tracking straight down the runway.

I just know some purist is going to mention the crosswind landing gear on later model C-195's. Yes, it has some castor built in, but it wasn't meant to allow the airplane to be landed in a crab - it just keeps a slight sideload from becoming an instant groundloop.

6 comments:

Aviatrix said...

Good post. I love crosswind landings. A normal landing, once you've set up, the airplane practically lands itself. But in a crosswind, you know you've participated!

Casey said...

Hey, have you seen the C-195's? The gear is designed to be landed in a crab!!! :)

Hamish said...

Apparently you can land a B52 crabbed as well (not that I've any first-hand knowledge of this :-) ).

But for me, getting a tailwheel endorsement gave me a much better feel for the consequences of poor landings and how to set up a slipped landing that's not going to cause a ground loop... (not that my landings are necessarily any better now, but it's the thought that counts, I guess).

Denny said...

Most all airliners can withstand a crabbed landing... all the jets that I can think of...

The whale lands just like a normal airplane.... aileron into the wind, rudder to align.. do that at the last minute.

Sam said...

I'll grant that any airliner will withstand a poorly executed crosswind landing compared to, say, a Cessna 152 - but still not recommended!

Reminds me of the time I was sitting in the back of a TWA MD80 and we had some pretty strong sideloads on landing...the rather large lady I'd been sitting next to pretty much ended up in my lap.

Aaron DeAngelis said...

Just wanted to say "Thanks!" for using my post in yours!

Thanks!

Great entry! I've been waiting for the proper conditions to try crab & slip, but last flight there was no wind (and I was alone in the cockpit :-) ,) and the one before that, we had to land early from turbulence.

Next time, I hope!