Slip-Sliding Away

Altitude is your friend, sure, but not when you’re too high on final approach because you misjudged your perspective of the runway. Then again, you might make a conscious decision to stay too high if you have engine trouble and you want altitude in the bank until you’re sure you have the runway made. Or let’s say you’re trying to get over obstructions on the approach path to a short runway, and having full flaps isn’t quite enough. The solution for regaining your figurative footing that’s called for in these slippery situations is (as if I didn’t give it away already): the slip.

The well-known incident over 20 years ago involving Air Canada Flight 174 certainly stands as a great testimonial. The victim of having his fuel load calculated in pounds instead of the usual kilograms, both engines on Captain Bob Pearson’s 767 (which at the time was the most advanced jetliner in the world) flamed out short of the destination at flight level 410 over Red Lake, Ontario. The captain, who was also a glider pilot, made a dead-stick landing on an abandoned runway at the former Royal Canadian Air Force Base at Gimli, Manitoba (which was being used at the time, actually, for drag racing) when they realized they weren’t going to make Winnipeg, their first choice. However, he also realized while on final approach that he’d probably overshoot the 8,000 foot strip. (This was probably a few moments before those same drag racers and their awestruck audience realized that they were about to get an even better show.) What’d he do? He lowered a wing, and “kicked top rudder“. I imagine that there was probably a mixture of stunned silence and applause, shortly afterward (not to mention quite a bit more burned rubber when the crew stood on the brakes). Not too surprisingly, the media referred to this event as the “Gimli Glider“.

You’d applaud too, once you see what en E-ticket escalator ride you can get out of this maneuver, especially in aircraft with a good deal of rudder authority, like a Piper Cub. Of course, the old 1947-vintage J3 that I used to fly at that grass strip near Baltimore didn’t have flaps. And that’s probably the origin of this maneuver from the days of yore. Back then, who had ’em? A flap was a term used to refer to the buzz and commotion after somebody’s faux pas, or else what a flag does in a stiff breeze. I can remember the sensation of looking not too far off from directly out my side window at the rapidly approaching runway. If ever you need to increase your rate of descent without affecting your actual forward airspeed much, the slip is the way.

Inside Information: There’s a significant cosine loss of indicated airspeed because the relative wind and the pitot tube are no longer parallel and the ram air no longer rushes straight into the pitot tube. However, total airspeed doesn’t significantly increase, even with a cosine gain from the steeper glide path.

One operational definition says that it’s when the rate of turn is too low for the angle of bank. And then there’s the practical application of this mismatch: Basically, it’s a maneuver where you deliberately (hopefully, not unintentionally) present more of one side of the fuselage than the other into the wind: “flying sideways” if you’ll permit the exaggeration. You start by lowering a wing in the desired direction, using the ailerons in the usual way, and then apply opposite rudder. Yes, it’s cross-controlled flight, and yes, this is exactly what can precede the deadly spin.

The Tricks: Your indicated airspeed can be unreliable, and while your longitudinal axis is pointing somewhere else, you have to remember to keep your ground track pointed where you really want to go by adjusting bank angle. And you have to remember to keep the nose down so that you stay safely above a stall.

Warnings: I hasten to add that a slip is not an excuse to salvage a poorly planned approach. Also, even if you’re not doing it because you have to, beware of slips when the winds are gusty, or the geography you’re over-flying might generate mechanical turbulence.

Most pilots learn it, and it can show up on the check ride. Besides adding to your ability to lose altitude when flaps aren’t enough (assuming the airplane is not placarded against doing so), the slip has other uses, too. Remember it when

  • Something forward of your firewall ever puts you in the skywriting business (I’m talking about a loose hose, or heaven forbid, an engine fire), and you need to keep smoke (or flames) away from the cockpit.
  • If you’re ever faced with a forced landing, and you need to get shoehorned into someplace really tight (and you’re not exactly concerned about being able to fly it out again).
  • You can use a slip to help close a door that popped open in flight (but need I remind you that this is only a good solution if you’re comfortable with it and only if you’re at a safe altitude … and only if there’s not much else going on).
  • I rather doubt that I need to convince you of its utility during your next crosswind landing.

Do not use a slip (aside from the airplane being placarded against doing them with deployed flaps) when you’re low on fuel. If the tank is already almost empty, the fuel line coming from the tank could just start sucking air instead of fuel. This isn’t something you’re likely to find a quantitative look-up table for, either. (Let’s see…I have 12 gallons left, so the chart says I can slip with up to a 20-degree bank…) The very situation in which you’re likely to do this, i.e., close to the ground near the end of a flight, could conceivably make its practicality a bit academic, but most of us don’t flight plan our fuel with brinkmanship in mind.

An airline pilot even once admitted that although this tends to be a procedure kept close to the chest, even airliners use the forward slip from time to time during normal operations. Whenever they find themselves a tad high on the glide slope (even after they have deployed the flaps, that is) they’ll dip a wing a very few nearly-imperceptible degrees, and even savvy cabin attendants are none the wiser.

I’ll toss this one out, too: the slip isn’t the only way to get down quicker, without going faster; one name for that maneuver is the falling leaf. In it, you enter a power-off stall, using alternating left and right rudder to the point where there was a small heading change-as well as the theoretical incipient spin, truth be told. (There’s a bit of knuckle-biting involved, and I mention it for academic reasons alone. Unless you’re pulling hazardous duty pay, leave this one for easy chairs and clucking tongues-or else when the houses are still really small.)

THE BOTTOM LINE: As with any of the arrows of airmanship we carry around in our skill-set quiver, the slip is certainly worth remembering. It’s also worth understanding as far as what it can do, when we’re paying attention; but also what it might do, if we aren’t.