Proficient Pilot: Giving ’er the slip

The 1995 docudrama attempted to detail the events leading up to the safe landing of an Air Canada Boeing 767 that experienced a double engine flameout because of fuel exhaustion while at Flight Level 410 near Winnipeg, Canada. (The 767 had been loaded with pounds of fuel instead of a like number of kilograms.)

The dead-stick landing at Gimli, Manitoba, on July 23, 1983, was a masterful display of airmanship, resulting in part from Capt. R.O. Pearson’s experience as a glider pilot. While powerless and on final approach to the 8,000-foot-long runway of an abandoned air force base (then used as a drag strip), he recognized that the Boeing was too high and would likely overshoot. Pearson responded almost instinctively by lowering a wing, applying top rudder, and deftly slipping off the excess altitude.

Slipping was developed because early airplanes did not have wing flaps. A slip was the recommended way to steepen the descent without increasing airspeed. Although slipping is less emphasized today, it nevertheless must be demonstrated during some flight tests. Many of us, though, fail to subsequently maintain proficiency in slipping, a maneuver that belongs in every pilot’s repertoire.

Slips also can be used to further increase sink rate with flaps extended because flaps are not always as effective as we might need them to be. They also are an effective way to reduce airspeed (especially prior to a forced or crash landing), to assist in closing an open door in flight, or to divert smoke and flames away from the cockpit during an engine fire in a single-engine airplane. Slips should not be used, however, with a low fuel supply because this could unport a fuel pump or fuel line in a near-empty tank.

I never allowed my students to solo until they had mastered slipping, but not for the obvious reasons. I discovered that those proficient at performing slips (especially sideslips) made the best crosswind landings. This, I believe, is because learning to cross-control an airplane at altitude made them more comfortable with slipping near the ground while executing crosswind landings.

Inadvertent slipping most often occurs during normal flight when bank angle is too steep for the existing rate of turn. It usually is the result of improper rudder usage during turn entry or recovery, inadvertently applying top rudder while turning, or flying one wing low while holding a constant heading. The airplane simply flies somewhat sideways, and those inside the airplane tend to lean toward the low wing (as does the ball of a slip-skid indicator). The drag produced during such “sideways” flight is what helps to prevent airspeed from increasing while descending in a normal slip.

It is not common knowledge, but some airline pilots—especially old-timers—use mild slipping to assist in gently descending their aluminum giants onto the glideslope when caught high and after the flaps have been deployed. (Using spoilers to lose altitude often is prohibited following flap extension.) Such slipping seldom involves lowering a wing more than a few degrees, is remarkably effective, and is rarely felt by those in the cabin.

Pilots of yore used a more dramatic procedure to descend steeply without building excess airspeed. It was called a falling leaf, a maneuver in which an airplane in a glide is made to swing from side to side and settle like a leaf falling from the sky with little change in heading. It is performed by entering a power-off stall and then applying sufficient rudder to cause the airplane to barely begin an incipient spin. Opposite rudder is then applied before heading changes very much and is held until an incipient spin barely begins in the opposite direction. This cycle is repeated until the desired amount of altitude has been lost. The descent profile seems more vertical than horizontal because the aircraft loses altitude rapidly and with little forward speed.

Once a pilot is proficient at slipping, he might find a CFI willing to instruct him in fishtailing. This is an emergency maneuver that can be used to quickly dissipate airspeed during a potential overrun during a forced landing. It is performed by applying rudder in one direction and then the other while applying opposite aileron in each case to keep the wings level. It can be highly effective. This maneuver, however, should not be performed in large, swept-wing aircraft.

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