Safety Publications/Articles


Slipping out of the past

An old remedy is still potent

In aviation's early days, aeroplanes didn't have leading- or trailing-edge devices. Without these lift devices that altered the wing and allowed slower final approach speeds, pilots were forced to glide with the power off and often would overshoot the runway. Since engine reliability wasn't the best, go-arounds weren't always an option. However, our intrepid aviators soon discovered they could "slip" their aircraft down final approach with an increased descent rate without increasing the airspeed. Pretty cool.

But these early aircraft had more tricky characteristics than just bad engines and no flaps. Their tails dragged on skids, and their long noses limited forward visibility. Since most were flown from the rear seat, landing and taxi operations were challenging. But with a slip, pilots could not only push the nose out of the way on final approach, they could land in shorter fields.

One might think that when aircraft manufacturers started enclosing cockpits and adding a nose gear and landing flaps, the slip would become obsolete, but it didn't. Student pilots are still required to demonstrate slip proficiency on their private pilot practical test. Why? Because the FAA wants pilots to be masters of stick and rudder, not just automation. Of course, most aircraft now have additional lift devices that help with glidepath control, but slipping still has practical applications.

Slips generally fall into three categories: turning, forward, and side, but I'll add one more called the power-on slip. The extent of thrust and cross-controlling determines the slip category.

By definition, a slip is the result of intentionally cross-controlling the rudder and ailerons (left rudder and right aileron, for example). These inputs make the aircraft fly with one wing down along its longitudinal axis. Remember that a slip is an intentional flight maneuver, not to be confused with poor yaw control. Also bear in mind that anything other than coordinated flight will make your occupants slide sideways in their seats, thus smooth flight control inputs are necessary to keep them comfortable. Some aircraft are prohibited from slipping in certain flap configurations, because the flaps may block the airflow over the elevator. Be sure to check your flight manual before attempting any slipping maneuvers.

The turning slip is rarely used these days, but it still has practical applications. The purpose of a turning slip is to lose altitude in a turn. For tailwheel aircraft not equipped with landing flaps, the turning slip allows a higher descent rate while pushing the nose out of the way for better visibility. In dire situations for any aircraft, a turning slip may help position the aircraft for an emergency landing, keeping aircraft limitations in mind. The turning slip is performed power off with the application of aileron in the direction of the turn and opposite rudder to hold the slip. Any slip increases the drag, thus an appropriate amount of forward pressure is required to maintain a safe approach airspeed. The turning slip is controlled by aileron and rudder inputs. Decreasing rudder pressure will return the aircraft to coordinated turning flight, thus canceling the slip. Decreasing aileron will transition the aircraft into a forward slip, keeping one wing down and the nose to one side. Transitions from any slip must be accomplished smoothly. Always maintain sufficient airspeed throughout any slipping maneuver.

The forward slip is performed just like the turning slip, but the heading remains constant. Its purpose is to lose altitude without a corresponding increase in airspeed, which would be the case if you simply pushed forward on the yoke.

The sideslip is commonly used in crosswind landings. Here, the rudder is used to maintain centerline heading while the ailerons are used to maintain the centerline track and keep the upwind wing low. Larger aircraft require earlier transitions because of their higher approach speed and greater mass. In some cases, a slight power adjustment is needed in a sideslip, to compensate for the increase in drag. Again, sideslips take practice, but mastering them will ensure consistent crosswind landings.

Parachute jump pilots and glider tow pilots use power-on slips to prevent shock cooling the engine. In this case, the pilots leave their power in mid-range, then add aileron and opposite rudder to induce a slip. The higher the power setting, the more bank angle is required to descend the aircraft. Of course, the steeper the bank, the faster the airplane falls from the sky, which is the intent of pilots who are paid by the sortie. Their goal is to land quickly while gradually cooling the cylinder head temperatures. As they near the traffic pattern, they reduce power and smoothly transition back to coordinated flight.

Now let's consider another application. Some faster aircraft have a tendency to aerodynamically yaw, or Dutch roll. While many of these aircraft are equipped with yaw dampers, their minimum equipment list may permit dispatching with the yaw damper inoperative. In such cases, it might be good to know how to stop a wagging tail. To do this, simultaneously ease in a slight amount of rudder and opposite aileron, hold until the yawing stops, and then smoothly neutralize the controls. If done properly, your passengers will never notice. Reducing the cruise speed may prevent frequent recurrence.

Every flight instructor knows that slipping still works today, minding any aircraft restrictions. If you haven't explained the nuances of this valuable maneuver to your clients, it's worth taking the time in the briefing as well as in the air. By understanding the slip's benefits, your client is more likely to learn and practice the maneuver.

While all slipping maneuvers require finesse, they are easily mastered. The key to successful piloting is to make smooth control inputs while maintaining a light grip. Don't be mechanical. Feel the aircraft and listen to the air flowing over its surface. If your aircraft manual permits slipping the aircraft with the flaps extended, then practice slipping approaches when the opportunity permits.

Mark W. Danielson is a retired Navy pilot who currently flies for FedEx. He has been a CFI for 26 years and has flown more than 11,000 hours.

By Mark Danielson

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