January 2009Features

Slip Solution

Forward slips made easy

The forward slip is an important skill that every pilot should be prepared to use as an effective way to lose excess altitude in a hurry. Whether you simply find yourself too high on an approach, or it's an emergency where it is imperative to get the airplane on the ground as soon as possible, a forward slip might be your best solution.

Photography by Mike Fizer
Reduce power
Forward slips should be thought of as a last resort to lose altitude on final approach. The intent of the maneuver is to lose excess altitude without gaining airspeed, so the procedure is much the same in every airplane. The first step is to reduce throttle to idle (right). Not doing so would be wasting the energy in the slip.
Lower flaps
Next, if your airplane allows, fully lower the flaps (above). Full flaps increase drag and allow steeper approaches without increasing airspeed.
Full, deflected rudder
What it looks like
The key to a proper slip is full rudder deflection (far left). This is what swings the nose to the side, causing the unusual sight picture (left).
How to mess up a perfectly good slip

Here is a list of common errors that occur most frequently during checkrides.

Forward slip ineffective because of:

  1. Failure to reduce power to idle.
  2. Failure to add or increase flaps when available.
  3. Failure to apply and maintain full rudder deflection.
  4. Failure to use appropriate rudder/aileron combination during a crosswind situation.
  5. Failure to reduce pitch to maintain proper safe gliding airspeed.
Simply stated, a forward slip is a maneuver allowing the aircraft to lose the most altitude in the shortest amount of time without gaining excessive airspeed. This is accomplished by reducing thrust to a minimum while increasing available drag to the maximum. Before flaps were typically found on light aircraft, forward slips were treated as a flap substitute, and they were a much more commonly used tool during normal approaches. But now that we have flaps installed on most training aircraft, forward slips seem to have become a lost art.

Clearly, a properly planned and executed approach should not require the use of a forward slip to lose excess altitude. But if you do find yourself higher than desired, for whatever reason, your first corrective action should be to reduce power, to idle if necessary. If you're still too high, you should increase drag by selecting full flaps--and, if applicable, lower the landing gear and set the prop control to full forward (high rpm). Still too high? How about doing some S-turns, traffic permitting? (If you're on approach to a tower-controlled airport, get the controller's permission before doing S-turns.)

If all these corrective measures still leave you too high, aside from the obvious go-around option, a forward slip could be your last best option in salvaging the approach. All too often, however, an applicant on a checkride will attempt to perform a forward slip without first reducing power or adding flaps, which of course produces a negligible increase in the rate of descent.

If your aircraft has flaps, combining their use with a forward slip will increase your rate of descent even more because of the increased drag provided by the flaps. And since the objective of a forward slip is to quickly lose as much altitude as possible, flaps can and should be used in combination with a forward slip unless otherwise prohibited in the pilot's operating handbook (POH). This, for example, was the case for some of the earlier Cessna training airplanes with flaps that could be extended to 40 degrees. In these aircraft, when full flaps were used in combination with a forward slip, an airflow disturbance across the horizontal stabilizer and elevator could result in a dangerous loss of pitch control during the approach. The POH stated, "Slips with full flaps prohibited."

In later models the maximum flap travel was limited to 30 degrees and this restriction was removed. (The POH for the 1986 Cessna 172P still says to "avoid slips with flaps extended.") However, even though slips with full flaps are no longer prohibited, you may still see a caution in your POH to avoid slips with full flaps for a variety of reasons, including the possibility of airframe buffeting or pitch oscillations. Some manufacturers include a caution to avoid prolonged slips with low fuel to prevent fuel ports from becoming uncovered. These important factors should be considered when deciding whether to use flaps in combination with forward slips or whether to use a forward slip in a particular situation.

In addition, there often seems to be confusion over the amount of control deflection needed and which rudder/aileron combination to use. To help answer this question, it helps to first review how we compensate for crosswinds on approach and consider the difference between a forward and a sideslip (see "Crosswind Tutorial," April 2006 AOPA Flight Training). When approaching to land on a crosswind runway, a pilot would normally begin the final approach using a crab to compensate for the crosswind. In a crab, the airplane is simply flying in coordinated flight on a heading that allows the ground track to align with the runway centerline. The singular difference between the airplane's heading, or longitudinal axis, and the ground track is the wind correction angle for those crosswind conditions.

At some point during the final approach, a transition from crab to sideslip for the landing flare and touchdown should be made. For a sideslip the pilot applies only as much rudder as is necessary to align the longitudinal axis with the runway centerline, and also applies just enough aileron to continue tracking the runway centerline. That is, control your ground track left or right of centerline (drift) using aileron inputs while varying rudder inputs as necessary to keep the aircraft nose lined up with (parallel to) the runway centerline.

With this understanding, it might be helpful to think of a forward slip simply as an exaggerated sideslip during a crosswind landing. Instead of using only partial and varying amounts of rudder inputs as in the sideslip, with a forward slip maneuver the rudder application should be fixed and steady at full deflection until the forward slip is no longer necessary.

If you are doing the forward slip correctly, do not be concerned when you see the nose of the airplane pointing toward the downwind side of the runway instead of aligned with the runway centerline. Apply aileron inputs as necessary to maintain the desired ground track just as during a normal crosswind landing. However, with full rudder deflection, you will need additional aileron inputs to counteract the increased yaw caused by the full rudder deflection. This is normal and desirable.

By fully deflecting the rudder, you will maximize its effectiveness in producing additional drag. Since the forward slip's primary objective is to maximize altitude loss, a direct indication of how well you're doing can be read directly from the vertical speed indicator (VSI). Similar to a crosswind approach, you should apply aileron deflections during the forward slip. Remember, the greater the control deflections (rudder, ailerons, and flaps), the greater the drag and the higher your rate of descent.

Regarding airspeed during a forward slip, rudder effectiveness increases with airspeed, so maintaining a higher airspeed will also require a greater aileron deflection. The result will be increased total drag. This all translates to a higher rate of descent and therefore a more effective forward slip maneuver. Remember, the greater the bank angle you can sustain during a forward slip, the more drag you are creating, which increases forward slip effectiveness.

Be aware that during the forward slip, depending on the location of the static port, airspeed indications may vary considerably. This is caused by static pressure inaccuracies caused by the relative wind striking the side of the airplane, thereby causing your static port to act like a mini pitot tube. For example, if your static port is located on the left side of the fuselage, a slip using right rudder will cause the perceived static pressure to be higher than actual as ram air is forced into the static port, resulting in your indicated airspeed being less than actual. Therefore, it would normally be advisable to maintain an airspeed comfortably within the middle range of the white arc (flap operating range) to avoid being either too close to a cross-control stall or a flap overspeed condition.

Keep in mind that if you're really, really high on final approach, even during a full forward slip, it is still possible to maneuver the airplane in S-turns to give you more time to lose excess altitude. Even though rudder deflection will remain at its full travel limit, ailerons and elevator will still be available to maneuver the airplane for S-turning as necessary. This added capability might be useful during an emergency descent.

If it's been a while since you practiced and you're too nervous or embarrassed to try forward slips in the traffic pattern, don't worry. You can review this maneuver in the practice area. Begin by fully configuring your airplane as you would on final approach (with landing gear down and prop lever full forward, if so equipped). Adjust power to maintain level flight and normal approach speed at around 3,000 feet above ground level while lining up with a road or railroad track to simulate a runway.

When you're ready to start down, simply reduce power smoothly to idle while simultaneously reducing your pitch attitude to control and stabilize your airspeed safely within the white arc on the airspeed indicator. Then roll aileron toward the upwind side of your ground reference line to maintain the desired ground track as you apply and hold full opposite (downwind) rudder. Plan to descend at least 500 to 1,000 feet before simultaneously leveling the wings and releasing rudder pressure while readjusting pitch to the normal gliding attitude. Then try one in the opposite direction exactly as above for the next 500 to 1,000 feet of descent before leveling off and returning to straight-and-level cruise flight.

At first, accomplishing forward slips might seem uncomfortable, awkward, or even unnatural, but with a little practice, you'll be able to keep this important tool in your bag of pilot skills and techniques. And being comfortable and proficient with forward slips will make this task a breeze on your checkride.

Bob Schmelzer is a Chicago-based United Airlines Boeing 777 captain, and a designated pilot examiner. He has been a flight instructor since 1972.