8.forward-and-side-slips. Forward Slips and Side Slips
A slip is a controlled flight maneuver in which the airplane is intentionally flown in an uncoordinated condition — the longitudinal axis is no longer aligned with the relative wind. Slips are produced by deliberately cross-controlling: applying aileron in one direction and opposite rudder. Two distinct slips serve two distinct purposes during approaches and landings: the forward slip for losing altitude without gaining airspeed, and the side slip for crosswind landings.
The Forward Slip
The forward slip is used when an airplane is too high on final and the pilot needs to dissipate altitude rapidly without increasing airspeed. It is especially useful in airplanes without flaps, when flaps have failed, or when a steeper-than-normal descent is needed to clear an obstacle. By presenting the side of the fuselage to the relative wind, the slip dramatically increases parasite drag, steepening the glide path while airspeed remains controlled.
Procedure to enter a forward slip on final approach:
- Reduce power to idle (typically already at idle in a power-off approach).
- Lower a wing with aileron — the slip is usually made into the wind so the lowered wing keeps the airplane tracking the runway centerline.
- Simultaneously apply opposite rudder to yaw the nose away from the direction of bank.
- Adjust pitch to maintain a safe airspeed — the slip will produce a nose-low attitude relative to the apparent horizon, but the airspeed indicator may be unreliable in some airplanes due to disturbed airflow over the static port.
- The ground track remains aligned with the original flight path, but the longitudinal axis is angled away from it.
To recover, simultaneously release rudder pressure and level the wings with coordinated aileron and rudder, returning to coordinated flight before the flare.
Key cautions:
- Some airplanes prohibit or limit slips with full flaps extended due to possible elevator blanketing or pitch oscillation. Always consult the POH/AFM. The Cessna 172, for example, contains a placard cautioning against slips with flaps fully extended.
- Airspeed indications can be unreliable due to skewed pitot-static airflow.
- The slip must be discontinued in time to establish a stabilized, coordinated condition before the flare and touchdown.
The Side Slip
The side slip is the standard technique — along with the crab method — for landing in a crosswind. Unlike the forward slip, the airplane's longitudinal axis remains parallel to the runway centerline, while the ground track is maintained directly down the centerline by banking into the wind.
Procedure for the wing-low (side slip) crosswind landing:
- On final, lower the upwind wing with aileron just enough to stop the airplane's drift toward the downwind side.
- Apply opposite (downwind) rudder to keep the nose aligned with the runway centerline.
- Adjust the bank angle continuously — more wind requires more bank, less wind requires less.
- Hold this cross-controlled condition through the flare and touchdown. The upwind main wheel touches first, followed by the downwind main, and finally the nosewheel.
- After touchdown, progressively increase aileron deflection into the wind to keep the upwind wing from rising.
If the bank required to stop drift exceeds what produces an acceptable margin from a wingtip or propeller strike, the crosswind component exceeds the airplane's capability and a go-around to a more favorable runway is warranted.
Comparing the Two
| Element | Forward Slip | Side Slip |
|---|---|---|
| Purpose | Lose altitude without gaining airspeed | Counter crosswind drift on landing |
| Longitudinal axis | Angled to flight path | Parallel to runway |
| Ground track | Same as original (aligned with runway) | Aligned with runway centerline |
| Typical use | Power-off approach, obstacle clearance | Final approach and touchdown in crosswind |
Both maneuvers are mechanically the same — aileron one way, opposite rudder — but the geometry relative to the desired flight path differs. In the forward slip, the longitudinal axis is yawed off the flight path; in the side slip, the flight path itself is bent so the airplane moves through the air at an angle while the nose stays aligned with the runway.
Common Errors
- Insufficient rudder, allowing the bank to turn the airplane rather than slip it.
- Excessive rudder beyond what the available aileron can balance, leading to a heading change.
- Failing to compensate for unreliable airspeed by referencing pitch attitude.
- Holding the slip too long on a forward slip and entering the flare uncoordinated.
- In the side slip, allowing the upwind wing to rise after touchdown.
- Slipping with full flaps in airplanes that prohibit it.
Proficiency in slips is required for the private pilot practical test and remains a fundamental piloting skill — particularly valuable when an engine failure forces a power-off approach to a confined field.