For the second time in my flying career I had flaps fail Friday (in the same airplane in which they failed about 12 years ago). The flaps failed in the up position when preparing to land… my student went to put the flaps down and nothing happened; checked the circuit breakers.. no dice. So, time to land without flaps. In terms of severity of equipment failures, this really isn’t a big deal. All private pilots are required to demonstrate their ability to deal with this failure. My recounting of the failure, prompted a question about flap failure and slips. Let’s work through the what and why
Why do we use flaps?
Flaps do two things that are helpful:
1. They change the shape of the wing in a way that increases the coefficient of lift; this allows the wing to generate the same lift at slower airspeeds. We should notice this effect by the seeing that the lower end of the white arc (flap range) is lower than the low end of the green arc. Why is that useful? For takeoffs, it can help get us in the air earlier. For landings, it can reduce the speed at touchdown, which can translate to shorter landings and less overall energy in the landing.
2. They add drag. Many of you may now be saying… drag, isn’t that a bad thing? Don’t we work hard to eliminate drag? For much of our flying we do want less drag, but on landing, it can very useful. Without that extra drag, it can be difficult to approach at a steep enough angle to clear obstacles without picking up airspeed and significantly increasing our landing distance.
The best way to really understand this is to try landing without flaps. First, remember that you’ll need to add some speed to your normal approach speed (I add 10 kts on a Cessna 172), since you can’t fly as slow safely without flaps. Now on approach you are usually left with either gaining speed as you try to point the nose toward the ground or approaching at a shallow angle that can be quite dangerous if there are towers or trees before your landing point.
How do we handle landing without flaps without flying a very shallow approach?
A slip (for the purposes of landing) is a maneuver designed to increase drag. From most perspectives, it looks like we are flying (partially) sideways. We intentionally fly uncoordinated, with ailerons used to roll the airplane one direction and a rudder used to angle the nose the other direction.
My guidance is to use full rudder while performing a slip and then use whatever amount of aileron is necessary to keep you aligned with the runway centerline. If there is much of a crosswind, you’ll need to plan to have the wing down from the direction of the wind. If there isn’t any significant cross-wind, I normally plan to keep the bank direction that I used to turn final.
The slip is used to help lose altitude or airspeed, but should come out of the slip prior to actual landing. Be aware that it can be hard to realign to “straight” after the slip.