Subscriber question:
"Is slipping an airplane with full flaps allowed or not? I fly a club Cessna 172 and one instructor told me it's fine while the other said it's forbidden." —Kyle S.
Jeff:
“I have to answer this with a question: Which model Cessna 172 are you flying?
The model matters because the answer is found in your POH and the guidance has changed over the years. In the POH for a 1956 Cessna 172 you’ll see, ‘Slips are prohibited in full flap approaches because of a downward pitch encountered under certain combinations of airspeed and sideslip angle.’ By 1977 the POH for the 172N model said, ‘Steep slips should be avoided with flap settings greater than 20° due to a slight tendency for the elevator to oscillate under certain combination of airspeed, sideslip angle, and center of gravity loadings.’ Check the POH for a late-model 172S and you’ll likely see, ‘Steep slips with flap settings greater than 20° can cause a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings.’
So that’s a prohibition in 1956, a strong caution in 1977, and an FYI in the 2000s. Over that time there were aerodynamic changes, including limiting the maximum flap travel from 40° to 30°.
Three important takeaways apply no matter what make or model aircraft you fly. One is that your POH rules and it’s the first place you should go for prohibitions on your specific aircraft. That includes any updates, ADs, or other changes that should be included since the POH was published.
Second is that these textual descriptions are usually found in the Amplified Procedures, which you’ll find in your flight manual for both normal and emergency procedures (usually after the checklists). Too many pilots breeze through this text or skip it entirely. Don’t be one of them. The amplified procedures often contain the details you need to make informed decisions on applying those simple checklists in the real world.
Third is that procedures evolve but ‘tribal knowledge’ doesn’t always keep up. When people tell you something must be done one way or another, ask for a source you can reference yourself.
You’d be surprised what the kinds of things you might find out.”
Do you feel OK about slipping your airplane with full flaps?
