Ask the Captain: Is it dangerous when flaps malfunction?

— -- Question: On a recent flight from IAD to LAX, our flight attendant announced that the plane's flaps were not functioning and asked the passengers to assume an emergency position for landing. The plane then landed without incident -- but surprisingly, it was the gentlest, smoothest landing I may have ever experienced. It felt like landing on pudding. Why was that? Did you ever have to do a flaps-free landing? And how dangerous is it to land without flaps?

— Submitted by Veronica Gould Stoddart, USA TODAY Travel

Answer: Yes, I have had flaps fail to operate and landed with them retracted. The landing airspeed is higher without the flaps. The amount of additional airspeed varies with the type of airplane and the weight. Pilots practice zero-flap landings in the simulator and have checklists for this abnormal condition. The higher airspeed results in a longer landing roll, precluding the use of a short runway. The checklists indicate the exact amount of additional runway needed. Once the pilots calculate the required runway, they can determine if proceeding to an alternate airport is required.

In one example, we were landing in Dayton, Ohio, when the flaps failed. We briefed the flight attendants and passengers that we would be landing at a higher speed and that the emergency vehicles would be following us as a precaution. After completing the required checklists and calculations, we landed without incident.

I would certainly not characterize it as dangerous. While it is unusual, this type of landing is one of many that pilots train for.

As for the landing being smooth, the position of the flaps does not make a smooth landing more or less likely. The touchdown firmness is more attributable to the ability of the pilot in slowing the descent rate at the right time. The Dayton landing mentioned above was a smooth one.

Q: I have always enjoyed your column and read it with interest. I was wondering why the flaps are set differently on takeoff than on landing. Also what do the flaps on the wing that pop up do? I have seen them come up a little ways as we were making our final approach to the airport.

— submitted by reader spedteacher

A: Good question. The flaps for takeoff are set to produce extra lift while not adding a lot of drag. During landing, the flaps are extended much farther, which allows a lower airspeed for approach and landing, but the drag is much greater.

Determining the proper flaps for takeoff and landing also depends on the runway length, airport elevation and weight of the airplane.

Here are a couple of examples of how take-off flap settings could vary. For a Boeing 737 at a high-altitude airport with a long runway and heavily loaded, a minimum flap setting would be used (depending on the model either flaps "1" or "5"). The airplane accelerates on the ground to a higher speed (using more runway) to an airspeed that allows it to climb if one engine fails. On a short runway this is not possible, and a flap setting of "15" is used. The extra flaps produce lift at lower speed, allowing liftoff to occur earlier. However, it may not be possible to carry as much weight (passengers, fuel, and/or cargo) while still being able to climb in the event of an engine failure.