Why do pilots set takeoff flaps before taxiing?

 

Why do pilots set takeoff flaps before taxiing?

Well…two reasons.

One, as the airline pilots already on this thread have pointed out, is they set takeoff flaps as soon as the plane is free of the terminal building because it’s in the pre-taxi checklist, and the reason it’s there is because too many planes - one is too many, but there were quite a few - crashed and killed everyone aboard because they tried to take off at flap-set speed with the flaps stowed. You CAN get a plane off the ground successfully with no flaps but you’ve got to be going far faster than with them deployed - the Boeing 737’s no-flaps takeoff speed is 70 knots higher than its with-flaps speed.

The other is to be able to quickly go back to the terminal and let everyone off the plane if they don’t work. Flaps on a large jet are generally extended and retracted with hydraulic pressure, which means there are quite a few things between the flap handle and the flap surface like pumps, valves and plumbing. In other words, there are a lot of reasons they won’t deploy. If you get all the way out to the runway and THEN discover that your flaps won’t extend, you’ve got to drive all the way back to the terminal - and in some places like Denver and Dallas/Fort Worth, that can be a VERY long drive. It isn’t good customer service to get everyone all excited about leaving then have to tell them, “sorry, the plane isn’t fit to fly, we have to go back to the terminal” when you’re already on the runway.