Flaps up and flaps not up

[Danial9874]

Flap up switch control rate of stab trim.When flaps are up low speed trim engageWhen flaps are not up high speed trim engage What is the different between "Flaps up" and "flaps not up" ? Flaps up = all flap retracted ?Flaps not up = flap in extend position ? Thanks

[+TSRA]

For the 737 the way the FCOM words things is a little more clear: "The main electric and autopilot stabilizer trim have two speed modes: high speed with flaps extended and low speed with flaps retracted."

So, at flaps at 0, you get low speed trim. With the flaps at anything other than 0, you get high speed trim. In other airplanes you'll see the phrases you've described "flaps up" and "flaps not up". Read that exactly as written. "Flaps up" is 0. "Flaps not up" is any position that is not 0. The best time to see this at my operator is in the after landing flow. The flow has you reset the stab trim to 5 units, and we do this as the flaps are coming up to take advantage of the high speed trim.

But, if you wait to reset the stab trim until the flaps have moved to something small, say 5 degrees, you can see the transition from high speed to low speed happen as soon as the flaps stop at the 0 position. The problem is it's now low speed and if you're not careful, you could miss something else - like starting the APU and putting it on the busses, and then the Captain doesn't check for "two blue" before turning the engines off, or he did but because we had the lights dimmed, he didn't see and the plane swaps to battery power, making the cabin go dark. At night. In the rain. Ask me how I know. So, yeah. High speed trim. Great thing.