Some aircraft you cannot use the speedbrakes/spoilers with any flaps extended. Such as the 727. The primary reason was due to the sink rate of the 727 with flaps 30, let alone flaps 40, that would develop with the combination. There is no structural reason.
And yes, I did on a few occasions use the spoilers with flaps 30 in 727, as long as you remained aware of the sink rate and reacted according you did not have any problems.
One thing we used to do in the sim during recurrency training was after we had finished all the requirements, the instructor would place us at the outer marker at 10,000, 250kts at LAX and tell us to see if we could land without changing headings. You can do it, believe it or not.
First put the gear down while extending the spoilers, reverse all three engines and then start putting the flaps down on speed. You end up with a hell of deck angle, trust me. The closest to the end of the runway I ever landed was about 2,000 feet past the end of the runway.
Now, there is no way on earth I'd try that in a real 727. You can really trust me on this.
Oh, one other reason that one would have the speedbrake/spoiler extended with flaps down in other aircraft than the 727 would be to keep the engines spooled up for adequate bleed air flow for anti-icing in icing conditions.
Last edited by con-pilot; 25th August 2010 at 19:33.