Idle check during run-ups
Like everybody else, I am a good boy and do my engine run-up checks, including a check that the engine idles at about the right speed.
If I'm perfectly honest, I've never given enormous thought as to exactly why this was important, until last week.
My job, as most of you probably know, involves testing (and analysing the testing) of small piston-prop aeroplanes. Well, last week we were doing some take-off and landing performance tests of a known type, fitted with an engine we've not seen on that type before. It happened that this aircraft has a slightly disfunctional throttle mechanism, and as a result it was idling at about 30% higher revs than would be normal. This wasn't enough to make the aircraft roll forward with the brakes off, but was about enough to keep it moving once started if you didn't apply the brakes.
No big deal? We didn't think so, until after a day's flight testing I went off and analysed the flight test results.
One thing it did was reduce the stall speed by about 3 knots, wihch is arguably nice to have, but again no big deal.
But, what this bit of residual thrust did was increase the amount of float on landing quite substantially, and also reduce the braking efficiency a bit (since the brakes now had more to work against).
The upshot was an increase in landing distance of roughly 60%: considerably more than any safety factor you'd normally apply to landing distances.
So, somebody has to redesign a throttle lever, I'll have more testing and analysis to do.... and I come away from this with significantly enhanced appreciation of the importance of idle RPM being within limits on my run-ups.
Just thought I'd share that.
G