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

The MCR01 is very light and does not have very good breaks. The extreme landing routine used by a number of owners is to switch the engine off after touchdown, stop the aircraft and restart to clear the runway. One owner who took me for a flight insisted it made a 50% reduction in landing roll!

I hope to run some tests of my own later this year……

Rod1

I was taught my my instructor on my very first lesson that that was the reason for the idle check! Then again, I learned to fly at Netherthorpe where any lengthening of the landing run could lead to an introduction to the "arrester hedge" :O

Another very good reason for the check is to ensure that with zero airspeed the donkey won't actually stop. Unlikely on final with airspeed and a windmilling prop but not unheard of in aerobatics; spin entries, stall turns, tail-slides, etc!

Stik

Me too Waldo :ok: Although I have to say I mentioned this to another instructor (different FTO) during a checkout and he just shook his head and frowned :rolleyes:

Very interesting Genghis . . . Never had it properly explained to me by any of my instructors! Im quite amazed at the figure of 60%!

You learn something new . . .(you know the rest) :ok:

I never had it explained to me, but I just assumed it was to make sure the engine didn't stop...particularly important in something like the C150 with a Continental engine, as I've had it come close to stopping at that point a couple of times. :eek: But I never thought to notice if the engine was idling too fast! Thanks, Genghis, now I know, I'm not likely to hit the hedge if I ever go to Netherthorpe...well, I might, but I won't have that as an excuse. :)

Thanks for that Genghis :ok:

I too believed that the check was done purely to ensure that the engine wouldn't potentially stall on final etc. I didn't really consider the problem of an engine developing higher revs.....................but I will now.

PD

How did you know what the landing distance would have been if the engine you had never seen before was set to idle at the lower speed? Just looking at the 60% figure, I'm sure you are correct in the distance increases. Did you come in slower because of the lower stall speed? How much would that have helped?

Because we'd previously tested the same aircraft type, at the same weight, on a similar surface, with the same test pilot flying at the same speeds (within a knot anyhow) - but a different engine fitted (which used a different throttle mechanism and thus idled properly).

Lower approach speeds is a possibility, but it means a different manual for each engine type, possibly uncomfortable attitudes, and also an aircraft which might behave differently when being landed at idle, and with the engine dead (e.g. deadsticking after an engine failure). For all those reasons, we didn't.

G

Ah, that would account for why my landing run seemed to go on for ever last week. We had just got the aircraft back in the air after 7 months in bits and I was trying to get the tickover speed set right. Whereas I realised that it would obviously make a difference I did not expect that much.

No offense Gengis, but I'm amazed that's the first time you had call to figure that one out :confused:

However, now I'm thinking about throttles ... What do you consider the best way to have a spring return set on a cabled throttle? Set for full on, or full off in the event of a disconnection? I know it's a rare occurance but what if? Both ways have advantages and disadvantages. As an ex glider pilot, I favour off, but a WW1 pilot (not many around I admit) used to a "blip switch" might prefer full on? What do you reckon?

SS

Had a problem similar to this on Tatenhill's Robin, left throttle
had less movement than the right!
Instructor commented that I should be closing the throttle all the way, I said "I am" as we floated down the runway.
When I used the right throttle and flew left handed, all was OK
as idle dropped another 100rpm which made all the difference.

7700

I understand that this has been known to manufacturers for many years.

Then again, if the aircraft is fitted with a Jab...

When I was learning to fly, my instructor told me that if I was ever running out of runway and heading for the hedge I should "cut the mags" because it would make a significant difference to the ground roll.

The thrust from a prop at idle rpm is still significant; try walking behind one!

Worth bearing in mind that most idle checks are carried out with a cold-ish engine, so if it's a bit fast when cold, it'll certainly make for an interesting arrival when it's warmed up after a sortie....:p

Shortstripper wrote
I think that I'd grasped, somewhere amongst my couple of aeronautics degrees and 90 types the importance of closing throttle to land - it was the magnitude of the effect of a slightly high ground idle that took me by surprise, not the fact that it was there. (Also this was a Rotax 912 engine, which has generally pretty low residual thrust due to the stepdown gearbox.)

As to the other question, it's a good debating point, but my money is on a "fail open" throttle. My reasoning is that I can always climb or fly fast with a fully open throttle, then simply kill the engine when safely in glide range of a good landing field. With a "fail closed" throttle you basically have a slightly extended normal glide range and that's it.

G

Optimum idle/minimum RPM with turboprops and jets can also be significant and often more difficult to determine with the types of instrumentation available.

Turboprops running at constant speed often only have a minimum turbine inlet temperature and/or pressure ratio to go by with questionable accuracies at the low end. Then most of any excess thrust roll out problems can often be cancelled out by going into reverse

I've experienced high idle landing distances before and always check to ensure a reasonable throttle closed idle RPM.

Not just on singles either. Asymmetric idle in twins can be a pain in the arse.

To reduce idle speeds (and hence taxi-speeds) we often cut onto one mag.
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Oh, should mention that is on floats - as once the donkey starts up ....... we're moving and there's no going back ..... or stopping ......quickly..... :p

That also explains why that Offspring song (No Brakes) always springs to mind at startup. :cool: