Originally Posted by
rkgpilot
Sorry to be a bit dumb, but a couple of questions:
In your simple fixed pitch prop trainer/tourer, the only way to know the engine is making full power at the start of the takeoff roll, is to know what the minimum staic RPM limit is. Any RPM below this value at the start of the takeoff run should be grounds for an immediate reject. Contrary to the assertions of one PPL poster here, it is very difficult to "feel" a underperforming engine, but the static RPM check will give you hard information.
What exactly does 'static RPM check' mean?Or 'minimum static RPM limit'? How do you do this check? Is it something I'm already doing, but under a different name?
Magneto Check: When checking the mags the difference in the mag drop between the the two mags is just as important as the maximum drop noted for each mag. A large difference in mag drop between the two mags indicates a problem with the ignition timing and should be investigated before further flight. The POH should have the limits for both maximum allowable drop and max difference allowed, which will always be considerably less (only 50 to 75 RPM). Similarly I have met many pilots that did not realise that no mag drop is actually bad as it indicates a fault of the magneto grounding circuit or mag switch and again is a reason to abort the flight
I do understand this, and always pay attention to this pre-take off. However, what about a mag check/dead cut check on shutting down?
I sometimes forget this and frankly, can't remember the difference between the dead cut and mag checks.
Can somebody please explain this? I know it helps ensure that mags (and therefore props) are not left 'live', but I can't remember quite how this is so.
Thanks!
To achieve the redline maximum RPM on an aircraft fitted with a fixed pitch prop the aircraft has to have significant forward motion (near flying speed). This is because relative airflow is needed to unload the prop. If the aircraft is stopped or only moving at very low speeds the full throttle RPM will be at a value significantly below the redline this is called the maximum static RPM. For the C172M , for example the redline RPM is 2700 but the static RPM range is 2300-2410 RPM (as per the POH normal operations section in the "power check" paragraph. The accptable static RPM will vary by airplane type and even in aircraft of the same model, as some aircraft have more than one pitch of prop approved. The significance of the static RPM is because at the start of the takeoff roll the only way to know if the engine is developing full power is to check for an indication of min static RPM on the RPM guage. I teach my student to slowly go to full throttle over about 3 seconds and as soon as the throttle is full in to check the RPM guage for static RPM and that oil pressure and oil temp are at the correct values. If there is any anomalies than a safe and easy low speed abort can be carried outtemp are at the correct values. I call this the "good engine check".
With respect to the Mag check. The runup check is to check for correct magneto oeration prior to flight. The "dead mag check" is to ensure that when the engine is shut down the mags are not still live. There are 2 ways to do this. One is to turn the mags to tehe off position and lsiten for the engine quiting and then quickly turning the mags back on. The disadvantage is if you are not quick enough the engine will backfire when the mags are switched on. The other method, which I prefer is, is to simply switch to each mag at about 1000 RPM. The drop as each cuts out is obvious and proves the mag is groundign out (ie turned off). The ignition key should also be given a tug as the key should not be removable with the left, right or both mag selected. The only way to remove the key in a properly operating mag switch is when the mags are in the off position.