Low Suction

[RNAVapproach]

Had a navigation flight planned today but discontinued the flight due to a very low suction reading. The needle had not moved since the start up and there was a hissing sound. Obviously perhaps a small leak but just wanted to see if others would have done the same? It was a CAVOK day, no IFR, just a VFR nav.

The suction is a particular problem with this a/c and have been told since that it usually comes up during run up/ during flight.

Safe call or Over cautious? I will let the jury decide?

RNAV

[IO540]

The problem with ignoring this is that the reason for the lack of vacuum could be a sheared vac pump drive shaft, which if it affects the accessory drive on the back of the engine, is going to cause an instant and total engine failure.

Very unlikely, and the vac pump shaft has a plastic section which is meant to shred if the vac pump jams, but I would not play with this.

Much more likely a hose has come off, but you need to be sure (which should take all of a few minutes).

[welliewanger]

Good call. If there is any doubt, there is no doubt!

If you're doing the kind of flying which requires suction instruments (for VFR that's only really the DI) then you'd better make sure that they work. A DI will appear to work with low suction, however it will precess more easily causing you to wander off heading. It creeps up on you and you don't notice it until it's too late. If you're instrument flying it can get even more tricky and because people are taught to trust the instruments it's easy to follow that slowly toppling AI - trust me, I've done it!

However (there's always a caveat!) it gives you an opportunity to learn more and progress. Now that you know you can't trust the DI, how about trying the nav flight only using the compass. It's perfectly possible and has so many opportunities to learn more. For example:
- Compass errors (during climbing, descending, accelerating decelerating and turning)
- It's in an awkward place so you can't look at it so often. This is actually a blessing in disguise. Once you're on heading, can you maintain it without looking at the compass / DI? It should be easy. Just find a spot in the distance that's lined up with the rivets on the nose of the aircraft and fly to it. That'll increase your lookout, reduce your workload and make it all much more fun.

Now that you've had an opportunity to think about it, discuss it with your instructor and try a nav flight with the DI covered. It'll take you out of your comfort zone, but that's the way we learn.

Enjoy

[IO540]

If there is any doubt, there is no doubt!I wouldn't simply say that, because one might be simply ignorant :)

The point I was hoping to make is that one needs to establish the full extent of the problem, before saying it doesn't matter.

Another example might be a non-working instrument panel lamp. You change the bulb but it still doesn't work. Most people would not worry about it. Only a bulb... But what if the cause is a damaged wire harness, rubbing against a metal bulkhead, with a much more vital wire about to short to the airframe, and .... there is a fuel leak nearby?

discuss it with your instructor

Sadly, IME, instructors often do ignore problems like this.