i was wondering if anyone could tell me what, if any effect an engine failure would have on vacuum driven instruments. having never had an engine failure i dont really know how well the prop continues to spin with no power. so even at low airspeeds does the prop continue to windmill aat a sufficiant speed to drive the vacuum pump? or are the instruments considered to be unrelaible? would it be the same for fixeed and variable pitch props?

Good point, I've never really thought about it, but you've got me thinking. I imagine that it depends on why the engine fails - catastrophic major component failure, like a crank / complete seizure then I guess not.

The props will normally spin fairly fast with no power, in fact if you 'fail' and engine in a twin, by switching off the fuel, then its difficult to actually see which engine has failed by looking at the prop. Of course when you feather it, it becomes obvious, but to restart you normally put the prop forward, give it a bit of a kick with the starter then the windmilling does the rest, so I assume it windmills pretty near or above idling speed.

EA

And I suppose the next question would be how long the gyros will continue to spin (at a speed which will enable the instruments to be useful) after the engine quits, if we take the worst case of a catastrophic failure which prevents the engine from windmilling? Or should you immediately assume they are unreliable, and switch to partial panel techniques (assuming we're in IMC)?

FFF
---------------

In my book if the engine fails the engine driven vacuum pump fails and the vacuum driven AI stops. I would not rely on it in IMC with a failed engine no matter how fast the prop was spinning.

Solution is partial panel (you are in current practice aren't you?) or electrically driven AI as back up.

Should one not have a vac gauge and a gyro warning light?

thanks for the answers. i dont have an IMC rating in fact i'm still trying to get my PPL. all my flying to date has been done in an aircraft with electric gyros so vacuum systems isnt something i have really covered (outwith the tech exam). the question was raised by a project i am currently doing at college.

good point about the vacuum gauge. but would it be common for that to remain within working limits on a windmilling prop?

AndyB79,

Vacuum-powered instruments are often driven by a vacuum generated by an external venturi on older aircraft. This is obviously completely independent of the engine and won't fail - if the airspeed decays to the point that it doesn't work then you've either landed or crashed :E

SD

The answer is "YES" you will lose all vacuum driven instruments. Just look at the Vacuum gauge on idle or even slightly higher power settings!!

Having had a Vacuum Pump failure, in IMC, on an ILS it isnt funny. In fact, had we not been near cloud base, I dread to think of the potential outcome. For what its worth I have an ATPL!

I will now never, normally, fly a single in IMC because, when you have your engine failure, you will lose your Attitude Indicator your Directional Indicator and possibly your de icing equipment. It is rare for singles to have 2 Vacuum Systems.

If you survive all this you will then be faced with a forced landing with, possibly, an exteremly low cloud base (otherwise why not stay below?).

Why do you think God made aeroplanes with 2 engines, 2 altimeters,2 Vacuum systems, 2 Generators, 2 VORs, 2 Radios and even 2 pilots?

I really do think that people who fly singles in serious IMC are stretching their luck sometimes.

UV

I would have thought that ATPL (IR) training would include partial panel flight, using the electric TC. Also normally an aircraft used for real IFR would have a second (electric) horizon.

10540

Partial Panel is tested on Licence renewal but usually only on some of the smaller types. It is limited to basic exercises and does not include any approaches.

Trials in simulators, carried out in the USA, giving unexpected Vacuum Pump failures to experienced, but unsuspecting crews, frequently resulted in disaster.

There have been a sucession of accidents, here in the UK, following Instrument failures (for whatever reason). The latest and most notable, of course, being the Beoing 747 at Stansted.

In my opinion an instrument failure in IMC (for whatever reason) is an extremely serious event.

Quite a few aircraft cleared for "real IFR" as you put it do not have a second Artificial Horizon of any description, even though they may have two engines! In this case they will, of course, have two Vacuum Pumps, but what about the singles??

UV

Just look at the Vacuum gauge on idle or even slightly higher power settings!!
I thought the regs for IFR flight say that the instruments (AI) must erect within 5 mins at idle (ie. while taxying).
Why do you think God made aeroplanes with 2 engines, 2 altimeters,2 Vacuum systems, 2 Generators, 2 VORs, 2 Radios and even 2 pilots?
To double the chance of an accident :D Seriously though, many "new" A/C like newish 172's have two vac pumps, but i agree its lack of PP training that kills people. I think anyone in their right mind who flies singles in IMC should practive PP stuff, if not then its their arse on the line.

I was doing PP stuff in a Seminole, and as we started transitioning the clouds while being vectored to the ILS my instructor "failed" an engine. So now, in IMC, partial panel, trying to identify, verify and "feather" the bad engine, with only PP instruments was "fun". Still, I learned a lot, it proves it can be done.

One other point, I saw a vac pump failure on a seneca, which casued the whole system to fail for some reason.....Don't know why, but its worth remembering that it can happen.

Cheers
EA

Cyer

My preferred solution is to have an electric AI as backup. However, probably the most dangrous thing of all is the time it takes to recognise that you have a vacuum failure. If this is too long you are probably in an unusual attitude which you are going to have to get out of partial panel or using the backup AI all in IMC. (A VMC vacuum failure should be a non event)

There is no doubt that an IMC vacuum failure without backup is a full on emergency ( and could be one even with backup).

In simple single engine aircraft with one vacuum pump the problem is not too bad.
At most cruise altitudes you will only have a 5 or 10 minutes before you hit/land on the ground anyway.

From my ATPL studies I remember that the gyro's should remain usable for 30 minutes in commercial operations.

I had a simulated gyro failure after take-off followed by airways leg, followed by ILS approach all partial panel. It shows that it can be done but it wasn't easy.

As UV has indicated, It is the gradual failing of the gyro instruments that I don't know how I would handle in real life.
I would like to at least be tested in a sim to see how long it takes me to get the aircraft inverted :}

FIS.

SSD

The venturi is indeed independent of the engine but not independent of airspeed! We have a vac gauge and it drops out of the green at anything less than normal cruise. It's also temperature dependent, you get less vacuum on a hot day than you do on a cold one.

Reading this thread stirred a memory. I used to share a Fairey Swordsman powerboat with a friend. When we rebuilt the panel we put two dirty great red lights on it. The idea was to fit a tee piece into the oil pressure gauge take-off on each engine and plumb in a pressure switch. A dirty great red light is more attention grabbing than a drop in pressure on a gauge.

I sold my share to my partner before this was completed. The inevitable happened. A year or two later he had still not finished it, an oil hose burst and an engine seized because he hadn't spotted the gauge dropping to zero.

We have low voltage warning lights but we still don't seem to have progressed to providing them for loss of oil pressure (or vaccum for that matter). And if you'd like to fit that simple safety device costing but a few pounds to your aircraft sir that will be loadsamoney please.

Mike

Hi, Mike.

Thanks for that - I thought that once you were at flying speed, not just cruise speed, that it was operational.

How does that work in a climb, then? Or do you not rely on your DI / AI except in straight and level?

:confused:

SD

Sorrry SD, referred to you as SSD.

We are a strictly VFR only Luscombe on a Permit. We have two of the large venturii, one each side just forward of the doors. These feed into a regulator that taps off 2 inches of Hg for the turn co-ordinator. The remainder of the vacuum is fed unregulated to the DI and AI (no electrical backup instruments).

The instruments won't really get up to speed until in level flight so we set them after climbing to cruising height. After that further climbing would reduce the vacuum but I suspect they would continue to function for some time at reduced pressure, certainly I've never noticed any abnormal precession.

Performance will obviously vary on different aircraft and with different instrument fits. I suspect that ours are not positioned in the best place. They're far enough back to possibly have some turbulent airflow off the cowl and also will suffer from blanking if the ball is off-centre. I've seen other installations where they are mounted side by side on the belly where they will get a cleaner airflow.

I understand a major problem with venturi fed instruments was their susceptibility to failure in icing conditions (just when you need them most).

Mike