Manifold Pressure Gauge

[WorkingHard]

I have a manifold pressure showing at 33 when engine stopped and also when running will not reduce below 29 (just occured on 3rd take off today). Now apart from a faulty gauge can any one suggest any other reason? Is it possible for the indicator needle to "slip" on the spline for example? All suggestions welcome.

Thanks

[Big Pistons Forever]

The manifold guage will show atmospheric pressure when the engine is stopped. The only way it could show 33 in is if you were 3000 ft below ground level, therefore the guage is clearly broken. At the risk of being rude, I have to ask why did you even start this airplane let alone fly it three times ?

[WorkingHard]

BPF - yes I would surmise the gauge is now faulty but i was asking for any other possible causes. BTW I did say it occured on the 3rd take off which was after we reached about 1000ft and about 20 miles from home base so we continued. Dont think you are rude but may i suggest you read the first post completely

[Big Pistons Forever]

Working hard

Sorry I misunderstood your post because I assumed the guage was reading 33in at start up not shut down. Anyway the guage is history and has probably expired from a perished diaphram or less likely an accumulation of guck inside which has gummed up the mechanism. There is nothing you can do to fix it other than having it overhauled or buying an exchange unit. The good news is exchange overhauled guages are not that expensive.

[Pilot DAR]

With one exception, the manifold pressure gauge will be repairable, at a suitably qualified facility. The exception is an instrument of a vintage of 50 years or so, whose dial and/or pointer are painted with radioactive paint so as to glow. Those instruments are generally refused by instrument shops these days. Be careful even throwing them out, they are considered hazardous material. These are recognizable by pointers and/or graduations and numbers which are not bright white. They may be slightly greeny, or dingy brownish. DO NOT retain such an instrument for the mantle, it is a hazard out of the aircraft. If the markings are bright white, you'll be fine.

As for the problem at hand, yes, the internal mechanizm can gum up and slip. They are nearly always able to be cleaned and made to work well again. There are no non-metallic parts in most of these instruments, so nothing perishes in there, just gums up (small amounts of gasoline do get in there, and leave gunk over time). There is also a tiny adjustable orifice at the inlet to the instrument, which can gum up, though this will cause sticking, not an over indication.

Do not rely on the instrument for any further flying until fixed, very particularly if the aircrft is a turbo. Have a shop remove the instrument and send it for overhaul. While they're in there, the manifold pressure line should be blown out to the engine. Other than that, if the line is in good shape, there's nothing else to worry about.

Pilot DAR

[SNS3Guppy]

Often with a manifold pressure gauge exhibiting this behavior, a little grease has entered the bourdon tube which connects the instrument with the induction manifold. The pressure trapped in the instrument creates a higher than normal reading.

Sometimes mechanics will use teflon tape when sealing the "b" nut on the back of the instrument; this is a common, but poor practice. The tape can work out of the threads on the b nut and cause a blockage. Generally the first step in trouble shooting this is to disconnect the manifold pressure line at both ends, and blow it out using shop air from the instrument-end. If this doesn't correct the problem, then submit the instrument for repair work.

[WorkingHard]

Thanks guys, the repair man has it. It seems likely at this stage it is simply an accumulation of muck that has caused the problem.

[Zulu Alpha]

The inlet to the gauge has a restrictor to dampen the movement to the pulses from the induction.
If you disconnect the pipe at the back you can put a screwdriver into the hole and undo the restrictor screw about 1/8th of a turn. Then reconnect the tube and run the engine and you should find that the problem is fixed. If the needle vibrates then tighten the screw again slightly.

If this doesn't fix it then it will need to be professionally dismantled and remantled together with payment of the normal £x00.



ZA

[Pilot DAR]

screwdriver into the hole and undo the restrictor screw about 1/8th of a turn.

Yes, though my experince has been that there are two forms of this little screw, and the less good type, a very small slotted one, is right down the hole. If you try too hard, and it is gummed up too much, half the head breaks off inside the hole, and the job just got a whole lot more complicated (ask me how I know). The better type has a phillips head screw, with the head visible, and is much more oops resistant. If you are going to venture down the screw hole in search of the first type, some suitable solvent is an important prerequisite!

[WorkingHard]

PPRUNE is an absolute gold mine of information. I am happy to leave my engineer to look after the mechanics of the aircraft as his knowledge is far greater than mine. I thought it interesting about the advice to not fly the aircraft with a faulty gauge. I am not sure I agree in this particular instance as it is only a gauge and if all other indications are correct then continue. My ears and eyes will suffice in place of a faulty gauge at least on a temporary basis but of course that is also dependant on which instrument is ault.

Thanks to one and all for the information.

[WorkingHard]

PPRUNE is an absolute gold mine of information. I am happy to leave my engineer to look after the mechanics of the aircraft as his knowledge is far greater than mine. I thought it interesting about the advice to not fly the aircraft with a faulty gauge. I am not sure I agree in this particular instance as it is only a gauge and if all other indications are correct then continue. My ears and eyes will suffice in place of a faulty gauge at least on a temporary basis but of course that is also dependant on which instrument is at fault.

Thanks to one and all for the information.

[Big Pistons Forever]

Originally Posted by WorkingHard

I thought it interesting about the advice to not fly the aircraft with a faulty gauge. I am not sure I agree in this particular instance as it is only a gauge and if all other indications are correct then continue. My ears and eyes will suffice in place of a faulty gauge at least on a temporary basis but of course that is also dependant on which instrument is at fault.
.

You are the PIC so of course it is your decision but some points to consider

Without a working MP guage you do not have

1) A direct indication that the engine is developing full power on takeoff

2) Indication of the development of carb ice

3) A valid Certificate of Airworthiness as the MP will be a required instrument

4) Valid insurance as a result of point 3

5) abitility to conform to any MP vs RPM restrictions

[SNS3Guppy]

I am not sure I agree in this particular instance as it is only a gauge and if all other indications are correct then continue.

I experienced a complete oil loss in a single engine turboprop a few years ago. The engine continued to sound and feel normal, and all indications, save one, were normal. The only instrument indicating the problem was the torque meter. According to the manufacturer, the engine is capable of running a half hour with no oil. The only problem is that while the engine runs fine, there's no power to the propeller, even at full rated RPM...and consequently no thrust. I ended up on a hillside.

The quick response here is likely that it's irrelevant...after all, it's a turbine, and we're all private pilots here, we don't talk turbines, yada, yada. The example is relevant, however, because it's an example of a situation in which a single instrument...the primary power instrument, was the only thing telling the story.

I've had a power loss in a twin commander while descending in which no change in yaw occurred. The only indication was the manifold pressure gauge. No feel, no sound difference...just a gauge indication.

Don't make the mistake of thinking you can disregard your required instruments in favor of your own eyes and ears. You may not know as much as you think you do, and that ignorance could cost you dearly.

Your aircraft isn't safe, nor legal without the required instrumentation. The airworthiness certificate becomes invalid so long as it's not in compliance. You don't have all the information necessary to safely operate your aircraft.

Get it fixed, then fly. Any other way is irresponsible.

[englishal]

3) A valid Certificate of Airworthiness as the MP will be a required instrument

4) Valid insurance as a result of point 3

Your CofA doesn't suddenly become invalid and hence insurance invalid due to the failure of an instrument in the air.

If the prop is left fully forward then the RPM cold be used as per a fixed pitch prop to get you home.

[Big Pistons Forever]

Originally Posted by englishal

Your CofA doesn't suddenly become invalid and hence insurance invalid due to the failure of an instrument in the air.

If the prop is left fully forward then the RPM cold be used as per a fixed pitch prop to get you home.

I read working hardpost's as implying he was prepared to continue to fly the aircraft without a working MP guage. If you takeoff knowing that guage is no working than the aircraft most certainly does not have a valid C of A and therefore the insurance is also likely invalid.

[Pilot DAR]

If the prop is left fully forward then the RPM cold be used as per a fixed pitch prop to get you home

Well, with the prop fully forward, you won't demand more power from the engine than it can safely produce, but you might not get home as expected. In some conditions, to prevent overspeeding the engine, you might not be able to demand enough power to maintain level flight. This would very certainly be the case in a twin operating on one engine.

Does this mean you should not continue your flight home? Of course not! Generally, with some conservatism with power management, you'd have no problem completing a normal flight with no harm to anything.

Beginning another flight in that condition would, as mentoned, be another matter. Intending to takeoff in an aircraft with a known defect, and not in accordance with an approved minimum equipment list, puts huge responsibility on the pilot making that decision. Things like a U/S nav light for day only flight don't get most people too excited, but a primary engine instrument is more important than that!

There are many worse instrument failures than a manifold pressure gauge reading wrong. (Like my passenger suddenly shreeking in pain after takeoff in a C185 amphib. When I looked over, his bare legs (wearing shorts that day) were covered in very hot, black engine oil, from the oil pressure line which had just burst, behind the panel, over his knees! Do I want to go back to floating, with an engine I know I cannot start again? ) Some instrument failures require an immediate change to the flight plan, MP gauge... not so much..