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Cessna 172 suction gauge tolerances

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Cessna 172 suction gauge tolerances

Old 17th Apr 2012, 14:09
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Cessna 172 suction gauge tolerances

The Cessna 172 1980 Model 172N Information Manual states the desired range of suction indicated is between 4.5 to 5.4 inches of mercury (see Airplane & Systems descriptions). A suction reading below this range may indicate a system malfunction or improper adjustment and in this case the indicators should not be considered reliable.

Under Normal Procedures before take off there is a requirement to test the magnetos at 1700 rpm and to check the suction gauge. Presumably that means to ensure the suction reading at 1700 rpm is within the allowable range.

Occasionally one sees a C172 where at 1000 rpm the suction gauge is not only below the minimum of 4.4 inches of mercury but so far below as to be off the clock. Normally after engine start it is good airmanship to check the suction system is operating correctly. At idle power even with well below the lower limit readings, in some C172 the gyros are erect and readings of the artificial horizon and directional gyro seem normal. Furthermore, when turning during taxiing gyro indications seem normal even though the suction gauge is almost zero reading.

Question: At idle rpm assuming 1000 rpm set, should the suction gauge read between 4.4 and 5.4 inches of mercury? Or are those limits only valid at 1700 rpm and above? The question arises when the aircraft is certified for IFR and the gyro instruments must be reliable before take-off. Idle power readings below those limits while taxiing for take-off may not ensure the gyros are reliable before entering IMC after lift off.
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Old 17th Apr 2012, 14:14
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Have you checked that both the Vac system filters are clean? they should be replaced at regular intervals and don't cost the earth so it is worth doing to avoid changing the gauge in error.
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Old 17th Apr 2012, 14:48
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Have you checked that both the Vac system filters are clean?
Your point is valid of course. But many pilots fail to write up perceived defects for fear of retribution from the operator or owner. This cavalier attitude is common. Not all operator/owners are kind and understanding. Thus the original question aimed at giving pilots something to hang their hat on in terms of suction indicated readings at selected rpm. The C172 Information manual does not make that clear and it is left to individual opinion as to what constitutes a defective suction pump or gauge.
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Old 18th Apr 2012, 04:02
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Hello All,
I am guessing you are in a training environment. Let me say.... you should always write up a item that doesn't meet limitations as published. If your instructor says it is ok....ask why? If they say don't worry about it....change instructors. A pilot's job... is to fly a airworthy airplanes. This requires the ability to say "no" if the airplane is not up to standards. In my carrier, as a pilot, I have written up hydralics, electrical, stand by instumentents....etc. Not because I didn't want to fly but because I was required to fly an airworthy airplane. Remeber.... you should always ask questions about why an instrument is not registering a correct indication. You can only gain knowladge by asking.

Dustertoo
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Old 18th Apr 2012, 04:33
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Surely the gauge would indicate a high suction pressure if the filters are blocked. Are they not on the atmospheric side of the gyros to protect them for airborne debris and if they were blocked, instrument indication would severely lag actual changes to attitude.
I would think the gauge range indicates the range, within which, the pressure needs to be to give gyro speeds that are sufficient to indicate changes of attitude and rate of turn with specified rates.
I think that's the most I've ever said on this forum, hope it wasn't all wrong!
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Old 18th Apr 2012, 05:18
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Surely the gauge would indicate a high suction pressure if the filters are blocked.
Sounds about right to me.
The vac gauge should show 4.5 - 5 at the RPM that the POH designates, normally around 1500 to 1700 rpm.
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Old 18th Apr 2012, 06:23
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JAKL & blackhand, better hit the books lads. A blocked filter will indicate less than the required 4.5 inches minimum.

This is what can happen to you if not paying attention.

http://www.atsb.gov.au/media/807221/ao2007047.pdf
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Old 18th Apr 2012, 06:37
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Don't forget that there is a suction/vacuum relief valve in the system. This is what controls the head of mercury (inches not hectors pascals) in the system and is adjustable. There is also a little gauze filter in the instruments themselves which if the aircraft is fairly old can become blocked and cause the instrument to malfunction. Remember air is drawn through the AH and DI by the vacuum pump through a filter usually mounted on the inside of the cabin and exhausted by the pump in the engine bay.
Some vacuum pumps have only a life of 500 hours so the MO should be looking at this and be changing the filters regularly.
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Old 18th Apr 2012, 07:13
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Brian

On page 14 of that report there is a diagram showing a typical suction system. The arrows indicate flow and it looks like the filter, if it were to become blocked or partially blocked, would cause a suction pressure indication as high as the relief valve would allow and the reduced flow of air across the gyro rotors would cause the gyros to spin slower.
Is that the case? And would a leak on the suction side of the gyros also cause slow speeds but a lower pressure indication?
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Old 18th Apr 2012, 12:55
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I've found that the needle is normally well into the green on single-engined aircraft at around 1500rpm.
I agree that congested inlet filters are usually the cause of low pressure. I once saw one that hadn't been changed, IIRC, for seven years according to the installation label and the regulator garter filter was brown and literally crumbling. The hoses weren't in much better condition.
System leaks or a fading vacuum pump can obviously be another reason for low pressure and are sometimes masked by adjusting the regulator without properly looking into the cause.
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Old 18th Apr 2012, 15:02
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So the pilot need not worry if the suction needle is never in the green during the taxiing to the runway but are in the green during run up to 1700 rpm? And that the gyros will be reliable for take off into the murk?
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Old 18th Apr 2012, 19:42
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And that the gyros will be reliable for take off into the murk?
No.

Check out the attitude erection system for air-driven AHs.
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