How many G's can a manifold pressure gauge withstand

Big Ed

Big Ed

N50247 - '79 Super D
Joined
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Tampa, FL
The answer, apparently, is less than 4. Or at least that was the result of my trial with a sample size of 1 today:

man pressure (2).webp

So looking at Aircraft Spruce, and holy crap the TSO gauges are expensive. Over $1000! A non-TSO gauge is less than $300. What say you all?

I suppose if I am going to spend 1 Grand on a manifold pressure gauge, I should be smart and put it towards an engine monitor that displays MP. I'm about to spend $2K on a 4 cylinder CHT-EGT gauge. That plus the $1K on an MP gauge would put me into a fancy CGR-30.

CGR-EGTCHT-MainScreen_2873x.webp
But the thing is, I really like steam gauges, and I also don't like the idea of having a primary instrument dependent on electrical power when I have a non-powered option. Again, what say you all?
 
Who do you plan on telling what kind of gauge you will put in? Bet the $300 one is better than the one that failed.
 
Ed,

You've been going through that panel piece-meal, maybe it's time to start from scratch? Or just replace the broken gauge with another one until you take the plunge with a new panel?
 
Big Ed said:
don't like the idea of having a primary instrument
Click to expand...

Would you consider the manifold pressure a primary instrument? I'm not sure I would. I only have about 100hrs in complex aircraft, so I certainly still have things to learn. However, my take on manifold pressure is it's something I refer to only when I make engine adjustments. Those are infrequent enough that I would feel comfortable with the manifold pressure indication being a small part of something larger.

My theory on instruments, which is somewhat born from experiences developing UI for robots and other tech, is that the big gauges should be the ones which you can expect to monitor in real-time. Airspeed, attitude, altitude, turn and bank, g-meter, etc... Things which just need to catch your attention only when something is wrong-- e.g. oil pressure, vacuum, alternator, etc...--, should be as small and unobtrusive as possible so as not to distract you from flying the plane. I figure a stall-spin crash is a greater threat than an oil-pressure loss, so I want to prioritize my attention on the things which can devolve from GOOD to BAD in the course of two or three seconds. (Of course, there should be a way for the quasi-static instruments to alert you that you need to pay attention NOW, but that's different from an analog gauge which you will monitor in a continuous manner across the course of your flight).

With this in mind, I would say that an engine monitor which pushes all the quasi-static gauges into one face is a great thing, so long as the engine monitor logs data which can be downloaded for analysis after a flight. Looking at the CGR-30 face, the only thing which I would want to change is to have RPM be its own instrument. This is because on my 7ECA I have to watch engine RPM while doing acro. YMMV since you have a constant-speed prop.

BTW, if you want to stay with steam gauges, but agree with what I said above, there are TSO'd triple instruments which UMA makes. You simply choose from (almost) any of their 1" dials and they can place them all into a standard 3 1/8" gauge face.
 
If you are experiencing stall/spin scenarios, I submit that you should cover all your instruments and practice at altitude.

Ed, would your mechanic even know if a different gauge showed up?

One of my Decathlon buddies said his airplane would go into an immediate spin if the ball was even slightly out of center at stall. When queried closer, he said that at stall he pulled the stick allthe way back and held it there. No kidding.

I have tried to make the Decathlon go uncontrollable (moose stall, etc) - mine recovers with slight relaxing of back pressure - no mean tendencies at all.

When I bought it, the TSO manifold gauge was reading about 20" too high. Flew it home that way without incident, then replaced the gauge as above. No problems for the last 18 years.
 
Very interesting discussion!

I would categorize instruments by how you use them: Fly, Set, or Monitor.

"Fly" instruments are those you must reference to maneuver. Eg the six pack for IFR. Those should be large, centrally located, and logically laid out to facilitate a scan pattern.

"Monitor" instruments are those that you check periodically to confirm correct system operations. They can be small and out of the way, and in a display format that calls attention to deviations and trends. You don't care what your exact oil pressure is; you just care if it is well into the green range and steady.

"Set" instruments are those you reference to make primary system inputs such as power settings. Eg MP and RPM. Those can be off center since they are not scanned. However, the scale needs to be sufficiently precise and large enough to set accurately and quickly, so you don't fiddle with throttle and prop.

I refer to MP every time I touch the throttle. As I get older and my near vision deteriorates, I need a larger format so that I can read without glasses. I can do that with a 3 1/8 steam gauge. Not sure I could with a 2 1/4 gauge. A large format 2 1/4 digital display would work too, but honestly I don't like the aesthetics of the EI digital gauges with the LED arcs.

That's my issue with an engine monitor for MP. Most of the displays try to cram as much info as possible onto a small screen. On most of them, the MP display is too small for me to read without glasses or squinting.
 
Now that I am flying acro several times a week, I am giving more thought to optimizing my panel for that.

Nobody talks about an acro scan, but we should. When flying a sequence, you need to check airspeed, altitude, and G's during pullout, plus RPM if you have a fixed pitch prop. You must be able to locate and read those gauges in a fraction of a second, in order to identity and respond to excessive speed buildup. This becomes more important as maneuvers get more complex and the floor of the acro box gets lower.

For example, the current Sportsman sequence contains a half cuban with a 2/4 roll. That is proving very challenging to fly well in an older Decathlon, because of the roll rate. By the time you set the inverted 45, do the 2 point half roll, and set the upright 45, it is very hard not to be in the yellow on recovery. Get a little steep on the 45's or slow in the roll and Vne is quite possible when learning. You gotta keep one eye on airspeed, one eye on G's, and a hand on the throttle. The third eye is on altitude so you don't bust the box floor.

The only time I use the ball is at slow speed and high power, eg on takeoff. For stalls and spins, watching the nose on the horizon works better for me. For rolling into and out of turns, I can feel it.
 
Hiperbiper said:
What size gauge do you have? I have several 2 1/4" MP gauges on the shelf.
Part 91 no TSO required.

Chris
Click to expand...

I have a 3 1/8 gauge, but have an empty 2 1/4 space next to it.

I've been trying to understand when TSO/PMA/STC is required but seems impossible to get a straight answer.
 
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Bob Turner said:
... experiencing stall/spin scenarios...
Click to expand...

Statistically, this is one of the biggest mortal dangers while flying. Continuing education is one important lever we can pull to keep safe, and smart instrument layout is another. It's the Swiss cheese model of failure analysis and best practices put everything going in our favor.

Big Ed said:
I would categorize instruments by how you use them: Fly, Set, or Monitor.
Click to expand...

Nicely put!
Big Ed said:
I've been trying to understand when TSO/PMA/STC is required but seems impossible to get a straight answer.
Click to expand...

My lay understanding, which might be flawed, is that:
  • TSO: This means the part can go into any aircraft, no questions asked. If a TSO'd part is misbehaving AND it is internally functioning correctly then the problem is the airframe.
  • PMA: The FAA has authorized a third-party to manufacture a look-alike part. So if ACA stopped making P/N 12345, you could start selling your part which is an identical replacement for P/N 12345.
  • STC: This is a modification which is certified exclusively to either a single plane or an itemized list of planes. E.g. an STC for your airplane to become IFR, or an STC for all 7ECA to install vortex generators. If there's no STC for your specific plane model, then you're not allowed to install the mod/part.
So using the above definitions in the context of your gauge:
  • PMA would be when a third-party manufacturer is making the identical manifold pressure gauge as featured on the original A/C type certificate. The part is a direct replacement because it's legally the same part.
  • TSO would be a manifold pressure gauge which is certified to go into any airplane. After all, they're pretty universal!
  • STC is where the part isn't certified for ALL airplanes, because for whatever reason the manufacturer thought it would be easier if they specifically targeted some, but not all, planes. Dynon does this with around 600 models individually approved.
Thus, if I'm right (and that's a big if!) your replacement gauge can be any of the above, so long as your plane falls into whatever certification the gauge has.
 
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In another thread I mentioned that the local FSDO had serious heartburn about my non-TSO radio installations. They were convinced that non-TSO was the same as bogus parts, and told me I would have to remove them.

My last e-mail remains unanswered. It has been a month and a half since I heard from them.

I have found an AC that states that if a com meets RTCA requirements then I have "acceptable data" which is all you need for a minor alteration.

I have found no regulation requiring a TSO radio. Even transponder equipment need only "meet" TSO requirements (c.f. 14 CFR 91.215). And by golly, I found a definition of "meets" in Order 8300.16A Appendix1.

So no wonder there are so many opinions - the FAA has not yet nailed this down, and all of their documents lean in my favor.
 
So far as I'm concerned from my reading of the FAR's for part 91certified ops aircraft the two pieces of equipment mandated to meet and be tso'd are the transponder and ADSB units. Everything else in a non-revenue aircraft is subject to AP/IA interpretation...
TSO is an FAA production standard. It provides no installation authorization by itself. The authorization to use the parts come from the manufacturer via the Maintenance Manual, Illistrated Parts Catalog and TCDS.
Anyway Ed, I'll send you a servicable 2 1/4" Mp gauge if you want it...
Chris
 
Hiperbiper said:
So far as I'm concerned from my reading of the FAR's for part 91certified ops aircraft the two pieces of equipment mandated to meet and be tso'd are the transponder and ADSB units. Everything else in a non-revenue aircraft is subject to AP/IA interpretation...
TSO is an FAA production standard. It provides no installation authorization by itself. The authorization to use the parts come from the manufacturer via the Maintenance Manual, Illistrated Parts Catalog and TCDS.
Anyway Ed, I'll send you a servicable 2 1/4" Mp gauge if you want it...
Chris
Click to expand...

That's my understanding too. However, the interpretation of this always breaks down into vagueness when it gets to this part: The authorization to use the parts come from the manufacturer via the Maintenance Manual, Illistrated Parts Catalog and TCDS.

There is nothing anywhere in the parts catalog, maintenance manual, or AFM specifying the gauges to be used. For example, here is the relevant snippet from the parts manual. Note the part number is blank for the MP gauge:

part number.webp

So ... what does that mean? I'd be perfectly happy to throw a non-TSO MP gauge in my airplane. However, as a general principle I want an A&P/IA logbook entry for anything done that is non-owner maintenance, so that I do not have problems come sale time. My IA is very good about letting me work under supervision and signing off on my stuff, but he does want proper documentation. So what do I give him for a non-TSO gauge?

On the 2 1/4 gauge, thanks, that would be awesome. If nothing else, I can install it to keep flying while I get my 3 1/8 gauge repaired. Also, I've been wanting to test my eyesight on the smaller gauge, as it would free up a large hole for me to rearrange my 6 pack. I would really like my G meter to be directly below my airspeed indicator. I think that would facilitate monitoring in a dive pullout. So I would like to move the RPM to the far left side of the panel, move the VSI under the altimeter, and move the G meter under the ASI. That would leave an unused spot under the AI for later use by an HSI.

IMG_20200914_164756.webp
 
Disagree on the transponder. Read my references carefully - they say transponders must "meet" TSO requirements. "Meet" expressly does not mean "have a TSO", according to the FAA (not according to me).

Always prepared to be proved wrong, but I would need to see the regulation.
 
This is from FAA Order 8300.16A Appendix 1:

10. Meet Minimum Technical Standard Order (TSO) Established Standards. Means that the equipment need not have TSO approval, but that it meets requirements set by the TSO.

And here is the FAR on transponders:

§ 91.215 ATC transponder and altitude reporting equipment and use.
(a) All airspace: U.S.-registered civil aircraft. For operations not conducted under part 121 or 135 of this chapter, ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate, or the appropriate class of TSO-C112 (Mode S).
 
Bob Turner said:
This is from FAA Order 8300.16A Appendix 1:

10. Meet Minimum Technical Standard Order (TSO) Established Standards. Means that the equipment need not have TSO approval, but that it meets requirements set by the TSO.

And here is the FAR on transponders:

§ 91.215 ATC transponder and altitude reporting equipment and use.
(a) All airspace: U.S.-registered civil aircraft. For operations not conducted under part 121 or 135 of this chapter, ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate, or the appropriate class of TSO-C112 (Mode S).
Click to expand...
OK. Copy and understand.
This is why I stated in my post it's up to the AP/IA to either ok or say no. Ed's IPC has no part number for the MP gauge because ACA used many different gauges thru the years and it just ain't that darned important. It's up to the person signing the logs and if they are happy with the old " Form, Fit, Function" guidance that used to be taught in A&P School.
Chris
 
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