How to validate the accuracy of electronic instruments

[bcool]

I'm looking at an aircraft that has been recently redone with an entire suite of electronic instruments including Aspen PFD & MFDs and EI MVP-50P engine gauges.

My question is how would an A&P doing a pre-buy for me validate the accuracy of the configuration of the equipment? For example, how would they be able to verify that the MP shown on the gauge is the true MP; likewise, the temps, pressures, fuel flows, etc? And the info presented on the Aspens is accurate, like airspeed, etc?

I guess I'll need to find someone who's familiar with the equipment...but considering the equipment is somewhat rare that may mot be the easiest thing to do, especially within the range allowed to reposition the plane for the pre-buy.

 

[ateamer]

Review ATC tapes and see if people compliment the previous owner on the accuracy of his instruments.

 

[flyingron]

Review ATC tapes and see if people compliment the previous owner on the accuracy of his instruments.

How would your mechanic verify the accuracy of non-electronic instruments in a prebuy? Answer: they are not equipped to do so for many instruments (and it's near impossible to check some on the ground in situ anyhow).

The normal check for the MP is to see if it reads something approximating the ambient air pressure with the engine off. Tachs are a little easier although the usual way is to just test them against a known good reference (like a strobotach or one attached to the ignition).

 

[Clark1961]

You'd be looking at one very expensive pre-buy if you wanted everything checked at that level.

The aircraft would have to go to an Aspen dealer to check the installation and calibration.

If you really wanted to be sure the temperature readings were correct all the thermocouples would have to be removed and tested then installed. Count on at least one being broken when removing.

Are you getting the picture? You're asking for a Q/C level that is beyond what is done on installation.

 

[weirdjim]

You will forgive the analogy, but it sounds to me like a first time "buyer" trying to verify that his new companion is a virgin.

Jim

 

[Mistake Not...]

It's not even slightly unreasonable to wonder if the important parts of the machine you're spending as much as a house for, work. The OP didn't ask the mechanic to check the torque on the panel light bulbs...he asked about the expensive stuff.

But, unfortunately in aviation as it currently is, that sort of detailed inspection / verification isn't done. Airplanes tend to be this weird mix of high tech and low tech. You could find out, but it'll be expensive.

Also, unfortunately, this board has more than a few people who think an obvious answer implies a stupid question and requires a "witty" reply.

 

[kyleb]

I'm looking at an aircraft that has been recently redone with an entire suite of electronic instruments including Aspen PFD & MFDs and EI MVP-50P engine gauges.

My question is how would an A&P doing a pre-buy for me validate the accuracy of the configuration of the equipment? For example, how would they be able to verify that the MP shown on the gauge is the true MP; likewise, the temps, pressures, fuel flows, etc? And the info presented on the Aspens is accurate, like airspeed, etc?

I guess I'll need to find someone who's familiar with the equipment...but considering the equipment is somewhat rare that may mot be the easiest thing to do, especially within the range allowed to reposition the plane for the pre-buy.

These are the kinds of thing YOU can do under your mechanic's supervision. Put a mechanical gauge in-line before the MP transducer. Same thing for the fuel and oil pressures. Have your mechanic put things back "right" (or verify that they are right) when you're done. Check your voltmeter with a little $4 digital voltmeter that fits into the accessory power supply in your panel (AKA the cigarette lighter).

For CHT, grab yourself one of those little IR thermometers, run the engine, shut it down and verify some temperatures. Airspeed? Fly alongside an aircraft with a known good ASI. For altitude, do the same. It isn't hard, just time consuming.

 

[Silvaire]

If you're going to fly IFR you'll need a biennial Pitot/Static cert, that will verify altimeter and airspeed as well as encoder for transponder.

 

[weirdjim]

These are the kinds of thing YOU can do under your mechanic's supervision. Put a mechanical gauge in-line before the MP transducer. Same thing for the fuel and oil pressures. Have your mechanic put things back "right" (or verify that they are right) when you're done. Check your voltmeter with a little $4 digital voltmeter that fits into the accessory power supply in your panel (AKA the cigarette lighter).

For CHT, grab yourself one of those little IR thermometers, run the engine, shut it down and verify some temperatures. Airspeed? Fly alongside an aircraft with a known good ASI. For altitude, do the same. It isn't hard, just time consuming.

How do you know the mechanical gauges are calibrated any more than the aircraft instruments. I'd trust the aircraft voltmeter over the Chinese voltmeter. The IR thermometer comes uncalibrated and how do you know the other aircraft has a good ASI, that your's isn't correct and theirs wrong.

Jim


.

 

[Clark1961]

If you're going to fly IFR you'll need a biennial Pitot/Static cert, that will verify altimeter and airspeed as well as encoder for transponder.

never had airspeed verified....

 

[Silvaire]

never had airspeed verified....

it's easy enough to do from a pressure reference, problem is that due to differences in aircraft rigging and other factors that may affect pitot tube alignment you can't verify that is what you're getting in flight. I suppose you could do it with GPS in opposite directions or maybe average speed around a 2 minute coordinated turn.

 

[Stewartb]

The reason aircraft instruments are so expensive is that they must meet the TSO which assures they comply with exacting tolerances and quality control. It's assumed they're accurate because they're engineered to be accurate.

 

[kyleb]

How do you know the mechanical gauges are calibrated any more than the aircraft instruments. I'd trust the aircraft voltmeter over the Chinese voltmeter. The IR thermometer comes uncalibrated and how do you know the other aircraft has a good ASI, that your's isn't correct and theirs wrong.

Jim


.

You don't. You're looking for reasonably close numbers between two independent gauges. Do you really care if your CHT's are 375 or 380, or if your ASI is off by a couple of knots?

If you come up against a substantial difference between gauges, you'll have to bring in another reference source to figure out which of your gauges is "off". That's problem in the first place. Few of us have a dead on balls accurate set of reference gauges that were calibrated and certified minutes ago by the Bureau of Weights and Measures.

 

[Silvaire]

On the rare occasions that I weigh an aircraft I borrow scales from a Sprint Car racer buddy. There are four scales but I only need three. So I stand on all three of them individually and figure if they all give the exact same result they are good because what are the chances of all three being off by exactly the same amount?

This is why old airliners had three inertial navigation units. If one does not agree he gets tossed out because it's virtually impossible for two of them to go off the exact same amount at the exact same time.

So you'd think you could buy two cheap voltmeters from different manufacturers and use that same logic but in reality it's most likely that the guts of ALL cheap voltmeters, regardless of brand, come from the same smoky factory in China.