Hobbs reading vs Flight Meter

Hi again guys,

My Hobbs has always registered a higher figure than the Flight Meter, as it should. But I’ve noticed that for a few months the two reading are identical.

Is there a software switch in the radio (G430) somewhere that can be turned on/off?

it occurs to me that this might be a rather cute ploy on the part of the AMO to increase his turnover as the MPI is based on the Flight Meter reading (in my country)…

Dion

There is a pressure switch on the pitot line, located in the bowels of the center console. The switch, rated at 20mA, tends to fail with the contacts welded together because for some unfathomable reason it’s being used to switch a relay coil (50mA inductive load) rather than the smaller current that the meter itself draws.

So get the switch replaced. It will cost you more in labour than parts. Oh, sometimes adjusting the switch will also work, but since they’re cheap replacing is a good option once access is gained, which is a pain.

via COPAme
Samsung SM-T820

Hi Clyde,
Ah, that makes sense! So I guess they use the positive pitot pressure to start the Flight Meter rather than altitude. Nice and simple!
But I know how damn expensive it is getting the cockpit floor/console opened up!
Funny, relays seem to be a weak point on SR’s…have had the flap relays fail a couple of times as well.
Thanks for the tip
Dion

I think we have the same problem. We just flew an SR20 from TX to NY and Hobbs and Flight meters the same.

Did you pursue a fix? Details? Cost?

Thanks

Bob

N624CP

Last September my local mechanic (not a CSC shop) charged me 5 hours of labor to install a replacement switch. I ordered the switch myself from www.tcairparts.com. Cost about $100. It was my third replacement switch, they seem to last about 300 hours.

I keep track of hobbs and flight time on a spreadsheet, with the difference calculated for each flight. Two or three flights in a row with no difference, and I have the switch replaced. I went a year with a bad switch before I learned it was something to pay attention to.

Wally,

One of the reasons IMHO that the switches fail is that the switch is used to control a relay which in turn switches power to the meter. Now this is crazy because the relay coil draws more current than the meter itself, and in fact the relay coil current exceeds the rated current of the switch. So I suspect that removing the relay from the circuit and switching the meter directly via the pressure switch would improve the reliability. Checking the circuit for your plane and measuring some currents would be the first step.

Clyde

Cirrus must have designed it that way for a reason, and that wiring is part of the type design.

If I were to guess, it is to have an air-switch failure mode that would cause the timer to free run (matching the Hobbs) rather than stop (losing track of maintenance time). If it stopped, you would no longer have a good idea of when the next time-related required maintenance would be due, causing airworthiness questions.

I would not recommend re-wiring that circuit.

No doubt, but was it a good reason? Since the circuit as designed violates good engineering principles by stressing a component outside its rated limits, I think it falls into the category of “it seemed like a good idea at the time”.

The potential failure modes are no different with or without the relay, and adding extra components increases the risk of failure. I seriously doubt that Cirrus engineers spent any significant time analysing potential failure modes of a subsystem that has no safety implications. The suggestion that it would be difficult to keep track of required maintenance without the flight meter functioning is a straw man. Older Cirri were delivered without a flight time meter installed, they manage to get maintained ok.

That doesn’t preclude an A&P from determining that a change to that wiring is a minor alteration.

You’d also need to replace the air switch part from a normally closed switch to a normally open one.

It appears the type design has the relay energized on the ground and de-energized in flight.

Check the wiring diagram.

Good pickup. That may explain why there is a relay there, and does lend credence to your theory that the design was intended to fail in a particular way. I wonder if there is a pin-compatible relay with a lower energising current - I checked the specs on the relay and switch some time ago and my recollection is that the relay coil drew 50 mA and the switch was rated for 20 mA. The relay is rated to switch 40A which is massive overkill. I assume the engineer who designed it just used the relay that was already on the shelf at Cirrus (it’s the same part as the flap relays e.g.)

I just measured a relay and the coil draws 110mA at 28V. Hardly surprising that the switch has a high failure rate - I confirmed that it is rated at 20mA max, and should have an RC snubber for inductive loads.

There is a normally open version of the same switch available.

Hello Wally,

Many thanks to you and all who’ve chimed in. So, co-owner and I are newbies to owning an SR20…very new! If you or anyone knows the part number(s) and a description of the location it would be great. Our mechanics are good, but not familiar with Cirrus.

I’ll look for the manuals on the Cirrus site, but any pointers would be much appreciated.

Thanks again all.

Bob

Bob - You and your partner are in for some good times indeed. You already know where to go for answers like part numbers - the Illustrated Parts Catalog (IPC). It sometimes helps to also cross reference the diagrams in the Aircraft Maintenance Manual (AMM) or the Wiring Manual. There’s never a better time than now to get acquainted enough with them to find your own part numbers. I know it’s a slog to start with, and it never gets downright easy for us Peter Pilots like it does for Cirrus maintenance shops.

As for your part number, I’d tell you if I knew. I don’t even recall what it is on my SR22, and I’ve never looked it up in the SR20 IPC (it might be the same, but maybe not). Perhaps others who would know might read this and chime in. But don’t wait for them too long - the answer is in the books. Good luck!

Wally, I’ve been trying to figure out how to get those for a 2008 SR20 Gen 3. Cirrus support site lists them for our serial number, but doesn’t give a link like the POH and other manuals. A purchase from Cirrus?

Bob

Robert, the manual links are on this site, in various members’ forums. I am not going to post the links in this thread because it is publicly visible. Cirrus does not publish the links, but has also not password protected them, and I’m not going to jeopardise that situation.

The Cirrus part number for the pressure switch is 14128-001.

Clyde

Clyde (and all),

Thanks! I found a disk the prior owner sent along with all the manuals on them and will research more. Many thanks for the part number Clyde.

Stay healthy/safe everyone.

Bob

Got the pressure switch. Going to assist mechanic on install. We can see it’s hard to get at from the right side as the AMM describes. Any techniques out there? Accessible from left side of console?

As always, many thanks to all,

Bob

Got it installed. Besides coming in from the right side, we came through the left side at the circuit breaker panel. Still difficult, but coming in from the left as well kept it to about a 1.5 hour job.

AMM says from the right side, but this really helped.

Bob

I have just replaced my second pressure switch in less than 100 Hobbs hours. Is there any way to talk to Cirrus and ask them to come up with a better part (as suggested in this thread above)? It takes around two hours to replace the damn thing, so the whole repair is not a negligible cost.

Val