IFR Check

[kandjw]

As we started doing these checks on the new Skyview in my RV6 I told the tech about the Dynon procedure to have the Skyview turned off while he changes the altitudes. He said had never heard of that procedure but would do it.

As he started the checks he stopped and said he could not do it this way because he needed to watch the transponder altitude readouts each hundred feet between the test points, and could not do that with the Skyview powered down. Is there a need for this each hundred feet checking?

I questioned it from two aspects. First, I have the idea somewhere that with the Dynon the transponder readouts were driven digitally by the EFIS to match the altimeter. So the real check is of the altimeter. Second, I wonder about the each hundred feet requirement. I normally watch this process and in the past I do not remember the techs doing more than running it to the thousand feet check points, stopping at each, tapping the altimeter once in a while and noting the encoder output.

He got interrupted and we will resume tomorrow afternoon. Any suggestions about this would be appreciated. Thanks.

 

[RVDan]

This is a common misconception. 

The altitude requirement is from 91.411(a) No person may operate an airplane, or helicopter, in controlled airspace under IFR unless--
[(1) Within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each automatic pressure altitude  reporting system has been tested and inspected and found to comply with appendices E and F of part 43 of this chapter;

That takes us to Part 43 Appendix E para. (c) applies to the Mode C altitude data:
(c) Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Integration Test. The test must be conducted by an appropriately rated person under the conditions specified in paragraph (a). Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C at a sufficient number of test points to ensure that  the altitude reporting equipment, altimeters, and ATC transponders perform their intended functions as installed in the aircraft. The difference between the automatic reporting output and the altitude displayed at the altimeter shall not exceed 125 feet.

You will note that nowhere does it say one must check every 100 ft.  You will notice that the only quantified requirement is the 125 ft difference between the displayed altitude and the altimeter in the aircraft.  It just says "sufficient number of points" With Skyview, the displayed data and the encoded altitude come from the same source, and are handled digitally so no error can be introduced in the data by the system.  The only thing he should have to do is to verify one or at most several points (where he stops when doing the altimeter test) to make sure the encoded altitude data is the same as displayed.  It is not necessary to track the altitude as it changes. 

He should be also doing 91.413 (b) Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.

Same end requirement.

If he says he has to watch it track ask him to show you where in the regulation it says that.  Maybe he proves me wrong.

 

[Dynon]

I suspect that he might think he needs to check each hundred feet because with the older "gray code" interface found on most blind encoder, one disconnected or misrouted wire will cause many altitudes to be right, but others to be wrong. By watching the altitude change over every hundred feet, you can catch this. So it's not a bad practice if you've got a gray code encoder converter. But SkyView uses a serial data output, and simply isn't susceptible to that sort of error. Part 43 Appendix E only requires that the thousands be tested explicitly.

 

[RVDan]

Agreed on the grey code, actually there are, I think 7 specific altitudes that need to be checked to verify all bit lines in grey code are connected and operating.  Also the Part 91 rule used to read that if the altitude source or altimeter were not TSO approved, that you had to do a correspondence check through all usable altitudes and that drove testing every 1000 ft.  But no more.

Dynon, could you point me to where you see Part 43 appendix E requiring 1000ft checks for the mode C altitude source?  We have had some pretty hefty discussions on these checks with the FAA and have never found anything other than the 125 ft correspondence requirement, and of course enough checks to verify integrity of the grey code. 

 

[Dynon]

Dynon, could you point me to where you see Part 43 appendix E requiring 1000ft checks for the mode C altitude source?  We have had some pretty hefty discussions on these checks with the FAA and have never found anything other than the 125 ft correspondence requirement, and of course enough checks to verify integrity of the grey code. 

Well (b)(i) says "(i) Scale error. With the barometric pressure scale at 29.92 inches of mercury, the altimeter shall be subjected successively to pressures corresponding to the altitude specified in Table I", and table I is in thousands of feet.

There's then a bunch of hysteresis and friction tests that talk about simulating difference descents and other conditions, but those tests are also concerned that the system gets to correct after a settling time, and not what happens along the way as things are changing.

 

[kandjw]

Thanks for the responses.  Finished the check tonight with the tech agreeing to the Dynon procedure with checks at 1,000 foot increments up to 20,000.

As he re-powered the Skyview at each altitude we got the big red "X" with the message "ADAHRS Fail - multiple sensor failures" or words like that. 

Called Dynon support and learned that the sensor failure occurs if Skyview sees a pressure mismatch between the AOA and Pitot.  The tech had pitot and static both connected through his test equipment - evidently to keep the pitot and static at the same pressure as he raised the altitude.  But the AOA port was not equalized.  He was able to finish the tests by checking each altitude quickly before the "X" appeared.

Something to be aware of.  It would not have occurred to either of us that was the problem. 

I questioned the Dynon phone support person about why the same tech and I had not had the problem with my RV12 we did a year or two ago.  I think the Dynon person told be this check between the pitot and AOA is a new one with version 11.  Dynon, please correct me if I got this wrong.

Again, thanks for the help in getting this task done.

 

[RVDan]

Dynon, could you point me to where you see Part 43 appendix E requiring 1000ft checks for the mode C altitude source?  We have had some pretty hefty discussions on these checks with the FAA and have never found anything other than the 125 ft correspondence requirement, and of course enough checks to verify integrity of the grey code. 

Well (b)(i) says "(i) Scale error. With the barometric pressure scale at 29.92 inches of mercury, the altimeter shall be subjected successively to pressures corresponding to the altitude specified in Table I", and table I is in thousands of feet above 3000 ft. Even for altimeters, B(2) gives allowance for alternate checks when an ADC is involved instead of a standard mechanical altimeter. Lots of latitude on how you get there with an integrated system.


There's then a bunch of hysteresis and friction tests that talk about simulating difference descents and other conditions, but those tests are also concerned that the system gets to correct after a settling time, and not what happens along the way as things are changing.

(b) applies to the altimeter, not the altitude source for mode C, which is what the OP was inquiring about.  Agree if they are doing the altimeter checks, even then though, table 1 is not every 1000 ft-

 

[dynonsupport]

To correct one subtle point, you said "... the sensor failure occurs if Skyview sees a pressure mismatch between the AOA and Pitot".

The ADAHRS Multiple Sensors failure will occur only if the system sees a significant mismatch in pressure between AOA and Pitot shortly after the SkyView display boots up and you're not in flight. Beginning in v11.0, SkyView does a "sanity check" for ADAHRS sensors, and one of the tests it performs is comparing pitot and AOA, and if significantly different, fails the ADAHRS.

We understand the rationale for connecting both pitot and static ports when doing an altitude check, but in SkyView, connecting the pitot port isn't necessary (no damage will occur if the pitot port isn't connected when applying pressure to the static port).

Thanks for the responses.  Finished the check tonight with the tech agreeing to the Dynon procedure with checks at 1,000 foot increments up to 20,000.

As he re-powered the Skyview at each altitude we got the big red "X" with the message "ADAHRS Fail - multiple sensor failures" or words like that. 

Called Dynon support and learned that the sensor failure occurs if Skyview sees a pressure mismatch between the AOA and Pitot.  The tech had pitot and static both connected through his test equipment - evidently to keep the pitot and static at the same pressure as he raised the altitude.  But the AOA port was not equalized.  He was able to finish the tests by checking each altitude quickly before the "X" appeared.

Something to be aware of.  It would not have occurred to either of us that was the problem. 

I questioned the Dynon phone support person about why the same tech and I had not had the problem with my RV12 we did a year or two ago.  I think the Dynon person told be this check between the pitot and AOA is a new one with version 11.  Dynon, please correct me if I got this wrong.

Again, thanks for the help in getting this task done.

 

[cmgolden]

Is there a written procedure for the IFR checks specifically on a (dual ADAHRS) Skyview system?   

Pages 5-12 to 5-14 in the Install manual give the "warm up" and "power off between pressure changes" advice. But because this will be the shop's first Dynon pitot-static test, it would be great to be able to hand him a step-by-step procedure from the manufacturer that fully complies with the IFR requirements.

(E.g. he asked if I knew how to calibrate the altimeter if they find an error).

 

[swatson999]

I don't know about everyone else, but for my static check, all we did was power it up and let it warm up for a few minutes, then just treated it like a regular static check (no powering off, etc.), and it came out well within specs all the way up to 25K. My guy says that's all he ever does and the Dynon units essentially never fail a static check (as long as no leaks in the system, etc.).

Sounds like 'the instructions are making it a lot more complicated than it needs to be.

 

[thibault]

Steve,

The answer is right in the installation manual, pg 5-13, Version 12.2:

"Dynon ADAHRS units use airspeed in the calculation of attitude (or GPS ground speed when airspeed is navailable). The internal rate sensors are monitored and calibrated in flight using feedback from the accelerometers and from airspeed to achieve a highly accurate attitude solution.

When a pitot or static test is performed the ADAHRS is being exposed to dynamics that are impossible to achieve in a real aircraft flight environment, namely, large airspeed changes without the accompanying accelerations and rotations. This false condition will cause a well calibrated unit to incorrectly adjust its calibration.  Furthermore, when a unit is in a test situation a change in applied pitot or static pressure will cause the attitude to pitch up or down.  This is expected.

Performing the following Dynon-prescribed pitot and static test procedure to a Dynon ADAHRS will prevent problems with the unit’s performance in flight."

 

[n97kd]

I had my system tested a couple months ago and "my guy" who said he has tested other Dynon systems said he has never had a  problem by slowly (very slowly) raising the altitude and slowly lowering it. We also connected his test box directly to all three ports on the ADHARS so pitot, AOA, and altimeter saw the same pressure.

I have flown my plane many hours since and haven't seen any ill effects using his procedure.

My question to Dynon: did this procedure mess up the calibration, if so, how would I know?

Thanks

Don Yoakley

 

[dbdurrett]

Might there be a way of creating in Setup Mode an entry for "Bi-annual IFR altitude/encoder test" that we go into and the tech could run his test the way they test every other altimeter. Protect the fine adjustment made to altitude in flight. All we need is to be assured of meeting the 125 foot criteria.
DonD

 

[krw5927]

Might there be a way of creating in Setup Mode an entry for "Bi-annual IFR altitude/encoder test" that we go into and the tech could run his test the way they test every other altimeter. Protect the fine adjustment made to altitude in flight. All we need is to be assured of meeting the 125 foot criteria.
DonD

This is a fantastic idea. Please, Dynon?

 

[cmgolden]

Agreed. Powering on and off really added substantial time to complete the checks. A simple wizard to walk through the required steps would be great!

 

[swatson999]

Steve,

The answer is right in the installation manual, pg 5-13, Version 12.2:

"Dynon ADAHRS units use airspeed in the calculation of attitude (or GPS ground speed when airspeed is navailable). The internal rate sensors are monitored and calibrated in flight using feedback from the accelerometers and from airspeed to achieve a highly accurate attitude solution.

When a pitot or static test is performed the ADAHRS is being exposed to dynamics that are impossible to achieve in a real aircraft flight environment, namely, large airspeed changes without the accompanying accelerations and rotations. This false condition will cause a well calibrated unit to incorrectly adjust its calibration.  Furthermore, when a unit is in a test situation a change in applied pitot or static pressure will cause the attitude to pitch up or down.  This is expected.

Performing the following Dynon-prescribed pitot and static test procedure to a Dynon ADAHRS will prevent problems with the unit’s performance in flight."

Well, I'll add that my airspeed and altitude have never varied from other aircraft flying as a group by more than 50-100' at any altitude we've used (up to around 14,000), so I'm pretty comfortable with the test...

I should add that I have a D6 as well, with pitot/static/AOA T-ed off of the same lines, and they're both within a few feet/knots of each other and both within the test requirements.

 

[thibault]

Steve and others,

I THINK that the quote from the install manual says that ATTITUDE calibration will be negatively affected, not pitot/static calibration.

Dynon,

Please speak up here!!!

 

[Dynon]

Fundamentally, it's the rate sensors - which are used in computing attitude - that can be temporarily affected by performing a static check without turning the display off in-between test points. The system uses flight dynamics to ensure that the way that the rate sensors are used in determining attitude. You can think of this as a dynamic component to the ADAHRS calibration that can be tricked if you fool the system into thinking it's flying when it isn't In no case does it affect airspeed or altitude accuracy at all. See the full background statement from the manual below.

Incidentally, as many of you have found, in the vast majority of cases you'll be just fine even if you don't follow these instructions. The most likely effect is that turn rate and/or attitude may be slightly off for a short period of time on the next flight after you've performed a pitot/static check while the system re-calibrates the system to its correct values when you're ACTUALLY flying (instead of fooling the system into thinking that you're flying).

------Installation Manual Background------


Dynon ADAHRS units use airspeed in the calculation of attitude (or GPS ground speed when airspeed is unavailable). The internal rate sensors are monitored and calibrated in flight using feedback from the accelerometers and from airspeed to achieve a highly accurate attitude solution.
When a pitot or static test is performed the ADAHRS is being exposed to dynamics that are impossible to achieve in a real aircraft flight environment, namely, large airspeed changes without the accompanying accelerations and rotations. This false condition will cause a well-calibrated unit to incorrectly adjust its calibration. Furthermore, when a unit is in a test situation a change in applied pitot or static pressure will cause the attitude to pitch up or down. This is expected.
Performing the following Dynon-prescribed pitot and static test procedure to a Dynon ADAHRS will prevent problems with the unit’s performance in flight.

 

[cmgolden]

A setup mode as simple is disabling temporarily Skyview's self-calibration, while showing baro settings and both ADAHRS altitude indications would ease IFR checks (and cut time by a large percentage).

I'd like to just risk having attitude off for a short period on the next flight except that flight might be in IMC, the very reason for having to do the checks in the first place.