Avidyne got very lucky. This AFM change is very involved (90% of the users will not understand what is actually meant and out of those who do, most would forget when faced with the conditions). The FAA could have banned all IFDs from use. Let’s hope the software fix will be out very soon.
It is released to the FAA awaiting approval. Hopefully soon.
Reading the amount of stuff on this on the US sites, it’s amazing this has not been discovered before. In the R9, it’s been present for years.
One comment made is that it shows a lack of a flight test programme, but I think it also shows how few people hand-fly approaches. If you fly them with the GPS coupled to the autopilot, and don’t look at the HSI/EHSI, you will never discover this bug.
So what do people do on IR checkrides?
I believe the bug was discovered by somebody doing a checkride, and the examiner wanted him hand-fly the approach, thus using the HSI deviation bar indications.
I believe the bug was discovered by somebody doing a checkride, and the examiner wanted him hand-fly the approach, thus using the HSI deviation bar indications.
Huh? Do you mean the A/P got the correct track while the HSI didn’t?
Huh? Do you mean the A/P got the correct track while the HSI didn’t?
As that’s a pure software thing I guess it is simply that one part of the software sends out the correct data to the A/P and another part of the software depicting the HSI messes up the display of the CDI. It’s probably a problem with data interpretation when encoding/decoding stuff between software processes.
My reading of it (limited as I don’t have an IFD540 so paying only limited attention) suggests that the ARINC429 lateral deviation data, as used by modern autopilots directly, or by roll steering converters which fake a heading bug to an old autopilot, is correct. It is the HSI deviation which isn’t. I am sure someone will correct me if this is wrong.
It appears to be related to when the GPS makes the change from FMS linear deviation guidance to the angular guidance provided by the FAS data block calculation. If the system switches to angular deviations at 2nm prior to FAF (as is per the TSO) with no consideration of inbound course, you’ll get the issue. One fix is to implement additional logic which looks for a turn at FAF in the procedure. If there is a change of track there, the switch to angular guidance is delayed until arriving at the FAF (at which point you will be centered on the lateral track).
