Does AFMS = Major Alteration ?

[Tom-D]

It indeed does, but



would mean a lot of avionics do not need an AFMS.

In particular an EHSI which does no more than the old HSI (in terms of its outputs) does not appear to meet the above requirement for an AFMS.

Is it possible to write to an FSDO and ask them these questions? Presumably the answer would be legally valid.

That is the premise that I would go on, the Sandel is nothing more than a Electronic HSI. it does not change the flight characteristics of the aircraft.

 

[John Collins]

It indeed does, but



would mean a lot of avionics do not need an AFMS.

In particular an EHSI which does no more than the old HSI (in terms of its outputs) does not appear to meet the above requirement for an AFMS.

Is it possible to write to an FSDO and ask them these questions? Presumably the answer would be legally valid.

Peter,

If you ask the FSDO, you get one man's opinion and it does not represent the FAA. If you want an official opinion, you have to write to the FAA counsel's office and it will take 4 months or longer to get an official response.

Usually, if the original STC required an AFMS, or a sample AFMS is provided, I would expect that the manufacturer determined that one is necessary, usually because of operating limitations. Often, the EHSI performs additional functions that are not performed by the previous HSI, for example it can control the CDI source, the autopilot source, and display other data.

 

[peter-h]

Usually, if the original STC required an AFMS, or a sample AFMS is provided, I would expect that the manufacturer determined that one is necessary, usually because of operating limitations.

Sandel don't actually say in their IM that an AFMS is mandatory (AFAIK).

They do provide a sample AFMS, however. I have no idea what the Cessna 421 STC says - I can see only the front page of it.

Does the manufacturer's opinion have precedence over the FAA? I didn't think it did (ref e.g. the old "mandatory" prop overhauls debate).

Often, the EHSI performs additional functions that are not performed by the previous HSI, for example it can control the CDI source, the autopilot source, and display other data.

I wonder if this offers a way to simplify the certification: install the EHSI so that is does no more control than the old one. Yes it can be installed to do a lot more but if one looks at what it actually does in terms of outputs, it isn't much. It is mostly a data display device.

If this is a valid approach, then I am sure than I am not the first to have realised it, which would explain why AFAICT these devices are being installed out there without an AFMS and apparently as a Minor alteration.

In my specific case, its output connectivity with the rest of the aircraft will be limited to the old HSI cable harness. Any extra wires (power, and a few inputs like the stormscope and the ADF) are inputs only and don't control anything.

It's true that I could go all the way, throw in a pile of relays, and bring in NAV2 and then have the ability to control the autopilot from NAV2. That would be a nice bit of system redundancy but it's a lot of signals to switch and I am not doing that.

I am considering bringing in NAV2, for display only (i.e. as an RMI; no OBS functionality as a already have a KI-204 doing that) but cannot see a way to configure the 3500 to limit its functionality to an RMI needle only, with no "CDI" presentation. The ADF, of course, has to work that way.

I already have a KI-229 RMI which is switchable NAV1/NAV2 and there is quite a bit of switching behind that.

Anyway, it's a fun discussion but all this may be moot since I have discovered that one can perfectly legitimately send the 337 (normal "field approval" route) to any US FSDO. Apparently it is normal for people to do this within the USA e.g. if their local FSDO does not have the specific expertise. This is the easiest approach, unquestionably legal, and I will try it.