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From: thomassen@avionicswest.com
Subject: More IFR/GPS
Date: November 16, 2005 1:34:22 PM PST
To: lml@lancaironline.net
My earlier note “Navigating in the IFR system in an experimental aircraft," stimulated lots of good discussion so I wanted to respond to some points that were raised. These comments fell into a few areas, Primary vs Supplemental use of GPS, FAA approval of equipment, and “The Rules”.
Primary/Supplemental GPS use
There were many comments on “ground based systems” for navigation – if you use them. Clearly the TSO 146 (WAAS) receivers don’t use ground based systems and yet they are legal for primary navigation. With them you don’t need to have VOR’s in range, and you don’t even need a VOR receiver in the airplane. However, as was pointed out, the controller won’t give you that route if he can’t see you on radar. But, if you go IFR VFR on top when out of radar coverage, you’re ok (and this is an IFR flight plan). Having the capabilities of primary GPS nav equipment is the key – and it doesn’t have to be certified. Experimental Chelton systems that use the certified Chelton Free Flight GPS and integrity software are prime examples of non-TSO’d GPS's that have the capabilities of LNAV-only TSO146 certified receivers.
If you have a TSO 129 receiver (Garmin 430/530/1000 for example) you are not allowed to use your GPS out of VOR range, because a RAIM failure will render the GPS useless. You do have an option to ask ATC for a clearance that might read, “fly heading abc until receiving the xyz VOR, then proceed direct”. What this has done is to make your directional gyro your primary navigation instrument, not your GPS, until you can receive the VOR. If they can’t see you on radar, you are out of luck. For these receivers, your primary system is always ground based. Note that when you file /G nobody knows if your receiver is primary or supplemental so it's up to you to play by the rules.
FAA approved Equipment
There were comments on local FSDO approval of equipment, a POH supplement, AC20-138A, and the like. These statements in my view were correct for certified aircraft. When you put equipment in certified aircraft the FAA controls what happens. Whether you make a major or minor alteration (file a 337 or not) equipment must be TSO’d and for IFR GPS’s the shop needs to write a supplement to your POH.
But what approvals are required for experimental aircraft? Here I think is the can of worms. The only time the FAA is supposed to get into the act is in the original signoff of your aircraft. If they don’t restrict you to VFR you should get a statement that makes 91.205 applicable to you if you want to fly IFR in the system. I know there is a lot of local variation in what the FAA wants to see in your airplane before they give you your certificate, but if you’ve jumped that hurdle and want to add an IFR GPS later, you as the manufacturer can do that without their intervention. I’m not aware of a requirement for a POH when you get your airplane signed off originally, let alone for a supplement when you put in your GPS. These things are required for certified airplanes.
There was a reference to a shop refusing to install a micro-encoder (not TSO’d) without a FSDO signoff. If this was part of the original FAA approval, and the FSDO wouldn’t give that approval without installing an inferior TSO’d unit, he may have been out of bounds in my view, but clearly in control. This is a typical case of inconsistency in the FAA. There are a lot of TruTrak autopilots in experimental airplanes, and they are not TSO’d . Nor is that fantastic engine monitoring system (MVP-50) from EI. Why not object to these on the same grounds? Objections should be based on the instrument requirements of 91.205 and nothing else.
The Rules
There were a number of references to AC20-138A as the rules for all aircraft with GNSS. I learned that Advisory Circulars are not rules, they are advice. They do not govern what you need to do to fly IFR. As I said in my article, I believe that Part 91.205 is both necessary and sufficient for IFR flight. The EAA summary I referred to in my article agrees with that. For those of you who haven’t seen that summary, I will send their pdf file on request.
Keith Thomassen
Avionics West Training
thomassen@avionicswest.com
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