# Using only GPS as your EFIS inputs??? Why u do dat?

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#### rv7charlie

##### Well-Known Member
Uh, no. First, go check out FAR 91 for what's required. Second, check out your aircraft's operating limitations, which might give you relief.
91.205 applies to standard type cert a/c; homebuilts have 'special' a/w cert. 91.205 only applies to homebuilts if operated at night, or IFR. Unless the FAA has radically revised their 'boilerplate' for homebuilt oplims in recent years, the language is there, though it's in typical triple negative legal doublespeak. I just checked mine; issued in 2004. The following isn't an FAA doc, but it does do a decent job of describing requirements.
If you're an EAA member, you could give them a call & get 'chapter and verse' on how day VFR homebuilts are exempt from instrument requirements.

Charlie

#### TFF

##### Well-Known Member
On another forum, a builder made the transponder correlation part of his build. He had access to the equipment. So this is arguable, but his stance is because he did it during the build, he can sign it off himself under his repairman certificate. Untested legally, but a worthy test of the rules. I don’t think anyone would get in trouble if deemed wrong, just a slap saying you interpreted the rule wrong. I would try if I was building something new.

#### pfarber

##### Well-Known Member
HBA Supporter

FAR 91.217 says -

(a) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder -

(2) Unless, as installed, that equipment was tested and calibrated to transmit altitude data corresponding within 125 feet (on a 95 percent probability basis) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter referenced to 29.92 inches of mercury for altitudes from sea level to the maximum operating altitude of the aircraft; or

(3) Unless the altimeters and digitizers in that equipment meet the standards of TSO-C10b and TSO-C88, respectively.

Keep in mind that even though an altimeter need not be TSO'd as mentioned above, it must have some type of production approval to be eligible for installation in a type certificated aircraft. Refer to FAR 21.303

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#### Marc Zeitlin

##### Exalted Grand Poobah
FAR 91.217 says -

(a) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder -

(2) Unless, as installed, that equipment was tested and calibrated to transmit altitude data corresponding within 125 feet (on a 95 percent probability basis) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter referenced to 29.92 inches of mercury for altitudes from sea level to the maximum operating altitude of the aircraft; or

(3) Unless the altimeters and digitizers in that equipment meet the standards of TSO-C10b and TSO-C88, respectively.

Keep in mind that even though an altimeter need not be TSO'd as mentioned above, it must have some type of production approval to be eligible for installation in a type certificated aircraft. Refer to FAR 21.303
Not sure what your point is here. The transponder and ADS-B altitude reporting must meet the requirements of 91.217, but that says nothing about the altimeter that the pilot uses to determine the aircraft's altitude. ALL the altimeters (static system) must be checked for IFR flight, per 91.411, but only the transponder and ADS-B's altimeter must be checked per 91.217 and 91.413, and the 91.413 transponder check does just that.

If you're not flying IFR, you don't need the 91.411 check.

#### pfarber

##### Well-Known Member
HBA Supporter
My initial question was if the GPS height was useful for anything since its usually the only sensor in low end EFIS.

basically the EFIS needs to have a barometric altimeter to be useful in navigation.

So it goes back to the 'why do EFIS displays do dumb things'.

Artificial horizons: garbage, don't need for VFR day, although night would be useful. Needs to be a GYRO thou, not silicon.
Anything altitude wise: useless unless they are linked into a barometric altimeter, useless in all cases.

So basically the hobby/DIY EFIS that are relying on silicon/GPS should be regarded as toys, and not useful navigation tools.

#### pfarber

##### Well-Known Member
HBA Supporter
91.205 applies to standard type cert a/c; homebuilts have 'special' a/w cert. 91.205 only applies to homebuilts if operated at night, or IFR. Unless the FAA has radically revised their 'boilerplate' for homebuilt oplims in recent years, the language is there, though it's in typical triple negative legal doublespeak. I just checked mine; issued in 2004. The following isn't an FAA doc, but it does do a decent job of describing requirements.
If you're an EAA member, you could give them a call & get 'chapter and verse' on how day VFR homebuilts are exempt from instrument requirements.

Charlie
If you want to do anything useful with an E/AB you will need to have ADSB-out and MODE-C. Mode-C is not dead. If you want to fly at night or in controlled airspace (other than D/E) you will need to have gear that DIY EFIS simply don't have, and almost certainly won't have a TSO.

Dynon and the EAA got STCs for a non-TSO EFIS (the now out of production D-10A). But this is also a $2k+ unit. Not the$500 ish DIY EFIS.

#### rv7charlie

##### Well-Known Member
Boy, am I glad you weren't around for the 25+ years & >1000 hours I flew homebuilts without an EFIS, ADSB-out, Mode C, or even a basic transponder. I'd have missed out on a lot of (quite legal) fun if I'd known that, including (legally) flying inside a Mode C veil to/from a private airport under the 'wedding cake', about 3 miles from the Class B airport, and to/from Sun&Fun multiple times, flying within two Mode C veils.

Now that you've filled me in on what you won't allow, I guess that I'll have to stop flying day-VFR in my non-ADSB-out RV with an experimental EFIS. Boy, I really hate that I started reading the Homebuiltairplanes forum....

Charlie

#### aivian

##### Member
My initial question was if the GPS height was useful for anything since its usually the only sensor in low end EFIS.
What do you mean by "EFIS" and "useful?" Any system which has attitude requires sensors other than GPS, and anything with a PFD requires pitot-static data. If any system is displaying a PFD based on inertial data I think we would agree that is not an appropriate thing to do.

Focusing on altitude, it depends on what is "useful" to you. You are correct if what you want to do is fly ATC assigned altitudes -- the ATC system is designed around pressure altitude and the hacks required to make pressure altitude work in this application aren't compatible with GPS. (It may actually be legal, I don't know, but I don't think it would be prudent. Similar issues have cropped up in my soaring club recently regarding staying out of class A)

If you want to know how much you clear terrain by then GPS is arguably better than pressure altitude -- it is *actually* altitude and doesn't require a pressure setting. I use GPS altitude in my sailplane as the basis for average climb rate and glide calculations. It is at least as good as pressure altitude in this application and the DIY hardware is much simpler (I would not refer to the navigator I use as an "EFIS" though). Admittedly, the whole aircraft and enterprise in this application is a "toy" and part of my personal ethos of having an aircraft that is as useless and fun as possible.

I'm not certain what you mean by "relying on silicon/GPS." Even certified EFIS systems determine attitude using solid state (either silicon MEMS or fiber optic) rate gyros + MEMS accelerometers + MEMS magnetometers + GPS, fused with a Kalman filter. Reading back to your original post, that is how I would interpret the description of the Flightview system.

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
Artificial horizons: garbage, don't need for VFR day, although night would be useful. Needs to be a GYRO thou, not silicon.
It's unclear what you're trying to say here. Only steam gauges use an actual gyro. Did you mean accelerometer-based AHRS (as opposed to GPS-derived attitude) instead of gyro?

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Uh, no. The reason is called "phase lead" by the control theory folks. Turn coordinator stirs in roll rate to tell you that you're starting to enter a turn, T&B indicates yaw rate directly. Lots easier to fly. My Ph.D. coursework was all about that kind of stuff in case you want to play the credentials game.
I learned on the T&B and never cared for the TC.

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Anyone here that has used a Narco Mark III coffee grinder radio ? I still have one.

A T&B and a Narco MK-3 and a ADF for IFR, what more would you want ?

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HBA Supporter
Yup.

BJC

#### pfarber

##### Well-Known Member
HBA Supporter
Boy, am I glad you weren't around for the 25+ years & >1000 hours I flew homebuilts without an EFIS, ADSB-out, Mode C, or even a basic transponder. I'd have missed out on a lot of (quite legal) fun if I'd known that, including (legally) flying inside a Mode C veil to/from a private airport under the 'wedding cake', about 3 miles from the Class B airport, and to/from Sun&Fun multiple times, flying within two Mode C veils.

Now that you've filled me in on what you won't allow, I guess that I'll have to stop flying day-VFR in my non-ADSB-out RV with an experimental EFIS. Boy, I really hate that I started reading the Homebuiltairplanes forum....

Charlie
Moderator Edit: Snipped some needless personal sniping.

An EAB for VFR day needs no instruments. As it was pointed out its no type certified. My question is that low end EFIS using GPS only is questionable decision since GPS altitude is not based off MSL.

Do pilots even realize that the EFIS is using GPS only data? For the minor amount of additional cost, you can add a barometric and temp sensor and program in a barometer adjustment. Like \$5.

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#### Marc Zeitlin

##### Exalted Grand Poobah
My question is that low end EFIS using GPS only is questionable decision since GPS altitude is not based off MSL.
Just out of curiosity, because I'm not familiar with them, which EFIS's use GPS only?

##### Well-Known Member
I’m not understanding your post, Ed.

No to what?

Are you agreeing with me that the T&B is much better than the TC?

What evoked that statement? I have posted anything about credentials.

BJC
I'm not following Ed's deal either. Maybe he had a not so hot start to the week?

BJC