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Encoding Altimeter

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bakerocb

Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

Posted: Mon Apr 21, 2008 3:13 am Post subject: Encoding Altimeter

4/20/2008



Hello Anonymous, You wrote:



1) "You seem to be saying that the only practical way to meet the

requirements is to use a TSO'd source for the altitude information the

transponder is transmitting."



That is close. Here is how I would phrase it: "At present the only practical

way to be in compliance with FAR Section 91.217, Data Correspondence Between

Automatically Reported Pressure Altitude Data and the Pilot's Altitude

Reference, is to use a TSO'd equipment source for the altitude information

the transponder is transmitting."



2) "That, of course, would eliminate valuable functionality of many

non-TSO'd electronic EFISs including the serial output to the transponder."



Specifically, the serial altitude data output to the transponder from a non

TSO'd altitude encoder within an EFIS would not, at present, be in 

compliance

with either 91.217 (b) or (c).



3) "Clearly, that isn't happening in the real world.  These are selling

well."



Correct.



4) "Are you against that?"



Not at all. Here are my positions:



A) People should make informed decisions.



B) If I can provide accurate information to people that will permit them to

make informed decisions I should do so.



C) The decisions that those people make, after I have given them the best

information that I have available, is theirs to make, not mine. The risks

that they chose to take, or not take, are theirs, not mine.



D) Most of the EFIS available to the amateur built community represent a

significant improvement in performance, reliability, and safety over

previously available flight instrument technology.



E) Most of the EFIS available to the homebuilt community contain a non-TSO'd

altitude encoder that is superior in performance, reliability, granularity,

and accuracy over altitude encoding equipment that was manufactured to

versions of TSO-C88 prior to TSO-C88b.



F) Amateur built experimental aircraft have flown thousands of hours using

EFIS with non-TSO'd altitude encoders feeding their transponders with no

apparent problems.



G) Hundreds of amateur built experimental airplanes are under construction

using EFIS containing non-TSO'd altitude encoders.



H) The FAA should recognize and accept the real world conditions described

in D, E, F, and G above.



I) The best way for the FAA to accept the real world conditions described in

D, E, F, and G above is to interpret the tests required by FAR Secs 91.411

and 91.413 (as appropriate) as fulfilling the requirements of FAR Sec 91.217

(b).



J) My initial attempts to accomplish H and I above with FAA HQ were met with

resistance and I ceased activity in this regard.**



K) If the FAA, and the people / entities who perform the tests required by

91.411 / 91.413, decided that every non-TSO'd altitude encoder in an EFIS

was not airworthy because it did not meet the requirements of either 91.217

(b) or (c) this decision would be a serious blow to the amateur built 

community.**



L) I do not proactively broadcast the information in K above, but if someone

asks a specific question on this issue or posts a position that I know to be

in error I revert to my positions A and B above.



M) I am open to improvements or changes in my positions.



'OC' Says: "The best investment we can make is the effort to gather and

understand knowledge."



**PS: The FAA decision makers who perpetrated the recent fiasco involving 

hundreds of airline flights being canceled and thousands of people being 

stranded over the issue of the exact spacing of electrical wire cable ties 

in the landing gear wheel wells of airline aircraft are capable of such 

thinking and actions.



------------------------------------------------------------



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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Mon Apr 21, 2008 7:01 am Post subject: Encoding Altimeter

One factor to remember is the operation of the encoding altimeter.  To send

the correct information it needs to be set at 29.92 in. hg..  That for all

intents and purposes means you would have to either turn off your Xpndr or

have a second altimeter for lower altitudes.  The big deal is every one

reports altitude to the same reference and the system as a whole operates

within parameters.



Noel



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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

reichec

Joined: 23 Apr 2007
Posts: 26

Posted: Mon Apr 21, 2008 2:34 pm Post subject: Encoding Altimeter

What?



The code that the encoding altimiter sends out is the altitude when you set 

the alitimiter to 29.92, thats called  pressure altitude.  No matter where 

you set the altimeter, the encoder portion still reports the altitude as if 

the thing is set to 29.92. That never changes.  You twist the knob to set 

your local atimiter setting so your dial reads zero when you are at sea 

level or actual field elevation for you locale.  The system is calibrated at 

29.92 and certified as to scale error +/- 25 feet when the altimiter is 

certified every 2 years.  You dont need 2 altimeters. Setting the dial 

anywhere you want on an encoding altimeter has no effect on the encoder 

portion of the device.  Some encoding altimeters have what is called a baro 

pot to send altimeter setting info to certain autopilots.  Anyhow. ATC 

corrects the number that your encoder sends out to the transponder and then 

out to ATC for local altitude changes in pressure. You dont need to do this 

for them and I certainly hope none of you have been leaving your encoding 

altimeters on 29.92 thinking that you are doing them a favor.

In fact I would discourage people from owning an encoding altimeter as the 

are a maintenance nightmare in some cases. They are expensive to overhaul 

and if it comes up at your 2 year inspection that either the altimeter half 

or the encoder half is out of spec, it makes the whole thing unairworthy. 

Its cheaper in the long run to stick with good certified altimeters like 

United instrument and encoders like the trans-cal SSD120-30A. 

ACK/narco/Ameri-king encoders are cheaply built and you get what you pay 

for.



Charles Reiche



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wgill10(at)comcast.net
Guest

Posted: Mon Apr 21, 2008 8:05 pm Post subject: Encoding Altimeter

OR...you could buy a Dynon and save even more during the IFR

recertification check. In fact, the avionics shops actually prefer the

Dynon due to their accuracy and simplicity.



Bill





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bakerocb

Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

Posted: Tue Apr 22, 2008 5:26 am Post subject: Encoding Altimeter

4/22/2008



Hello Bill, Thanks for your input. You wrote:



1) "OR...you could buy a Dynon ........."



Since the Dynon units do not contain a TSO'd altitude encoder operating the 

Dynon unit alone to feed a transponder in flight would presently leave one 

in violation of FAR Sec 91.217 (as interpreted by FAA HQ). There are two 

ways to approach this condition:



A) Purchase and install a separate TSO'd altitude encoder and use that 

encoder to feed the aircraft's transponder.



B) Decide that violation of 91.217 is an acceptable risk and feed the 

transponder from the non TSO'd altitude encoder contained in the Dynon unit.



2) ".....and save even more during the IFR recertification check."



My local  avionics shop charges a flat fee for the FAR Sections 91.411 and 

91.413 required testing. This fee has been the same no matter whether I do 

the labor of removing and reinstalling the altimeter and TSO'd altitude 

encoder, for bench testing and any adjustments needed prior to the airplane 

side testing, or they do that labor. (By the way, four years ago that flat 

fee was $150, now it is $300.)



3) "........the avionics shops actually prefer the Dynon due to their 

accuracy and simplicity."



I hope that the avionics shops continue this practice without any regard for 

what FAR Sec 91.217 says.



4) ".......the avionics shops actually prefer the Dynon due to their 

accuracy and simplicity."



I wonder about the complexity of removing and reinstalling an EFIS from the 

airplane compared to the complexity of removing and reinstalling an 

altimeter and separate TSO'd altitude encoder should any bench testing and 

adjusting be needed.



A) Maybe EFIS removal and reinstallation is no big deal, but I envision a 

bunch of connections, particularly if it is the type of EFIS that also 

includes displaying engine performance parameters.



B) Maybe the EFIS's have no means of local avionics shop adjustment.



C) Maybe the EFIS's would never need adjustment.



D) What has been the experience of EFIS operators that have actually been 

through FAR 91.411 and 91.413 checks, as appropriate, after flying for at 

least two years?



Would any EFIS owners and operators care to comment on these points?.



'OC' Says: "The best investment we can make is the effort to gather and 

understand knowledge."



---------------------------------------------------



Time: 09:05:34 PM PST US

From: "William Gill" <wgill10(at)comcast.net>

Subject: RE: Encoding Altimeter

OR...you could buy a Dynon and save even more during the IFR

recertification check. In fact, the avionics shops actually prefer the

Dynon due to their accuracy and simplicity.



Bill

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reichec

Joined: 23 Apr 2007
Posts: 26

Posted: Tue Apr 22, 2008 2:06 pm Post subject: Encoding Altimeter

I have no experience with the dynon other than I want my friend to put one 

in his rv7. But I can tell you that in installations such as this and like 

the G1000, its easier to certify devices in the airplane rather than 

removing them so you can be using whatever its readout is to certify it.  I 

have run across one GDC 74A (Garmin g1000 air data computer) that failed a 

test point just slightly out of spec but garmin allows us to recalibrate and 

electrically slide the scale so everything is within spec all the way up. 

Remember that down low the tolerance is +/- 20 feet for altimeter 

certification and up over 20k its well over +/- 100 feet.  Mechanical 

altimeters can and do sway their actual reading all over the place up at 

altitude, and generally the air data computer tpye devices are pretty darn 

close.

YMMV



Charlie



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wgill10(at)comcast.net
Guest

Posted: Tue Apr 22, 2008 5:54 pm Post subject: Encoding Altimeter

I have the Dynon and a mechanical altimeter...both remained in the plane

for testing and both altimeters met the requirements. However, the

transponder was removed for bench testing.



Bill



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