Airspeed Indicator accuracy and the importance of a true static "system"

[Gilbert Pierce]

Hmmmm! I guess the four shops that I watched do the certification didn't get it right.

[Jim Hann]

dgapilot,

My radio guy disagrees. 413(c) specifies who can do the check. 413(b) only applies if maintenance or installation has been done, so any time you have work done a new 413(b) check SHOULD be completed. Only 413(a) applies if everything is functioning normally is how he explained it to me. That was all my airplane has had before the new transponder was installed. Appendix F to Part 43 only mentions Mode C in passing in (c)2 for receiver sensitivity.

Again, that is what my avionics guy showed me when I asked during my 413 check.

Jim

Appendix F to Part 43 - ATC Transponder Tests and Inspections
The ATC transponder tests required by § 91.413 of this chapter may be conducted using a bench check or portable test equipment and must meet the requirements prescribed in paragraphs (a) through (j) of this appendix. If portable test equipment with appropriate coupling to the aircraft antenna system is used, operate the test equipment for ATCRBS transponders at a nominal rate of 235 interrogations per second to avoid possible ATCRBS interference. Operate the test equipment at a nominal rate of 50 Mode S interrogations per second for Mode S. An additional 3 dB loss is allowed to compensate for antenna coupling errors during receiver sensitivity measurements conducted in accordance with paragraph (c)(1) when using portable test equipment.
(a) Radio Reply Frequency:
(1) For all classes of ATCRBS transponders, interrogate the transponder and verify that the reply frequency is 1090 ±3 Megahertz (MHz).
(2) For classes 1B, 2B, and 3B Mode S transponders, interrogate the transponder and verify that the reply frequency is 1090 ±3 MHz.
(3) For classes 1B, 2B, and 3B Mode S transponders that incorporate the optional 1090 ±1 MHz reply frequency, interrogate the transponder and verify that the reply frequency is correct.
(4) For classes 1A, 2A, 3A, and 4 Mode S transponders, interrogate the transponder and verify that the reply frequency is 1090 ±1 MHz.
(b) Suppression: When Classes 1B and 2B ATCRBS Transponders, or Classes 1B, 2B, and 3B Mode S transponders are interrogated Mode 3/A at an interrogation rate between 230 and 1,000 interrogations per second; or when Classes 1A and 2A ATCRBS Transponders, or Classes 1B, 2A, 3A, and 4 Mode S transponders are interrogated at a rate between 230 and 1,200 Mode 3/A interrogations per second:
(1) Verify that the transponder does not respond to more than 1 percent of ATCRBS interrogations when the amplitude of P2 pulse is equal to the P1 pulse.
(2) Verify that the transponder replies to at least 90 percent of ATCRBS interrogations when the amplitude of the P2 pulse is 9 dB less than the P1 pulse. If the test is conducted with a radiated test signal, the interrogation rate shall be 235 ±5 interrogations per second unless a higher rate has been approved for the test equipment used at that location.
(c) Receiver Sensitivity:
(1) Verify that for any class of ATCRBS Transponder, the receiver minimum triggering level (MTL) of the system is −73 ±4 dbm, or that for any class of Mode S transponder the receiver MTL for Mode S format (P6 type) interrogations is −74 ±3 dbm by use of a test set either:
(i) Connected to the antenna end of the transmission line;
(ii) Connected to the antenna terminal of the transponder with a correction for transmission line loss; or
(iii) Utilized radiated signal.
(2) Verify that the difference in Mode 3/A and Mode C receiver sensitivity does not exceed 1 db for either any class of ATCRBS transponder or any class of Mode S transponder.
(d) Radio Frequency (RF) Peak Output Power:
(1) Verify that the transponder RF output power is within specifications for the class of transponder. Use the same conditions as described in (c)(1)(i), (ii), and (iii) above.
(i) For Class 1A and 2A ATCRBS transponders, verify that the minimum RF peak output power is at least 21.0 dbw (125 watts).
(ii) For Class 1B and 2B ATCRBS Transponders, verify that the minimum RF peak output power is at least 18.5 dbw (70 watts).
(iii) For Class 1A, 2A, 3A, and 4 and those Class 1B, 2B, and 3B Mode S transponders that include the optional high RF peak output power, verify that the minimum RF peak output power is at least 21.0 dbw (125 watts).
(iv) For Classes 1B, 2B, and 3B Mode S transponders, verify that the minimum RF peak output power is at least 18.5 dbw (70 watts).
(v) For any class of ATCRBS or any class of Mode S transponders, verify that the maximum RF peak output power does not exceed 27.0 dbw (500 watts).
Note:
The tests in (e) through (j) apply only to Mode S transponders.

(e) Mode S Diversity Transmission Channel Isolation: For any class of Mode S transponder that incorporates diversity operation, verify that the RF peak output power transmitted from the selected antenna exceeds the power transmitted from the nonselected antenna by at least 20 db.
(f) Mode S Address: Interrogate the Mode S transponder and verify that it replies only to its assigned address. Use the correct address and at least two incorrect addresses. The interrogations should be made at a nominal rate of 50 interrogations per second.
(g) Mode S Formats: Interrogate the Mode S transponder with uplink formats (UF) for which it is equipped and verify that the replies are made in the correct format. Use the surveillance formats UF = 4 and 5. Verify that the altitude reported in the replies to UF = 4 are the same as that reported in a valid ATCRBS Mode C reply. Verify that the identity reported in the replies to UF = 5 are the same as that reported in a valid ATCRBS Mode 3/A reply. If the transponder is so equipped, use the communication formats UF = 20, 21, and 24.
(h) Mode S All-Call Interrogations: Interrogate the Mode S transponder with the Mode S-only all-call format UF = 11, and the ATCRBS/Mode S all-call formats (1.6 microsecond P4 pulse) and verify that the correct address and capability are reported in the replies (downlink format DF = 11).
(i) ATCRBS-Only All-Call Interrogation: Interrogate the Mode S transponder with the ATCRBS-only all-call interrogation (0.8 microsecond P4 pulse) and verify that no reply is generated.
(j) Squitter: Verify that the Mode S transponder generates a correct squitter approximately once per second.
(k) Records: Comply with the provisions of § 43.9 of this chapter as to content, form, and disposition of the records.
[Amdt. 43-26, 52 FR 3390, Feb. 3, 1987; 52 FR 6651, Mar. 4, 1987, as amended by Amdt. 43-31, 54 FR 34330, Aug. 18, 1989]

[dgapilot]

Hmmmm! I guess the four shops that I watched do the certification didn't get it right.

I ran two different shops and worked at a third. All 3 did the full Appendix F and Paragraph C of Appendix E for a VFR test to comply with 91.413. The FAA in 2 different FSDOs required it in the QA manual procedure.


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[Jim Hann]

The FAA in 2 different FSDOs required it in the QA manual procedure.

(In my best Jamie Hyneman voice) There's your problem!

Same three letter reason I had to use an IA under a different FSDO to get a Field Approval, locally three IAs and a shop wouldn't even try, they have been shot down so many times for FAs at our local FSDO. So much for uniformity.

dgapilot, I hope you don't think I'm mad at you or don't believe you, I do. And yes, my avionics guy is a CRS.

I pulled out my logbooks and looked at the sticker/signoff. It says I/A/W Appendix F and Appendix E(c). Sooo, all I can surmise from his comments and yours is that our FSDO (and apparently Gilbert's) only require 1 data point for Mode C correlation, field elevation. Thanks FAA for making something else clear as mud. Ugh!

14CFR43 Appendix E (c) Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Integration Test. The test must be conducted by an appropriately rated person under the conditions specified in paragraph (a). Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C at a sufficient number of test points to ensure that the altitude reporting equipment, altimeters, and ATC transponders perform their intended functions as installed in the aircraft. The difference between the automatic reporting output and the altitude displayed at the altimeter shall not exceed 125 feet.

[Gilbert Pierce]

I will make sure to keep using one of the four shops that have done it in the past.

[MN_flyer1]

I think in a "nut-shell" that the only requirement for a VFR transponder check is that the altitude correlation between the altimeter and encoder is within 125 ft on the ground only. Hence the static system is not checked.

If you have an IFR check done then they have to pressure check the static system right at the connection to the static port. They have to pump a vacuum on the system with a calibrated meter attached to make sure the Altimeter & Encoder correlate within a "spec" to the calibrated altimeter all the way up through the encoder limits I believe.

I am not a avionics guy but I can see how some ugly stuff can happen. For a VFR check you wouldn't have to look at the plumbing under the dash.....

[dgapilot]

(In my best Jamie Hyneman voice) There's your problem!

Same three letter reason I had to use an IA under a different FSDO to get a Field Approval, locally three IAs and a shop wouldn't even try, they have been shot down so many times for FAs at our local FSDO. So much for uniformity.

dgapilot, I hope you don't think I'm mad at you or don't believe you, I do. And yes, my avionics guy is a CRS.

I pulled out my logbooks and looked at the sticker/signoff. It says I/A/W Appendix F and Appendix E(c). Sooo, all I can surmise from his comments and yours is that our FSDO (and apparently Gilbert's) only require 1 data point for Mode C correlation, field elevation. Thanks FAA for making something else clear as mud. Ugh!

14CFR43 Appendix E (c) Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Integration Test. The test must be conducted by an appropriately rated person under the conditions specified in paragraph (a). Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C at a sufficient number of test points to ensure that the altitude reporting equipment, altimeters, and ATC transponders perform their intended functions as installed in the aircraft. The difference between the automatic reporting output and the altitude displayed at the altimeter shall not exceed 125 feet.

Jim,no offense taken. We're all here for entertainment and to learn. So folks may have thin skin. I realize I'm, shall we say, opinionated, and come off a little strong at times myself. All is good.


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[Gilbert Pierce]

I think in a "nut-shell" that the only requirement for a VFR transponder check is that the altitude correlation between the altimeter and encoder is within 125 ft on the ground only. Hence the static system is not checked.
..

That has been my experience as well.
As an ex Navy avionics tech in a previous life I always visit with the guy doing the check and understand what he Is doing.

[Homer Landreth]

I think in a "nut-shell" that the only requirement for a VFR transponder check is that the altitude correlation between the altimeter and encoder is within 125 ft on the ground only. Hence the static system is not checked.

If you have an IFR check done then they have to pressure check the static system right at the connection to the static port. They have to pump a vacuum on the system with a calibrated meter attached to make sure the Altimeter & Encoder correlate within a "spec" to the calibrated altimeter all the way up through the encoder limits I believe.

I am not a avionics guy but I can see how some ugly stuff can happen. For a VFR check you wouldn't have to look at the plumbing under the dash.....

The confusion of these can be fairly easily resolved:

FAR 91.411 says "No person may operate an airplane, or helicopter, in controlled airspace under IFR unless - . . . . . .
and points to FAR 43 Appendix E
FAR Appendix E
The altimeter is subjected to a series of tests to insure it meets the minimum requirements for accuracy and repeatability.
The altitude encoder is checked against the primary altimeter and adjusted as required to insure data correspondence.
After the equipment has been tested and inspected and (if removed) reinstalled in the aircraft, a static system leak check is performed

FAR 91.413 says "No persons may use an ATC transponder that is specified in 91.215(a), 121.345(c), or § 135.143(c) of this chapter unless, . . .
and points to FAR 43 Appendix F.
FAR 43, APPENDIX F, describes the transponder inspection requirements. The tests are fairly simple and can be performed on the ramp in about 15-30 minutes.

So if you are going to operate IFR you you do Appendix E and Appendix F

If you are NOT going to operate IFR, you skip Appendix E and ONLY do Appendix F.

When you read the totality of these tests, you realize that Appendix E is really not a Transponder check at all, all those checks are
actually accomplished in Appendix F.

[Kelliots]

This is very interesting considering my latest situation. I replaced my ancient cave-man round transponder with a KT-76A in the panel under my KX-125. I then hooked it up to my ACK encoder, which has two very small screws which are used to adjust the encoder altitude readout for fine tuning. It works great but when doing in flight altitude checks with my local approach control, the altimeter readout was 100' - 200' higher than what they were reporting me as. On top of that, my King AV8OR GPS moving map is showing me 50' - 300' higher than my altimtere when set to current barometric pressure. So I adjusted the two little screws per instructions in the ACK encoder manual and checked with Tulsa approach again at 3500' and it was spot on with the altimeter, but the altimeter reading still doesn't agree with the GPS altitude. I thought the GPS WAAS accuracy was something like 3 meters, but then I got to thinking that was lat/long position accuracy and probably not altitude readout accuracy. Does anyone have information on the altitude readout accuracy of our moving map GPS units like the AVOR Ace? Also I've decided I am going to take my altimeter into the shop to have it calibrated, as I think that is part of the problem. kelliots '61 Colt