L-29 TFBs and PEC Mil-Specs

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Tower-fly-by has always been one of the most precise methods of altimeter calibration. As part of an exercise preparation at ITPS, I flew on a number of TFB test points in the Czechoslovakian Aero L-29 Dolphin to verify the limited numbers found in the AFM and check Mil-Spec compliance.

While the concept of the Flight Test Technique is pretty straightforward, the data reduction and analysis required for Mil-Spec compliance is not. Is there more than one Mil-Spec? Which one should be chosen? What do they require? Are they currently cancelled?

l29-TFB

In short, the MIL-I-5072-1 covers all types of pitot static tube operated instrument systems while MIL-I-6115A deals with instrument systems operated by a pitot tube and a flush static port, as is recommended by USN TPS. The quantitative numeric side of the tolerances described in the above two specs is exactly the same. USAF however, seems to have taken those specs a step further through  MIL-P-26292C(USAF), which puts more focus on high Mach number and supersonic region and defines the tolerances based on parameters derived from dimensional analysis (ΔP/qc) rather than absolute tolerances in knots and ft. For data presentation purposes, all specs require analysis of the errors to sea level conditions vs. indicated speed parameters, which again is not a single step process.

While all specs are currently cancelled without replacement, probably due to the many new and non-traditional air data systems and the air data computers used, they remain a good guidance especially for older airplanes.

Unsurprisingly, as the figure below shows, the L-29 failed to meet the  MIL-P-26292C(USAF):

MIL-P-26292C-L29

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2 Responses to L-29 TFBs and PEC Mil-Specs

  1. Bill Walker says:

    Before you cancel the aircraft Type Certificate ;):

    Most TFB data reduction methods assume all the observed error is a static error, and non is a pitot error. Have you any other data to confirm if there is, or is not, pitot error present as well? GPS method, for example? Might be a good student exercise here.

    In my experience, the only way to be sure you have correctly identified pitot and static error contributions is to compare aircraft pitot and static pressures to reference pressure sources (boom, trailing bird or cone, etc.)

    • aeroscience says:

      Dear Bill,

      Thanks for the excellent point!

      My Instructors at ITPS when I was a student taught me to always crosscheck my results and examine my assumptions before making any conclusions ;) . So here is my reply:

      These results have been confirmed by other methods as well. Both by a “modern” version of the ground course method (which determines airspeed and thus dynamic pressure error) and was in very good agreement to the TFB results, as well as some GPS test points. I just did not plot that data as it is part of the students report, not mine ;)

      Moreover, excellent point on using FTTs that compare to reference sources, I absolutely agree. MIL-Specs and AC25 actually only accept those techniques. However, consider that the TFB is in concept a reference comparison FTT as the indicated Hp is compared to the reference external gauge pressure altitude.

      On the assumption that Pitot error is zero, while we always question it by comparing FTT results, it is interesting to note that it is universally recommended for fixed-wing (Mil-specs, AC25, AC23) and my small flight test experience has shown that it really always works.

      Also, on the last line of your message you are referring to “trailing bird method”… You told us about the trailing bomb, but never the “bird” version… I think you need to come back for more! :)

      Always honored to have your posts on my blog.

      Cheers!

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