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From: al@qiclab.uucp (Al Peterman)
Newsgroups: rec.aviation
Subject: Re: Required Instrument Calibrations
Keywords: localizer glideslope calibrate else splat?
Message-ID: <1990Oct24.195029.2347@qiclab.uucp>
Date: Wed, 24 Oct 90 12:50:29 PDT
References: <13493@june.cs.washington.edu>
Organization: SCN Research/QIC Laboratories of Tigard Oregon
Lines: 63

In article <13493@june.cs.washington.edu> pablo@cs.washington.edu (David Cohn) writes:
>
>I've noticed what seemed to be a gap in coverage of regulations on
>inspections of equipment: While the static system (alt. and airspeed)
>and transponder (ballast) must be tested and calibrated every two
>years, and VORs must be checked for accuracy every 30 days, I could
>find no regulation anywhere specifying testing, or even error requirements
>for ADFs or that most crucial precision-dependent system of all, the
>localizer/glideslope!
>
>My main concern in this stems from the fact that the ADF in ol' 54088
>always seems to point about 15 degrees left of true bearing. I had
>originally assumed that this was within tolerance...


OK, I'll give some lowdown on avionics inspections.  As Pablo mentioned
there is NO regulations on part 91 aircraft for regular inspections of
the ADF or Glideslope.  There is also NO check made of the Localizer!
The localizer signal is converted to a deviation on the CDI by entirely
separate circuitry than the VOR signals, so accuracy of the VOR check
has nothing to do with good localizer operation!

Now on Part 135 aircraft there is a requirement for an annual check of
all systems, including avionics.  There should be an approved checklist
that tests all systems in all modes.  It's kind of fun to spend an hour
or two trying out all the switches in a Gulfstream G-II or a Lear 35...
and at the end of that checkout, all avionics have been tested.

For part 91 aircraft, the owner or pilot may wish to test the avionics
that have no "required" tests on a regular basis.  This can be done
either by using test gear, or with some careful observation of normal
navigation signals.  Any avionics shop will have a portable tester that
can easily test the localizer and glideslope for correct indication and
accuracy in a couple of minutes.  Many won't charge to check they are
working correctly.   The ADF is harder to "test" in the aircraft using
test signals, so the shop will use a known signal and see if the
indicated bearing is correct.  Typically ADF indications should be
within 5 degrees, and there are ways to correct for errors.  The 15
degree error mentioned is excessive.  It's easy to line your plane up on
a runway and see that the indication to a know compass locator beacon is
correct.  If there is an error, you might try doing a reading at each 90
degree heading, and checking your compass for accuracy.  Of course you
may need clearance to do compass swings on an active runway!  :-)

As a pilot, you can "test" your localizer by checking that the localizer
shows you centered on the localizer at the outer marker.  Anything even
approaching a one dot error should be considered reason to have it
checked.  Likewise, the altitude at the outer marker will verify the
glideslope as correct.  It's best to identify the OM by visual means to
be sure of the accuracy of the indications.  Assuming about a 5 mile
final approach from the outer marker the altitude should be within 100
feet for good accuracy.  Also dual nav systems can "cross check" to
see that both systems give similar (hopefully correct) readings.  Just
remeber that there are common failure points for dual nav systems,
such as antenna systems.

Now you see why I suspect Loran is as accurate as other navigation
systems..at least some of them do internal data integrity checking..


-- 
Alan L. Peterman                                   (503)-684-1984 hm
                       cse.ogi.edu!qiclab!al