Compass Reliability

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GAHorn
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Joined: Fri Apr 12, 2002 8:45 pm

Compass Reliability

Post by GAHorn »

The thread about swinging a compass reminded me of two embarrassing events in my flying career related to use of compass.

The first event was mostly idiotic. The second was potentially very dangerous.

The first event was in a Cheyenne II departed Atlanta Peachtree-Dekalb for College Station. During the relatively calm enroute phase my co-employee (MIke McGrath, fully qualified…we traded seats on alternate legs) and I were casually snacking on Planters Peanuts and Cokes. Almost absent-mindedly we noticed the dual-HSI appeared to “drift”. This became a discussion of curiosity and commentary about the recent instrument shop mx trip the airplane had undergone.
A new Primus radar had been fitted and the shop had taken the remote gyros out of the plane for calibration since the radar used the VG for stabilization. Since they were working on the VG, we’d authorized them to bench-test/re-calibrate the DGs also.
Now, while enroute we noticed both the HSIs (slaved to flux-valves connected to the DGs) were deviating from the magnetic headings/courses of our routing. (This was late ‘70s when RNAV-Direct was less common,… We were navigating primarily VOR-to-VOR along airways and jet-routes.)
After a few re-slaving operations using the tiny toggle-switch which corrects the HSI’s back to agree with the Mag Compass…. our heading to remain on-course had become pretty divergent …which we initially blamed on strong cross-winds at altitude. It looked like the winds-aloft forecast for FL240 really missed the boat! We made mention or it to Flight Watch as a PIREP.
After awhile I decided to put the peanuts away and toss the plastic cups we’d been drinking from into the small trash-can behind the right-seat. I asked Mike “You want anymore peanuts?”…. “No”, he replied.
I reached down to grab the plastic lid. to snap back onto the can which we had sitting on top of the glareshield…. pulled the can off the glareshield to snap the lid on it and store the can away in the cabinetry….. and saw the compass take a 30-degree “swing” as the peanuts were removed from the glareshield. (cardboard container…. metal bottom). 8O

(Yes, we had to re-slave the HSI’s and the “strong cross-winds at altitude Pirep” we’d issued earlier..?? …we decided to just not say anything more.)
:oops:

The second event occurred in my 170-B while at night while VFR. We (Jamie, myself, and our friend and now “Gone West” Member, Bob Edmondson) had departed David Wayne Hooks about 10PM headed West back to our home just west of Austin, We had been maneuvered by DWH tower out of the pattern, cleared to “proceed on-course, and change frequencies” from DWH tower …and as it was late night and no traffic on the radio out west of the Class-C…. We leveled-off just below an overcast at 7,000 MSL to cruise at 6,500 and switched the Com to monitor 121.5 while enroute.
There rapidly developed an undercast/ground-fog layer over the entire area….. apparently the DWH airport had been in a small area of no fog…but the entire Houston area was rapidly becoming fog-bound.
it was kind of “eerie”… as the city lights beneath us illuminated the undercast into a positively ORANGE GLOW. The overcast also reflected the city lights in a similar hue. It was almost as if we were in the Starship Enterprise …-flying-along …in this strange Orange atmosphere.
We had excellent visibility between the layers…. probably 10 miles or more….but there was nothing to see except these Orange cloud-layers above and below us as far as one could see.

About this time I noticed our “original” style AN-5735 Directional Gyro was indicating about 260-degrees heading…while our mag compass was showing 80 degrees.

I have the habit of setting my D.G. before takeoff to the runway heading…(confirming it agrees also with the mag compass)…. then immediately after lift-off… I confirm both the Art. Horiz and the D. G are “erect” and spinning with good vacuum… then I UN-cage them to agree with current attitude and heading.

But we had been thru a period of “maneuvering” immediately after take-off in the dark…and shortly after turning West we had entered this area between Orange-Layers… and had no more outside reference. We were flying in VFR conditions…but forced to rely upon cockpit instrumentation….in this eerie time-warp/orange-glow. To complicate matters…the cloud layer above was not perfectly level with the horizon..it slanted slightly to the north, which made concentration upon the Art. Horizon somewhat intense. (Not sure I can relate the odd situation with textual description alone…but suffice it to say it took considerable effort. And to further complicate matters…now my “hockey-puck” D.G. totally disagreed with my magnetic compass almost a full 180 degrees! What to believe!)

It would seem a simple matter to place the most confidence in the mag compass…. except that I’d taken off toward the North and all my turns toward home were to the Left, i.e., to the West. But my Mag Compass said East and my D.G. which I’d set immediately after lift-off said West. And with the horizon not visible and the overcast/undercast not being “level”…. it was a real challenge, let me assure you!

About this time I relized that IF the mag compass were telling the truth then We’d be heading straight back into the Class-C airspace of Houston Intercontinental at 6500’. 8O There was simply NO WAY for my brain to wrap around that because I’d only made a few turns to the west after departure. Bob and I discussed this and he was just as bewildered as myself. In the back seat Jamie had her head on a pillow trying to get some sleep….and I felt almost drunk with confusion.

I decided I needed help to sort this out, so I called Houston Approach and Identified myself as being “ VFR somewhere northwest of DWH, between layers with a conflicting heading indication” …and I asked for radar identification and confirmation of “track”. Houston was immediatly helpful, issued a “squawk”, requested “Ident”…and confirmed I was almost back over my point of departure and heading East. 8O

The mag compass was (as we’ve been taught to believe) absolutely correct.

I thanked Approach, re-set my D.G. and turned West, and remained on flight-following until they handed me off to Houston Center. By this time we were just south of Brenham, the “Orange Julius Glow” had disappeared…and in-fact a “starry starry night” had developed overhead. We had lost the overcast and there was only some patchy ground-fog below. The rest of the flight home went just fine…the forecast for home was true …(CAVU)…. and it was nother learning experience…even for this ten-thousand-hour ATP who received a much-needed slap in the face and reminder that perhaps he might occasionally suffer a D.G. failure.
Yes, it was discovered over the next few flights that the D.G. was indeed slowly dying and was sent for overhaul.

Single engine, at night, between layers …is IFR regardless of what the weatherman says. :oops:
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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