You first
Stephen Lindberg said:
A long time ago I read that the early air mail pilots, pre-Jimmy Doolittle/Sperry, who were caught on top would turn their DH-4 onto a magnetic heading of 180 and let down using the mag compass to indicate a turn. If the air was smooth and your luck was running hot and strong, this just might work. The dip error in the mag compass would cause it to lead turns when on a heading of 180, and therefore exaggerate any deviations from wings level flight. Anyone ever try this? Plan B would be to join the Silkworm Club, as did Charles Lindbergh. He landed his parachute, walked over to the wreck, and retrieved the mail sack, as I recall.
With a name like Lindberg you try it first?
I recall as an inst student my insturctor mentioned the wet compass on 180 could be used for wings level. So we tried to fly with it. The problem is with a C-172 and a hood, you can't help but see out, so it is academic. It was one of those things you thought, nice to know but not practical. I never tried in in IMC. Later as a CFII, I never emphasised using the compass for an emergency wing leveler. Unless you are going to practice it for real, it is way out there. Also practice of the technique is hard to achieve on some planes with the compass above the glare shield. May be in a simulator. This would be one for the myth busters.
I taught timed turns for partial panel, use standard rate and counting to your self, roll level, wait, wait, wait, for the compass to stopped spinning, than look, back to needle-ball-airspeed(_alt/vsi). Staring at the compass in a turn, IMC, partial panel or otherwise is a good way to loose control. It just bounces around to much, especially when you are turning. So really I only let my students look at it when level, that was the only practical use IMHO.
In the case of it being your only bank inst, if you get it to swing, you are in trouble while you wait for it to come back, if it ever does.
On a related note, an examiner tried to tell me the C-172 was so stable you could let it just descend and let go of the yoke, it would spiral safely down. Needless to say the demo went horribly wrong. As the bank increased the speed built, as the bank increased further the nose dropped, the more the bank got ........ well "divergent", more bank and more nose drop, spectacularly. I guess he was wrong.
Add a RV, fast and low drag to the equation, with little to no roll stability, more problems. If you lost it, you would be ............ well you know. Now I did try my Gramin with the HSI display. Covered up my RV-4s gyro and tried to fly as best I could. Now there was no hood and I was VFR. It seemed like I could do it, but than again I was VFR and I was relaxed.
The GPS DG may be more stable than a wet compass on 180, and it might be OK in a C-182? In a RV for real? No. Obviously the update rate may not be fast enough. If you had to do it, it would be a pucker factor 10 for sure. The problem with the wet compass or GPS is if you loose it, you can't really recover. I have watched enough wet compasses in practice with my students to know that compass turns are just not practical. In the RV the speed builds so fast you will be in trouble.
To add to the legend, I have heard people say an AFD was used or could be used as BANK instrument to save the day. If that was in actual and they survived, just using the ADF for bank, they had nerves of steel and big brass ones. I guess practice makes perfect. Like I tell anyone with just a TC or T&B for back up, you better practice needle-ball-N-airspeed, often, if you think you can pull it off in IMC.
I flew "partial panel" for real in a RV-4. Vacuum pump pooped out. I was of course in and out of IMC. I lived, but the T&B was swinging around, back and forth with the turbulence and the planes natural dutch roll. I broke out between layers in a few minutes, thankfully. I don't mind saying I was a little scared. I did not really know how bad much my old T&B (which was rebuilt and yellow tagged) to fly by. My bad. I should have practiced in that plane with that instrument. In a C-172 and a TC it was a non event, but the RV yaws just enough to wag the needle like a happy puppy dog, which I never really notice with stable Gyros working. It is not comfortable splitting the difference to guess if you are yawing or level.
So to your original question, flying by mag compass. I think it is impractical except for those who might practice it to proficiency and in optimal calm conditons in a super stable plane. I think with GPS the mag compass is almost as needed as an ADF, not very. In the Boeing I don't recall looking at it very much, oh yes to check the PFD, primary flight display heading, based on the IRS, inertial refrence system.
