primary flight instruments on partial panel

"Greg" wrote in message
ups.com...
In primary and supporting method of attitude instrument flying, which
do you consider the primary instrument for bank in the partial panel
situation (loss of both attitude indicator and heading indicator)?
Turn coordinator or magnetic compass?

The turn co-ordinator is your primary measure of bank, for a couple of
reasons. First, the compass doesn't measure bank angle - it just shows what
direction you're pointing in - whereas the TC does show the degree to which
you're banked. Second, and most importantly, because of the way it's put
together, the compass doesn't turn steadily, and so it'd be wrong to fool
yourself into thinking that the faster the compass was moving, the more
you're banked.

Have a go one day and you'll see. Take someone competent with you to look
out of the window, get to a safe place and height, make sure your DI's set
correctly, and do a rate-one turn through 720 degrees or more (go around as
much as you like, in fact). In the northern hemisphere at least, you'll note
that when the DI goes through 090 or 270, the compass stands a fair chance
of agreeing with the DI. As you go through north, though, the compass will
under-read, and as you go through south, it'll over-read. The only time you
can rely on the compass is when going in a straight line - which is why you
always check and adjust your DI when flying in a straight line.

Hence if you want to do a partial-panel turn in IMC, you fly straight and
check the compass, then do a rate-one turn (the turn co-ordinator shows you
the bank angle) for the required number of seconds - three degrees per
second for a rate-one turn, of course. You then level out, give the compass
a few seconds to stop wiggling, and then adjust your heading if you're a
little bit out. I seem to recall in the IMC rating test you have to get to
within five or ten degrees of the desired heading with one initial turn and
one "fine tune" adjustment - which is generally pretty achievable.

David C