Backup gyros - which do you trust?

There are several things you can add to help with the identification, In my
plane I have a low vacuum warning light (part of the precise flight backup)
mounted between the AI and DG. The AI is one of the sigmatec ones with a vacuum
flag, so that if vacuum is lost in the instrument but not in the system I still
know about it right away. These warnings cover identification of the more
common cause of loss of the AI. The other failure mode would be failure of the
gyro, in which case I don't believe you get the insidious gradual spin-down like
you do with loss of vacuum. I also fly with the GPS on the HSI page to offer
yet another source of redundancy.

Personally, I think the instrument scan typically taught relies too heavily on
the AI given its relatively low reliability. Unfortunately, the alternative is
a scan that works a bit more like a partial panel scan using the AI as
supporting, not primary. Such a scan is much harder to master and requires
considerable finesse to keep from chasing the needles. It is not one I would
expect to be able to teach someone just learning to fly by instruments.


--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

You have three sources of bank information in a typical panel: the attitude
indicator, the turn coordinator, and the heading indicator. If two agree and
the third does not, it is faulty. Add a fourth source and it makes
elimination that much easier.

Bob Gardner

"Steve House" wrote in message
...
I've been reading with interest the several threads where a number of
people
have strongly pointed out the advantages of a backup electric AI to
supplant
a vacum driven main AI. But I'm reminded of the saying "A man with a good
watch always knows what time it is. A man with two watches is never
sure."
So I'm toodling along in IMC with no outside horizon reference and I see
my
two AIs don't agree with each other. How do I determine which to trust?
If
I had a third, I could go with a 2 of 3 voting strategy of course, but
with
only two, what do you do to decide which is operating properly and which
one
has faulted? Obviously I can look for consistency with other
instruments -
does my DG or Turn indicator show I'm turning, does the VSI show a climb
or
descent - but what would be the best strategy given the various ways
vacuum
or electric driven instruments can fail?

The presence of two attitude indicators is especially valuable when they
disagree. That disagreement will direct your attention to the needle/ball
and basic flight instruments to help determine which one is correct. With a
single AI you could more easily follow a gyro error without noticing a
difference in the other basic instruments until it was too late.

--

Darrell R. Schmidt

B-58 Hustler History: http://members.cox.net/dschmidt1/


"Steve House" wrote in message
...
I've been reading with interest the several threads where a number of
people
have strongly pointed out the advantages of a backup electric AI to
supplant
a vacum driven main AI. But I'm reminded of the saying "A man with a good
watch always knows what time it is. A man with two watches is never
sure."
So I'm toodling along in IMC with no outside horizon reference and I see
my
two AIs don't agree with each other. How do I determine which to trust?
If
I had a third, I could go with a 2 of 3 voting strategy of course, but
with
only two, what do you do to decide which is operating properly and which
one
has faulted? Obviously I can look for consistency with other
instruments -
does my DG or Turn indicator show I'm turning, does the VSI show a climb
or
descent - but what would be the best strategy given the various ways
vacuum
or electric driven instruments can fail?

"Ray Andraka" wrote in message
...

Good reason not to depart in low IMC I guess. I agree, that the AI is a
weak
link and carries with it some risk. While nice to have, cost and panel
space
make the back up AI difficult in some aircraft.

I think it is a matter of priorities... in an airplane which is IFR
certified, it is hard to believe there is not some space or economic
compromise which could not be made if a pilot felt this were an important
enough issue. I know I will get diagreement on this as always, but I think
the electric AI comes first before an IFR GPS. Even C152s sometimes have
Garmin 430s/530s nowadays; an electric AI would make much more sense IMHO.

As an even better solution, Hal Sheevers of Sporty's has for quite some time
been lobbying the FAA to permit an electric AI to replace a turn
coordinator... it does not seem as if the issue is getting very far with the
FAA, but I do think that would be a very good compromise if we started to
see electric AIs installed in place of the turn coordinator on planes where
panel space is tight.


--
Richard Kaplan, CFII

www.flyimc.com

The problem isn't just finding a home for it, it is finding a home for it that
is in a spot where it will be in the pilot's primary scan. I don't think it
would do much good on the other side of the panel where it might only be
referenced once a minute or less. I also agree that an IFR GPS is not
necessarily a high priority. Frankly, that (the GPS) is a lot of money for very
little added capability. In the case of a GPS/Nav/Comm, it also introduces a
single point of failure for all of the electronic nav gear with no back-up other
than what might be in your flight bag.

It is a shame that many of the simulators do not fail the AI the in the gradual
way it fails for real. I think training with realistic failures is the best way
to be able to recognize the failure (some of the accident reports indicate even
with redundant AI's recognition is not guaranteed). Unfortunately, the failures
can't be realistically simulated in the aircraft (at least not without an
illegal mod to the vacuum system), so ground based simulators must fill in
there. I've hear that some of the high end simulators such as the Frasca's do a
realistic AI fail, but the PCATDs I've played with all just pop from working to
tipped over instantaneously. Any of the current breed of PCATD's do any
better? Mine is a really old version of Elite, from about 1995 or so.

Richard Kaplan wrote:

"Ray Andraka" wrote in message
...

Good reason not to depart in low IMC I guess. I agree, that the AI is a
weak
link and carries with it some risk. While nice to have, cost and panel
space
make the back up AI difficult in some aircraft.

I think it is a matter of priorities... in an airplane which is IFR
certified, it is hard to believe there is not some space or economic
compromise which could not be made if a pilot felt this were an important
enough issue. I know I will get diagreement on this as always, but I think
the electric AI comes first before an IFR GPS. Even C152s sometimes have
Garmin 430s/530s nowadays; an electric AI would make much more sense IMHO.

As an even better solution, Hal Sheevers of Sporty's has for quite some time
been lobbying the FAA to permit an electric AI to replace a turn
coordinator... it does not seem as if the issue is getting very far with the
FAA, but I do think that would be a very good compromise if we started to
see electric AIs installed in place of the turn coordinator on planes where
panel space is tight.

--
Richard Kaplan, CFII

www.flyimc.com

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

"Ray Andraka" wrote in message
...

The problem isn't just finding a home for it, it is finding a home for it
that
is in a spot where it will be in the pilot's primary scan. I don't think
it

Put it where the turn coordinator is located and the put the turn
coordinator off to the side somewhere.. the regs say you must have a turn
coordinator but do not say where the turn coordinator has to be on your
panel.


tipped over instantaneously. Any of the current breed of PCATD's do any
better? Mine is a really old version of Elite, from about 1995 or so.


The current Elite software allows a choice between instant or gradual AI
failure. They have a reasonable upgrade program for their software as well.


--
Richard Kaplan, CFII

www.flyimc.com

"Gary L. Drescher" writes:

In MS FS2002, the AI fails abruptly, in just a few seconds. But I've heard
that FS2004 (due late this month) has more-realistic gyro failures.

If you don't want to wait, FlightGear already has gradual gyro
failures. You can fail an individual gyro or an entire system
(i.e. vacuum or electrical):

http://www.flightgear.org/


All the best,


David

--
David Megginson, , http://www.megginson.com/

I thought one of the reasons for the turn coordinator is that it won't tumble in
unusual attitudes where the AI will. I once tumbled my AI in IMC, and it was
not a pretty thing. Gave me a real bad sense of vertigo. At the time I had a
needle and ball rather than a TC, but that was the instrument that let me sort
things out. I think I would still want either a T&B or a TC in my primary scan.

Richard Kaplan wrote:

Put it where the turn coordinator is located and the put the turn
coordinator off to the side somewhere.. the regs say you must have a turn
coordinator but do not say where the turn coordinator has to be on your
panel.


--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

Newer vacuum AIs have warning flags, dual vacuum pumps, and vacuum warning
lights on the annunciator panel. If I had an electric AI and a vacuum AI and
they disagreed markedly, I would be suspicious of the one having warning
lights and flags all over it.

Otherwise, comparing them to the other instruments, as you suggested, is the
best approach. AOPA Safety Foundation just ran some tests on how long it
takes a pilot to recognize that his vacuum system has failed, and how long
it takes pilots to recover from those failures. Many times it took over 90
seconds, which is way too long. Some never did recognize the failure.
However, none of them were convinced that the vacuum instruments were
working and that it was the other instruments that had failed. IIRC somewhat
less than half discovered the problem quickly and took quick action. Only
about 1/4 of them covered the failed instrument.

On 13-Jul-2003, Ray Andraka wrote:

The problem isn't just finding a home for it, it is finding a home for it
that is in a spot where it will be in the pilot's primary scan. I don't
think
it would do much good on the other side of the panel where it might only
be
referenced once a minute or less.


Our Arrow has a backup electric AI on the right side of the panel. It is
turned on whenever I am in or near IMC. I do not include it in my primary
instrument scan, but check it periodically to make sure it is on and stable.
I have fortunately never had a failure of my vacuum AI in IMC, but I have
practiced under the hood with the vac AI and DG covered, using the electric
AI for guidance. For me, it only takes a few seconds to get comfortable
with looking across the panel, and sure beats sweating bullets trying to
hold attitude and a reasonable course with only the TC, particularly in
turbulence.

So, the only issue is whether I could detect a failed vac AI before it leads
me (or the autopilot) into an unusual attitude. If the problem is (as is
most likely) a vacuum pump failure, I have a vacuum warning light right in
front of my eyes that would be hard to miss. But a failure of the gyro
itself could be trickier. I tell myself that if I can't keep a reasonable
course (per the DG) with a wings-level attitude (per the AI), then something
is wrong and I need to immediately refer to the TC and the electric AI to
sort things out.

-Elliott Drucker