Backup gyros - which do you trust?

"Steve House" wrote:
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."

This is a very interesting issue, to me. Reading the records of IMC
loss-of-control accidents is very unsettling to this single pilot IFR flyer
because of the cases where there *was* backup attitude instrumentation
available. Even when there wasn't, the pilots usually had at least the turn
coordinator to help keep the aircraft upright. It is too simple to chalk up
all these accidents simply to lack of proficiency. There is something else
going on - some human factors issue that has not been properly identified. I
suspect it may be related to task saturation. If so, instrument panel
clutter could be a contributing factor.

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?

My strategy is to include a yoke-mounted GPS displaying a synthetic HSI in
my scan. This works wonderfully well in training, but I am not sure how well
I would do in a real situation where my AI suffered a gradual failure.
--
Dan
C172RG at BFM

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

Ray Andraka wrote:
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.

You are probably right. When I did my initial IFR training, my
instructor was very big on partial panel work. As a result, I learned
to not rely on the AI, and I find partial panel approaches (in training,
anyway) almost a non-event.

The downside, is that I suspect I don't use the information the AI gives
me as much as I should. I tend to fly pitch by airspeed, not by the AI.
This probably makes me not as smooth and precise as I might otherwise
be. But I do have a lot of confidence that I can fly an approach on the
TC and ASI alone.

Roy Smith writes:

The downside, is that I suspect I don't use the information the AI gives
me as much as I should. I tend to fly pitch by airspeed, not by the
AI.

Are you sure that using the ASI for pitch doesn't make you smoother?
I think that a couple of knots difference is more noticeable than a
fraction of a degree change in the AI pitch indication.

My problem is that managing pitch with the ASI gets hard in turbulence.


All the best,


David

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

Ray Andraka wrote:
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.

Ray,

I'll speak to the latter.

A failing horizon gyro may not "spin down". But it can still
be insidious. Example: our AI had a period where, in level flight,
it would jump up and indicate a rather nose-high attitude. Fail
to catch it and you'd be in a rather steep dive. Then it would
go back to normal. Then jump up again....finally it broke and
unmistakably started spinning in a nauseating fashion, but the
"breaking" process could easily have caused a loss of control for
a pilot w/out a good cross-check (our failure happened VMC)

Cheers,
Sydney

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?

Bob Gardner wrote:
You have three sources of bank information in a typical panel: the attitude
indicator, the turn coordinator, and the heading indicator.

Um, isn't this leaving out something fundamental (and pretty
reliable, if difficult to interpret)?

Sydney (Be Expert With Map and Compass)

Dan Luke wrote:

It is too simple to chalk up all these accidents simply to lack of
proficiency.

I was just reading the NTSB report of the King Air that crashed in
Colorado, attributed to spatial disorientation after a partial panel
failure. It seems representative of the problem. The facts are
chilling:

- IMC, alt. 23,200 ft.
- Two person cockpit.
- Experienced pilot - 5117 hours total, 2520 in type.
- Partial panel loss due to AC power failure.
- Failure immediately indicated by flags on affected instruments.
- Remaining instruments, powered by vacuum:
Left - airspeed, turn/slip,
Right - airspeed, turn/slip, altimeter, attitude.
- Aircraft began gently increasing turn within one minute of failure.
- Time between instrument loss and impact - one minute, 33 seconds
- Flight path consistent with graveyard spiral

http://www.ntsb.gov/publictn/2003/AAR0301.pdf

There is something else going on - some human factors issue that
has not been properly identified. I suspect it may be related to
task saturation. If so, instrument panel clutter could be a
contributing factor.

One comment in the report was that the pilot might have had a tendency
to focus on a single problem, and mot paid attention to other things.
He could have been trying to troubleshoot the electrical problem, and
not handed control over to the copilot, who would have had a better view
of the remaining functional instruments.

In any event, it is amazing how quickly the pilot lost control of the
aircraft, considering how this should have been fairly routine: If an AC
inverter had failed, then the changeover to the remaining inverter is
accomplished with a simple flip of a switch, and should have been almost
a reflexive action. The failure would have been immediately obvious, so
it wasn't one of those insidious failures that people don't notice at
first. An experienced IFR pilot should have been aware of the need to
maintain attitude and yet lost control almost immediately. In reading
the report, it seems like such an avoidable accident, yet...

James Robinson wrote:

One comment in the report was that the pilot might have had a tendency
to focus on a single problem, and mot paid attention to other things.
He could have been trying to troubleshoot the electrical problem, and
not handed control over to the copilot, who would have had a better view
of the remaining functional instruments.

This is poor CRM if it is the case.

Did the report say anything about the training of the pilots? I
woulda thought they did regular sim stuff, where I assume the
instructors put you through the wringer on various failues.

The failure would have been immediately obvious, so
it wasn't one of those insidious failures that people don't notice at
first. An experienced IFR pilot should have been aware of the need to
maintain attitude and yet lost control almost immediately. In reading
the report, it seems like such an avoidable accident, yet...

Yeah, that's what gets me about so many of these.

Sydney

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?