Avionics News November 2014 - 41

Let's review some basic information on mechanical gyros.
and also course/deviation indicator. On higher-end
Until recently, mechanical attitude and directional gyros
models, the gyro may be remotely mounted and slaved
were standard in all instrument-capable airplanes - and
(using a magnetic flux sensor) to counteract precession.
probably still function as primary instruments in a majority
The additional complexity means more possible failure
of the general aviation fleet. These gyros may be driven
modes, which can complicate troubleshooting and lead
electrically or by air - and while you may occasionally
to expensive repairs. Similar issues exist with higher-end
encounter an antique with venturi-driven gyros, the vast
attitude indicators that include flight directors and the
majority of them (at least in piston applications) are powered
more complex attitude and direction indicators common
by an engine-driven vacuum pump.
in pre-glass turbine airplanes.
It's the pump that causes the
For the avionics shop, mechanical
major reliability problem for
gyro issues are most likely to appear
vacuum-driven gyros: Carbon from
when an operator enters with a
FOR THE AVIONICS
dry-vane pumps is released into the
malfunction - perhaps the HSI is
SHOP, MECHANICAL
vacuum system by design. These
slow to track a navigational aid, or
GYRO ISSUES ARE
pumps should be replaced every
the autopilot seems to spend a lot
500 hours, but it's not unusual for
of time "hunting." The underlying
MOST LIKELY TO
operators to defer replacement.
problem may be precession in a
APPEAR
Eventually, this leads to a pump
gyro that's going bad but has not yet
failure. Filters in the vacuum system
completely failed. The question, of
should prevent any material from
course, is what to do next.
the pump getting to the gyros, but
For the operator, the overriding
I have personal experience with a
concern at this point will most
badly contaminated vacuum system
likely be minimizing downtime. He
(in a former flight school airplane
wants the airplane fixed and flyable
that I made the mistake of buying)
- so this is a bad time to bring up
that required gyro overhaul as well
more long-range concerns. If the
as pump and filter replacement.
problem appears to be a bad gyro,
Even sealed electric-driven gyros, which are not
pulling it and sending it to a gyro shop for overhaul
exposed to any risk of vacuum system, eventually fail.
(or exchanging it for a new or rebuilt unit) is a logical
Typical mechanical gyro rotors spin continuously at
step - though in more complex systems, it may be worth
speeds above 10,000 revolutions per minute, and while
spending a little time confirming that only the gyro is
built with jeweled bearings, they will eventually wear
defective. A bad magnetic flux sensor - or problem in
out. That's why the Federal Aviation Administration
cabling - might produce some of the same symptoms
requires dual attitude indicators (or attitude indicator
as a precessing remote gyro, and lead to an unhappy
and turn coordinator) for airplanes to be operated under
customer if the replacement doesn't solve the problem.
instrument conditions - and that each have a separate
Even in relatively simple systems, there can be
power source (one electric and another vacuum, or
unexpected issues. A few years ago, I experienced a
all-electric with separate electrical buses and/or battery
tumbled attitude gyro in a piston single. There was no
backup).
warning - the gyro, while old, had always operated
The situation gets more complex in advanced systems
properly and showed no symptoms such as slow
- a mechanical horizontal situation indicator, for
Continued on following page
example, will include a gyro (may be vacuum or electric)

WHEN AN
OPERATOR
ENTERS WITH A
MALFUNCTION.

avionics news

*

november

2014

41



Avionics News November 2014

Table of Contents for the Digital Edition of Avionics News November 2014

Point of Communication
AEA Now
The View from Washington
International News and Regulatory Updates
Shifting redundancy gears
Member Profile
AEA Rate & Labor Survey
A Conversation With...
Member Profile
Real-world Avionics
Antenna Tech
Aviation Aces
Business Basics
Before & After
Meet the AEA board of directors
Avionics technician certifications
Legal Ease
Marketing Report Card
What's New
Marketplace Classifieds
Avionics News November 2014 - Intro
Avionics News November 2014 - Cover1
Avionics News November 2014 - Cover2
Avionics News November 2014 - 1
Avionics News November 2014 - 2
Avionics News November 2014 - 3
Avionics News November 2014 - Point of Communication
Avionics News November 2014 - 5
Avionics News November 2014 - AEA Now
Avionics News November 2014 - 7
Avionics News November 2014 - 8
Avionics News November 2014 - 9
Avionics News November 2014 - 10
Avionics News November 2014 - 11
Avionics News November 2014 - 12
Avionics News November 2014 - 13
Avionics News November 2014 - The View from Washington
Avionics News November 2014 - 15
Avionics News November 2014 - International News and Regulatory Updates
Avionics News November 2014 - 17
Avionics News November 2014 - 18
Avionics News November 2014 - 19
Avionics News November 2014 - Shifting redundancy gears
Avionics News November 2014 - 21
Avionics News November 2014 - 22
Avionics News November 2014 - 23
Avionics News November 2014 - Member Profile
Avionics News November 2014 - 25
Avionics News November 2014 - 26
Avionics News November 2014 - 27
Avionics News November 2014 - AEA Rate & Labor Survey
Avionics News November 2014 - 29
Avionics News November 2014 - 30
Avionics News November 2014 - 31
Avionics News November 2014 - 32
Avionics News November 2014 - 33
Avionics News November 2014 - A Conversation With...
Avionics News November 2014 - 35
Avionics News November 2014 - Member Profile
Avionics News November 2014 - 37
Avionics News November 2014 - 38
Avionics News November 2014 - 39
Avionics News November 2014 - Real-world Avionics
Avionics News November 2014 - 41
Avionics News November 2014 - 42
Avionics News November 2014 - 43
Avionics News November 2014 - Antenna Tech
Avionics News November 2014 - 45
Avionics News November 2014 - 46
Avionics News November 2014 - 47
Avionics News November 2014 - Aviation Aces
Avionics News November 2014 - 49
Avionics News November 2014 - Business Basics
Avionics News November 2014 - 51
Avionics News November 2014 - 52
Avionics News November 2014 - Before & After
Avionics News November 2014 - Meet the AEA board of directors
Avionics News November 2014 - 55
Avionics News November 2014 - 56
Avionics News November 2014 - 57
Avionics News November 2014 - Avionics technician certifications
Avionics News November 2014 - 59
Avionics News November 2014 - 60
Avionics News November 2014 - 61
Avionics News November 2014 - Legal Ease
Avionics News November 2014 - 63
Avionics News November 2014 - Marketing Report Card
Avionics News November 2014 - 65
Avionics News November 2014 - What's New
Avionics News November 2014 - 67
Avionics News November 2014 - 68
Avionics News November 2014 - 69
Avionics News November 2014 - 70
Avionics News November 2014 - Marketplace Classifieds
Avionics News November 2014 - 72
Avionics News November 2014 - Cover3
Avionics News November 2014 - Cover4
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