Flying around in a single-engine airplane usually means depending on its lone vacuum pump to turn some gyros. Even piston-powered twins have a weak link or two in their systems. Despite training in no-gyro procedures, recent high-profile accidents have demonstrated that failure of the vacuum system is both insidious and a bona fide emergency. In a follow-up to his recent article on vacuum system failures, AVweb's Scott Puddy takes a look at some of the choices available on today's market to back up your vacuum pump. There are a lot of options and a wide range of price points. Which is the best solution for you?
July 10, 2002
|About the Author ...
R. Scott Puddy was an ATP, CFI, CFI-I, MEI who taught out of the
Buchanan Field Airport (CCR) in Concord, California. Scott was type-rated
in the Beech/Raytheon King Air 300 series but regularly flew a V35 Bonanza
and practices law in San Francisco.
On the morning of June 18, 2002, Scott perished doing what he
loved: practicing aerobatics in a Yak-52, in the mountains of Brentwood,
He contributed many articles about flying to AVweb in recent
years and also worked as our features editor. His enthusiasm for
aviation and his intensity in pursuing it were simply extraordinary.
Even more extraordinary was his dedication to sharing his passion for
flying with others, by teaching and writing. He touched a lot of lives,
undoubtedly saved many, and his legacy of written words will continue
to do both for many years to come. Scott's warmth, wit, and keen
intelligence will be missed by all who knew him and worked with him.
has been written about the frailties of the vacuum pumps that power our
crucial flight control instruments. Most recently, AVweb published the
first article in this series ("How Can I Fail Thee?
Let Me Count the Ways"), which took a close look at some of the weak
links in vacuum systems and components installed on piston-powered airplanes.
At the end of that article, I promised to take a look at some of the
alternatives to back up these systems. For example, what, if anything, is
being done about their frailties? What can you do about it? How much will it
cost? Following are some answers to these questions.
Precise Flight Standby Vacuum System
(MSRP: $429, Plus Installation)
Precise Flight Inc.
manufactures the standby of standby vacuum systems. Inexpensive and elegant in
its simplicity, the Precise Flight standby vacuum system consists of a vacuum
line that connects the vacuum system to the engine intake manifold and a
cockpit-controlled shuttle valve that determines whether the vacuum pump or
the intake manifold is the operative vacuum source. The system also includes a
low vacuum warning light to alert the pilot that the vacuum pump has failed.
The system is STC'd for virtually all GA aircraft and, at a seemingly
reasonable cost of $429 (plus installation), there isn't a cheaper insurance
policy available. If you have nothing else on board, buy one.
Precise Flight system is not without its limitations and tradeoffs, however.
The main limitation is that despite its name it is a standby vacuum
"source," not a standby vacuum "system." When the vacuum
system is operating in the standby mode, it uses the same instruments,
filters, vacuum lines, and regulator as it does when operating in the normal
mode. If any of those shared components have failed, the standby vacuum source
may not restore system operation.
The significant tradeoff is available horsepower. The Precise Flight system
depends on the existence of a differential of at least four inches of Hg
between ambient air pressure and intake manifold pressure. The example Precise
Flight gives is:
If you are cruising at full throttle at
8,000 ft., MSL, you will have a manifold pressure of approximately 22 inches
Hg. It will be necessary to reduce power to approximately 18 inches Hg to
have a 4 inch differential available to operate the aircraft instruments
after a vacuum pump failure.
That equates roughly to a reduction from 75 percent to 50 percent in the
power available for cruise at 8,000 feet. Not too bad if you're flying a
Bonanza over sea-level terrain. However, it would be a different story if you
were flying over mountains in your fully-loaded Piper Arrow. Cruising at
12,000 feet, you'd have only about 18 inches manifold pressure available at
full throttle. In order to power the instruments, you would need to reduce the
throttle setting to 14 inches. You couldn't hold altitude at that power level
which would leave you with the choice of descending or flying needle, ball and
airspeed. Worse yet, you might be induced to use the minimum power setting
necessary to hold altitude and yet rely on an underpowered and unreliable
attitude indicator. Not the most attractive option.
Standby Electric Vacuum Pumps
(MSRP: $1,995-$3,255 Plus Installation)
you're willing to dig a little deeper into your pocket and are looking for the
protection of a standby vacuum source that will not reduce your plane's
service ceiling to mid-mountain elevations, you could purchase a standby
electric vacuum pump. The available systems include the Aero
Safe Guardian I Standby System, a vacuum version of which sells for $1,995
(the pressure system is $2,095) and a variety of "auxiliary air
systems" manufactured by market-leader Airborne
and available at prices ranging from $2,410 to $3,255, depending on the
Standby electric pumps rely on the traditional use of the electrical system
to back up the vacuum system. They are activated by either an automatic switch
or a pilot-controlled switch when the primary pump fails. A check valve
isolates the failed vacuum pump while the electric pumps powers the system.
Like the Precise Flight system, electric standby pumps merely provide a
backup vacuum source, not total systems redundancy. The electric pumps don't
rob you of horsepower, but they do cost more. They also exact a small weight
penalty. Be aware that there have been service difficulty reports mentioning
some installations with automatic switches that did not include an annunciator
light to warn the pilot that the system was operating on the auxiliary pump.
In such cases, the pilot could be unaware that primary pump had failed until
the auxiliary unit itself breaks down.
Standard Three-inch Electric Gyros
(MSRP: $1,795-1,995, Plus Installation)
If you are willing to spend what it takes to purchase and install a standby
electric pump and are even remotely concerned that your next vacuum system
failure might result from the breakdown of a component other than the vacuum
pump itself, you should at least consider installing an electric attitude
indicator (and perhaps an electric directional gyro) in lieu of a standby
extra set of electric gyros would provide total systems redundancy for your
vacuum system. Plug in an electric attitude indicator and you would have
attitude information available even if your entire vacuum system were to kink,
crumble or collapse. Whereas vacuum-powered attitude indicators cost about
$500, a new-manufacture, self-contained R.C. Allen three-inch electric unit
(14V or 28V) with internal lighting will run you $1,795 plus installation.
Think about it: complete systems redundancy for the attitude indicator for
less than the cost of a standby vacuum pump not a bad deal. If you wanted
to back up both the attitude indicator and the directional gyro, the equipment
cost would double.
Installation is the major drawback here. You would need to find space on
your panel for one or two fairly large instruments. Further, they should be
installed high enough on the panel to facilitate their use in emergency
circumstances. There are a lot of Cessnas flying with ample usable space just
to the right of the radio stack. Installation of the instruments there
introduces a parallax issue, but the instruments
can be used with training. There are a number of other planes flying with the
ADF in the upper left corner of the panel. That is an ideal location for a
standby attitude indicator. You would need to relocate the ADF or, if you are
carrying an IFR GPS, you may elect to remove it altogether. In other cases,
there just isn't enough room and you will need to explore other options.
Two-inch Standby Attitude Indicators
(MSRP: $5,700 - $14,969, Plus Installation)
room for a three-inch instrument? If you scrutinize the top left corner of
your panel, you'll probably find a spare two-inch cutout if you are willing to
relocate or remove a less essential piece of equipment such as the
panel-mounted clock or fuel flow gage. The problem? In the electronics world,
when you start thinking in terms of diminutive size you also have to think in
terms of copious dollars.
You can pick up an overhauled AIM
520 series two-inch attitude indicator (28V) for $5,700. A factory-new
unit (which may not be immediately available) would run $6,700. If you have
$14,969 that you don't know what to do with, you could step up to the
top-of-the-line J.E.T. AI-804
series two-inch unit (28V). J.E.T. advertises that its unit will provide up to
nine minutes of usable attitude information after a complete power loss. J.E.T.
also sells an emergency power source that can supply the unit for up to five
Standby Instrument Suite
(MSRP: $54,580, Plus Installation)
you elect to purchase any of the above systems and have to explain your
expenditure to your wife, husband or significant other, you might find it
useful to know how much you could have spent if you hadn't been so careful
with your money. The BF
Goodrich model GH-3000/3100 Electronic Standby Instrument System (ESIS)
sells for merely $54,580 (including magnetometer and magnetometer install kit)
and packs more information into a three-inch box than you can believe. (Take
note Precise Flight this is a standby system.)
The ESIS is in essence a miniature electronic flight instrument system (EFIS)
like you would find in heavy-metal cockpits. It replaces all three traditional
standby instrument indicators (attitude, altitude and airspeed) as well as the
navigation instruments. Its various displays include: attitude, altitude,
vertical speed, indicated airspeed, mach number, Vmo and Mmo, VOR data and
guidance, ILS data and guidance, DME data (including distance, ground speed,
and time to station), magnetic heading, and flight management system
information (distance to waypoint, desired track, selected course, bearing to
waypoint, to/from indicator, course deviation, vertical deviation, and
parallel cross track).
Okay, so it's a little spendy and the capabilities might be overkill for
your application. Still, how cool would it be to have a constant Mach number
readout in your C-172?
($276,600 - $?,???,???)
makes you nervous even to have a vacuum pump installed in your plane? Yes, you
still have options.
Until recently, you pretty much had to step up to turbine-powered aircraft
to find instrument systems that relied solely on redundant electrical power.
Recently, however, Cirrus
Design made vacuumless flight available to the common man when it
introduced its Cirrus SR-22. The SR-22's all-electric flight control panel is
powered by a dual-battery, dual-alternator, dual-bus electrical system. Priced
at $276,600 in its basic configuration, the SR-22 certainly is not the least
expensive alternative to the vacuum pump. On the other hand, its
fully-electronic panel comes complete with a 3,400-lb./180-knot airplane.
While this option is of little comfort as a back-up system when your vacuum
pump packs up for the night in your other airplane, the point is that
you wouldn't need that pump if you were flying the SR-22. Of course, having
that vacuum pump and a vacuum-driven attitude indicator might come in handy
some dark night when the electrical system fails.
So there you have it ... a host of choices to back up your vacuum-powered
flight control instruments. If you regularly fly IFR, PLEASE use one. You
don't need no stinking standby system? Read these ...
... and think again.