Recent high-profile accidents have demonstrated that even when redundant or back-up systems are present, they don't guarantee a successful outcome to a vacuum system failure. Indeed, the vacuum pump and its associated bits and pieces are some of the most failure-prone components with which pilots of piston-powered aircraft surround themselves. AVweb's Scott Puddy takes a close look at the vacuum system and its many weaknesses while offering some insights on what you can do about them.
January 10, 2001
|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.
the originator and dominant manufacturer of dry vacuum pumps, now
attaches this disclaimer to each pneumatic pump that it
FAILURE TO FOLLOW THE FOLLOWING INSTRUCTIONS MAY RESULT IN DEATH, BODILY
INJURY, OR PROPERTY DAMAGE:
1. A BACK-UP PNEUMATIC
POWER SOURCE FOR THE AIR DRIVEN GYROS, OR A BACK-UP ELECTRIC ATTITUDE
GYRO INSTRUMENT, MUST BE INSTALLED IN ALL AIRCRAFT WHICH FLY IFR.
2. ANY INOPERATIVE AIR
PUMP OR OTHER COMPONENT OF THE GYRO SYSTEM, AND ANY INOPERATIVE BACK-UP
SYSTEM OR COMPONENT, MUST BE REPLACED PRIOR TO THE NEXT FLIGHT.
3. THIS PILOT SAFETY
WARNING MUST BE PERMANENTLY RETAINED IN THE PILOT'S OPERATING HANDBOOK
FOR THE AIRCRAFT INTO WHICH THIS AIR PUMP IS INSTALLED.
Now wait a minute! Section 91.205(d) doesn't list a backup vacuum source as
required equipment for IFR operations. Why is Airborne attempting to rewrite
For years your instructors admonished you to "trust your
instruments." Why is Airborne now saying, "For God's sake, don't
trust your instruments"?
The reason? Dry vacuum pumps are designed to fail abruptly and without
warning. Evidently, some people (or their survivors ... or their lawyers) have
determined that a statistically intentional and precipitous failure of our
principal instrument system is unsporting.
Meanwhile, the National Transportation Safety Board (NTSB) has reported
pneumatic system failures as a factor in an average of two fatal accidents per
year over the past decade. Some of those accidents, including the
1999 crash that killed well-respected pilot and instructor Itzhak Jacoby, his
wife and his daughter, or the
recent one that resulted in the deaths of Missouri Governor Mel Carnahan, his
son and his aide, were widely reported by AVweb and other media.
Numerous other pneumatic system failures occur each year without causing
accidents, incidents, or sensational headlines. Those events may have escaped
your attention but are communicated to the FAA and your maintenance technician
through Service Difficulty Reports (SDRs) and manufacturer's Service
So what's going on? How frequent and how significant are these systems
breakdowns? Must you have a back-up vacuum system to fly IFR? Should you? Do
you need attitude instruments for VFR? Is a standby vacuum source sufficient
protection for you or is it merely an adequate means of insulating Airborne
from legal liability?
A System Under Siege
Several years ago, AVweb Editor-In-Chief Mike Busch wrote two
excellent articles about attitude indicators
and the vacuum pumps that drive them that are
well worth reading and reading again. Since then, the plot has thickened
considerably in the wake of three significant Airworthiness Directives (ADs),
multiple Service Bulletins, and a mountain of SDRs relating to failures of
various components of air-driven instrument systems. One of these ADs was
directed against ... you guessed it ... Airborne.
The Airborne AD resulted from a series of premature (between 0-400 hours in
service) "normal-mode" failures of Airborne pumps. Upon
investigation, it was determined that batches of flexible couplers bearing
manufacture dates of "12/97" or "5/6/98" were defective. Priority
Letter AD 98-23-01, issued October 29, 1998, mandated immediate
replacement of the targeted parts and restricted operations of affected
aircraft to daytime VFR pending compliance. More recently, Airborne issued Service
Letter 53 on July 12, 2000, relating to excessive wear of the teeth on the
drive spline of its clutch-operated dry air pumps which precluded the clutch
assemblies from properly engaging and disengaging.
competitor RAPCO was likewise the subject of an AD. The FAA issued Priority
Letter AD 97-16-10 on July 31, 1997, because of several reports of cracked
filter housings on RAPCO's in-line pressure filters. Used principally in
twin-engine aircraft that employ a pneumatic pressure system rather than a
vacuum system, those filters were reported to be failing within the first two
to six hours of service because the plastic was excessively brittle and
subject to cracking when exposed to a low humidity/high temperature
environment. More recently, Airborne issued Service
Letter 56 on April 19, 2000, which required inspection of certain of its
filters to remove loose particles of black elastomeric compound that may have
been left behind during the manufacturing process.
...Defective Vacuum Lines...
Also during this period, Cessna issued Service Bulletin SEB 96-10 on June
7, 1996 and Beechcraft issued a safety communiqué in August 1996 relating to
Imperial Eastman Hytron Redi-seal hose. Those vacuum lines failed internally
without giving any visible external indications. The red inside liner could
crumble, undetected, and contaminate the other vacuum system components.
...And Defective Valves
Meanwhile, even back-up vacuum systems drew FAA fire. Effective January 14,
2000, the FAA issued AD 99-24-10 relating to the
very popular Precise Flight Model SVS III Standby Vacuum System which was
installed on 51 models of Beechcraft airplanes, 170 models of Cessna
airplanes, 12 models of Mooney airplanes, and 74 models of Piper airplanes
(amongst numerous others). This is a repetitive inspection AD resulting from
shuttle valve failures which prevent the standby vacuum system from operating
after the primary system fails.
A Wake-up Call For General Aviation
These events are a wake-up call to general aviation. Any vacuum-driven
instrument system consists of the instruments themselves, a pump, filters,
valves and air lines to connect all of the above. As the events of the past
few years aptly demonstrate, each of those components is life-limited and
susceptible to failure.
The Advances To Failure...
to recent "advances" in equipment design, GA pilots relied on
external venturi tubes to power the flight control instruments. Venturi tubes
were pretty much maintenance-free, devoid of moving parts and worked well,
unless and until visible moisture was encountered at below-freezing
temperatures. In that event, the venturi tubes were the first place to look
for warnings of accumulations of ice. Fair-weather friends to be sure, but you
weren't supposed to be operating your Bugsmasher II in icing conditions,
The next power source was the "wet" vacuum pump. Those
engine-driven devices were functional, but inelegant and inappropriate for
some applications. Wet pumps have metal vanes and were lubricated by engine
oil. The pumps were long-lasting and could be overhauled once they
demonstrated signs of wear. However, the wet pump's discharge air contains an
oil mist requiring an air-oil separator to return most of the oil to the
engine. Even so, they tended to lubricate the belly of the airplane as though
in constant preparation for a gear-up landing. The oil mist also tended to
deteriorate rubber deicing boots in applications where the pump powered such a
system. Finally, wet pumps could not be used in positive pressure systems
because the oil would contaminate the system filters and the instruments
themselves. The move to dry pumps was on.
the early 1970s, general aviation switched to dry vacuum pumps manufactured
principally by Airborne and relative newcomer Sigma-Tek. Although there are
differences between the two, Airborne and Sigma-Tek pumps use self-lubricating
graphite vanes which are both the solution (no engine oil required for
lubrication) and the problem (the pumps tend to produce spec vacuum throughout
their lives and then disintegrate internally in a cloud of carbon dust).
Compounding the sudden failure mode is a frangible coupling that is designed
to shear abruptly in the event of overstress or sudden stoppage. In most
cases, the aircraft operator will see no sign of any problem until the vacuum
gage reading falls precipitously to zero.
is the expected lifespan of a dry vacuum pump? Although times vary from
installation to installation (and everyone seems to have "horror"
stories telling of dramatically shorter lifespans), some guidance is found in
the warranties the pump manufacturers offer. Airborne has a
two-year/1,000-hour warranty on its 215/216 series pumps (used on most
non-deiced single engine planes) and a one-year/400-hour warranty on its 240-,
400- and 800-series pumps (used on deiced and larger aircraft). Sigma-Tek
offers a two-year/1,000-hour warranty on its pumps. Airborne also specifies
that its pumps should be routinely replaced at intervals varying from 300 to
1,200 aircraft hours depending on the unit. For non-deiced singles, the
applicable replacement interval is 1,200 hours. Airborne schedules filter
replacement on much shorter intervals every year or 100 hours (whichever
comes first) for the vacuum regulator filter and every year or 500 hours
(whichever comes first) for the central air filter element.
...Back-Up Vacuum Sources...
As do other manufacturers, Airborne recommends (and sells) a standby
electric auxiliary vacuum pump for systems having only one primary pump. The
auxiliary pump connects to the primary pneumatic system through a manifold
downstream of the vacuum regulator. During normal operations, the auxiliary
pump is separated from the primary system by a check valve. In the event the
primary pump fails, the auxiliary pump is activated and the check valve
separates the failed primary pump from the vacuum system.
A more rudimentary but common standby vacuum source is a cockpit-controlled
connection between the vacuum system and the engine intake manifold. So long
as the pilot selects an engine power setting that is low enough to maintain a
differential between ambient air pressure and the intake manifold pressure,
there will be vacuum to power the instruments.
The main weakness of the electrical standby pump installation and the
intake manifold standby connection is that they provide redundancy for only
the vacuum pump itself. When the system is operating in the auxiliary 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. In
essence, installing a standby vacuum source places single-engine airplanes on
a par with light twins that have two pumps powering a single system and a
single set of instruments. In other words, the standby vacuum system does not
provide total systems redundancy.
Failures, Failures And More Failures
If, as Airborne suggests, you were to install a standby pump and routinely
replace your vacuum pump prior to its failure, you could minimize the risks
inherent in the design of dry pumps. However, you would be addressing only
that specific risk. Murphy's Law states that any component that can fail will
fail and at the least-opportune moment. Service
Difficulty Reports filed during the last five years provide ample evidence
that Murphy's law applies to general aviation. Here s a sampling of what can
go wrong with your vacuum system.
Vacuum Pump SDRs...
vacuum pumps fail in ways you might not expect. Vacuum pump-related failures
caused a loss of engine oil pressure in several cases. In four cases, the
vacuum pump housing literally split in two. In three instances, the vacuum
pump adapter cracked. In two other incidents the vacuum pump gasket was
Failures result from installation errors as well as from product defects. A
new Grob 115C suffered camshaft drive gear damage because the factory had
mis-installed the vacuum pump. In two other cases, vacuum pumps were
incorrectly installed in the field resulting in a losses of engine oil.
Operator error is also a factor. One pilot attempted to operate his C-172
without having the vacuum pump installed, with predictable adverse results.
...Dual Vacuum Pump SDRs...
Dual pneumatic pumps yield redundancy only so long as the pilot is able to
detect the failure of the first pump.
A rare partial failure of a dry pump in a twin-engined Piper evaded
detection because the minimal pressure (one inch) produced by the failed
pump, while insufficient to power the instruments, was sufficient to avoid
triggering the warning indicator.
In another case, because there was no annunciator installed on this
system, the pilot was unaware that the primary pump had failed and flew
for hours with the standby pump as his only available source of pneumatic
In a third instance, the contamination resulting from the failure of the
first pump obscured the system warning indicators. The pilot did not
notice the initial breakdown until the contamination disabled the second
pump resulting in a complete system malfunction.
...Vacuum Filter SDRs...
There were numerous failures of filters resulting from product defects,
installation errors and inadequate maintenance.
An in-line filter was installed backwards causing a vacuum pump to break
down after seven hours in service.
Another in-line filter disintegrated upon removal after having never
been replaced during the plane's 2,170 total hours in service.
Yet a third filter failed in flight after 2,898 hours in service.
A fourth in-line filter deteriorated and crumbled after two years in
The main vacuum filter was waterlogged in two cases the first because
of a leaking windshield seal and the second because it was exposed to
torrential rain while baggage was loaded into the nose cargo bay.
Several plastic in-line filters melted because of engine or exhaust
Other filters cracked and leaked, leading to vacuum pump failures in some
cases and low system pressure in others. In one of those instances, the
mechanic had not hard-mounted the filter. It shifted and was cracked because
it interfered with movement of the control yoke.
...Vacuum Valve SDRs...
number of valve failures made the list.
There were multiple cases of broken check valves that prevented systems
on twin-engine planes from developing pressure from operable pumps on one
side following a failure of the opposite side pump.
A Piper twin experienced low vacuum pressure because both relief valves
had stuck in a partially closed position.
A Piper Malibu suffered multiple failures of its standby vacuum pump
because a mis-set regulator caused the main pump to build up excessive
pressure that over-worked the standby pump. In another instance, a
pneumatic regulator leak resulted in inadequate system pressure.
...Vacuum Hose SDRs...
In addition to the problems with the Imperial Eastman Hytron Redi-seal hose
experienced primarily in single-engine Cessnas, the latex surgical tubing type
vacuum lines used in late-model Mooneys drew fire. There were several reports
suggesting that those lines tend to fold over and kink after a few years,
resulting in system blockage. Other reports reflected that retracting the nose
wheel in some Bonanzas was crimping air lines. There were also a few
miscellaneous instances of lines failing because of chaffing from air
conditioner ducts and the like.
...Vacuum Gage SDRs
Even the vacuum gages themselves can fail.
There was a report of a gage that was leaking internally, yielding
abnormally high indications which resulted in the system pressure being
set too low to power the instruments.
In another case, there was a system leak at the coupling to the gage
which resulted in an pressure indication that was lower than the actual
system pressure. Adjustment of the regulator to yield a normal reading
commanded excessive system pressure, causing a premature vacuum pump
What's A Pilot To Do?
Federal Aviation Regulation Part 91 allows you to fly IFR with no back-up
for any component of your air driven instrument system other than needle, ball
and airspeed. Airborne recommends and the FAA's
Safety Article, P-Pamphlet #8740-52 strongly suggests that at least a
back-up vacuum source should be installed in any plane that is flown regularly
under IFR. However, based on the above, you may determine that a mere standby
vacuum source does not provide sufficient protection for all the potential
failures that are inherent in the system. What's a pilot to do?
not consider what the regulations call for when true systems redundancy is
required? Turbojet airplanes operated under Part 135 or Part 121 are required
to have a third gyroscopic bank-and-pitch indicator (artificial horizon) that:
Is powered from a source independent of the electrical generating
Continues reliable operation for a minimum of 30 minutes after total
failure of the electrical generating system;
Operates independently of any other attitude indicating system;
Is operative without selection after total failure of the electrical
Is located on the instrument panel in a position acceptable to the
Administrator that will make it plainly visible to and usable by each
pilot at his or her station; and
Is appropriately lighted during all phases of operation.
These devices, sometimes known as "peanut gyros," are the latest
rage among many owners of high-performance piston-powered airplanes with one
effect being that the price for secondary attitude indicators has increased
and supply has dropped.
Are they the silver bullet? Are they STC'd for your plane? What do they
cost? What is the price relative to that of a standby vacuum pump? Where can
you mount one? (Hint you probably have a redundant piece of equipment
mounted in your panel that starts with "c" and rhymes with
Sorry, we're out of room but those are all good questions for next time.
Until then, brushing up on your partial panel skills would be a good idea.