Vacuum systems for general aviation aircraft have long proven their reliability based upon their simplicity and the durability of the few key components. In most light aircraft, the system consists of an engine-driven vacuum pump, relief valve, suction gauge and inlet filter. These components are packed together amidst a tangle of hoses and collectively serve to drive the critical gyroscopic instruments mounted in the panel. Some of the more advanced aircraft systems also include warning lights, oil separators and back-up systems. These add-ons are sometimes offered as standard equipment but are also available as options and/or approved modifications. The vacuum system is quite elementary in design yet its ability to function smoothly may become compromised when the basic maintenance requirements are neglected. The vacuum pump is the driving force behind creating suction for the gyros as it draws air from the cabin, through the inlet filter and regulator. The primary gyro operated flight instruments include the Attitude Indicator (AI) and Directional Gyro (DG, aka the Heading Indicator) as they are commonly known. Older turn & bank instruments and later turn-coordinators were dependent on suction until preference was given to those designed for electric operation.
A very basic diagram of the aircraft vacuum system components
When considering the evolution of the pump as the heart of the vacuum system, let’s look back before its development and understand that vacuum dependent gyros were set into motion through the use of a venture tube mounted to the exterior of the forward fuselage. The size of the installed venturi was determined by the number of installed gyro instruments. The venturi was essentially maintenance free yet eventually became less effective as our modern aircraft demanded a more robust system in order to compensate for changes in altitude and atmospheric conditions. That demand actually brought about the introduction of the “wet type” vacuum pump which was driven off the engine accessory housing. Prior to the 1970s it was common to have a wet type pump which was driven by oil pressure and required an air-oil separator to direct oil back to the engine as the resulting suction air served to drive the gyros. The wet type pump was adequate during its time yet things could get messy as oil would often find its way onto the exterior of the engine as well. There have been two main types of these vane pumps used over the years with the second type being the “dry type” vacuum pump becoming popular throughout the Seventies era aircraft.
Basic venturi operation pre-dating modern vacuum pumps
The industry swiftly moved forward with the dry type pump design which has remained the standard to the current day. This similar vane type pump design was now splined to the engine accessory case but did not require the pressure oil for operation or internal lubrication. This dry type of vane pump was instead dependent upon self-lubricating carbon graphite vane construction. The new pump brought about other challenges with gradual wear of the vanes and accumulation of carbon dust yet remained superior to the wet pump in many other aspects of function and design. The splined drive is attached to a frangible plastic drive coupling which is designed to shear instantly whenever the rotational drag of the pump exceeds the normal operating torque. This, in turn saves the engine from any hint of damage unlike other mounted accessories such as magnetos. The dry pump design has remained essentially the same over the years with the exception of improved material technology and wear indicator options. These pumps can also be easily overhauled or exchanged when replacement is necessary.
A sheared drive coupling in a dry type vacuum pump
Flight after flight, these dry vacuum pumps quietly and consistently do their job so well that the reliability factor allows us to become somewhat complacent with such simple tasks as replacing filters or deteriorated hoses. The pump itself is relatively maintenance free but the aircraft manufacturer usually documents a service limitation on the pump and/or regular testing with a wear indicator tool. The carbon vanes are subject to noticeable wear somewhere between 500 and 1000 hours of service dependent on the operating environment. Vane wear is therefore expected to accelerate as the pump accumulates time in service. There are repair and overhaul kits available for these pumps but it is usually best to replace a worn pump with an exchange unit from the manufacturer. The overhaul process includes re-conditioning the eccentric pump cavity as well as replacing the rotor vanes. When replacing the pump, it is also an opportune time to inspect the condition of the instrument hoses making sure that there are no loose contaminants or deterioration of the rubber hose material. Many high performance aircraft are also equipped with a stand-by vacuum system to ensure reliability even if the primary pump fails. Other contributors to pump failure besides time-in-service include heat stress, contamination, improper installation or operational stress from kinked or improperly installed vacuum hoses. Vacuum systems are generally universal across most aircraft types but you may also see pressure systems in Beechcraft models where the theory is reversed to push air through the gyros rather than pulling it.
A typical dry vacuum pump with a warning against potential vice damage
One of the foremost enemies of the vacuum pump is heat generated simply through normal operation. Anything that can be done to lessen the effect of this inherent condition should be of interest to the owner/operator. The introduction of vacuum pump cooling shrouds have become standard additions in many cases for this very reason. These add-ons can also be purchased for any pump that does not already have one. Once installed, the shroud assembly takes cooling air from a baffle opening and forces it through a scat type cooling hose to the shrouded circumference of the pump body. Simple as that.
A pump cooling shroud with hose and baffle mounting attachment
The vacuum system includes a relief valve (aka a regulator) located between the gyros and the pump. This component allows ambient air to limit the pressure differential across the gyros to about 5 in hg (inches of mercury). This value is dependent on pump output as determined by the engine RPM during ground operations. The normal operating value may not be indicated at lower RPM but will settle into the green operating range upon reaching 1700 RPM or so, then should remain consistent from that point on. The regulator can be manually adjusted to compensate for even a conservative amount of vacuum loss due to internally worn components. Keep in mind that these adjustments are often a temporary solution to symptoms of a gradual problem within the system. This is why it’s beneficial to understand the system and recognize any possible sources of defects. The amount of available suction is indicated on the suction gauge, also known as a vacuum gauge, to provide the flight crew with a visible indication. The vacuum gauge on a typical Cessna model is marked with a green operating range of 4.6 to 5.4 in hg.
Vacuum filters are an often overlooked part of the vacuum system. The main inlet filter is also referred to as the central filter (being central to the system) and is a canister type design with a pleated filtering element usually mounted to a bracket attached to the interior firewall. The filter represents the first line of defense for trapping any dirt and dust particles present in the ambient air source. Air is drawn in through the filter and fed directly to the gyros via rubber hoses. These filters are often recommended for replacement after 500 hours of service. The filters are relatively easy to replace yet sometimes difficult to access under the panel and they must be installed properly in order to prevent any unfiltered air from entering the system.
A common central air filter complete with hose fittings
The vacuum regulator is also mounted to the firewall and is equipped with a foam type garter filter, which is often replaced at or around 200 hours of service. These filter replacement figures are common to the industry but may be subject to alternative replacement schedules as determined by the operator and their approved maintenance schedule. Cessna have also introduced a recommended replacement schedule for the pump drive coupling to ensure that it does not shear prematurely due to the ongoing effects of service fatigue. As manufacturers focus more on preventative maintenance for vacuum systems, we can be assured that our applied maintenance practices in the field will go a long way towards promoting extended reliability.
The firewall mounted vacuum regulator and garter style filter
Modern electronic flight systems using digital technology in the form of multi-function flight displays (aka a glass cockpit) can effectively replace the gyro instruments rendering the entire aircraft vacuum system to be redundant. Once aircraft have been upgraded to these modern systems, the vacuum components are removed and the Weight & Balance amended to account for the adjustment in aircraft empty weight.
A modern panel using electronic attitude and heading flight instruments
In summary, we can see how aircraft vacuum systems have been proving themselves as a relatively simple yet reliable means for keeping the gyros turning. The minimal servicing and maintenance requirements have also proven that it doesn’t take much to keep the system operating unless some of the elementary steps described earlier have been neglected during scheduled maintenance activities. We are now witnessing another significant leap forward in general aviation with the modern options for glass technology to replace vacuum systems altogether. In the meantime, most operators will certainly remain dependent on good old vacuum theory for continued operation of their gyroscopic instruments and will continue to rely on a few necessary practices to keep them running for a long time yet.