IT HAPPENS EVERYDAY. An aircraft's vacuum system fails without warning. Suddenly, the gyro instruments critical for IFR stop working. Unless atmospheric conditions are crystal clear, the pilot is flying blind.

That's why we designed the SVS V Standby Vacuum System. It operates on the differential between manifold pressure and ambient pressure. When activated, the SVS V restores flight instruments to their full capability. The Early Warning alerts the pilot to vacuum failure before gyro instruments are affected. That leaves plenty of time to take corrective action.

The pilot activates the SVS V cable control which opens a bypass valve into the engine intake manifold, then adjusts the power setting by watching the instrument suLction gauge along with the settings placard on the panel. This allows vacuum to restore so the crew can find a place to land and replace the primary vacuum source.

How Does It Work?

The  Standby Vacuum System is designed to operate two directional gyros in the event of a primary vacuum source failure. The SVS allows the use of engine intake vacuum, in conjunction with a flight tested operating procedure, to supply vacuum to the primary aircraft instruments. This vacuum supply is limited by the difference between ambient air pressure and intake manifold pressure; the system is for emergency use only and is most effective below 8000 feet ASL.

A three way check valve called a shuttle valve ties into the existing vacuum system between the vacuum source and the vacuum regulator. One side of the valve connects to the vacuum source, one side to the intake tap, and the third outlet connects to the instruments. An on/off valve controlled by a push/pull cable activates the system. The pilot is informed of a vacuum source failure by a warning light. The warning light taps in at the vacuum source alerting the pilot of a vacuum failure at the time the vacuum source fails rather than when the gyros begin to tumble.

Manifold pressure exists in any engine intake system. There is a higher pressure in the manifold at higher RPM's (more power=higher pressure). Think of a cylinder at sea level. At sea level the pressure is approximately 29.92" hg at 59 degrees F. If the cylinder is pressurized to 40" hg and vented to the atmosphere it will push air out until is equalizes. If the cylinder is depressurized to below the atmospheric pressure in this case less than 20" hg, it will suck air in until is equalizes (creating vacuum). With that in mind, when the pressure in your manifold is 23" hg, the open vent (line connecting to the instruments) to the SVS will create vacuum (suck air into the intake manifold) trying to equalize to the ambient air pressure of 30" hg at sea level.

Ambient air pressure changes due to temperature, humidity and altitude. Ambient air pressure decreases as altitude increases. Using the more power=higher pressure equation, as we go up in altitude we have to lower aircraft power to create the differential that we need. Thus allowing the intake to suck air through the SVS system. This creates vacuum for flight instruments.

The SVS works off of a differential between atmospheric and manifold pressure. A typical vacuum regulator is set at 5.5 in. hg. Atmospheric pressure varies with altitude and temperature so the numbers used to represent the atmospheric pressure will vary for any given situation. For this example we will use the altitude of 5000?Lets assume we are cruising at 5000 feet and our manifold pressure is 22 in. hg and the vacuum warning light is activated. First thing to check is the vacuum gauge just to be sure that indeed we have lost our vacuum source. At this point we activate the standby vacuum system. We need at least 3.5 in. hg of differential to operate two directional gyros. The atmospheric pressure at 5000?s approximately 24.5 in hg. With a manifold pressure of 22 in. hg this only creates a differential of 2.5 in. hg. In order to create enough vacuum to run the gyros we need to reduce power to at least 21 in. hg. This creates the differential between atmospheric pressure and manifold pressure of 3.5 in. hg that we desire. The only time the regulator comes into play is if the vacuum exceeds the regulator setting of 5.5 in hg and then it reduces the vacuum to this setting. At 4000?he atmospheric pressure is nearly 26 in hg, with a manifold pressure of 22 in hg we have nearly 4 in hg in differential pressure. In order to create vacuum in most cases we have to reduce power. This is why we have to make adjustments to the engine rpm/manifold pressure. In a perfect world the above is true, however we have to take into some other considerations such as pneumatic friction loss, engine condition, installation, etc. This will affect the actual power settings that need to be made. This is why the flight test is important. During the flight test the actual engine settings are recorded on a placard and placed in view of the pilot for easy reference. The pilot who may have to use the system should accomplish the flight test. This will give him/her a good understanding of how the system is operated.