Vacuum Source

3105 NE Dennis Ct.
Bend, OR 97701
PH: 541-389-4884
FAX: 541-389-4885


Even a simple product like the Standby Vacuum Systems can get a little complicated. We have put together a list of the most common questions we receive and have grouped them for easy use.

Even a simple product like the Standby Vacuum Systems can get a little complicated. We have put together a list of the most common questions we receive and have grouped them for easy use.

How Does It Work?

Trouble Shooting

Installation Questions

Airworthiness Directive Questions

How Does It Work?

The Precise Flight 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 5000í. 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 5000í is 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 4000í the 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.

Trouble Shooting

1. Is my engine supposed to run rough when I pull the cable for the Standby Vacuum System?

Answer: The Standby Vacuum System allows for controlled vacuum leak from the engine ‚ At idle, vacuum leaks are evident from a rough running engine. The system will be used when the engine speed is above idle and the leak is small in proportion to the total aircraft engine airflow. The vacuum leak can be minimized by insuring that the vacuum regulator is working properly.

2. When I did the flight test the Vacuum never varied. Is that the way it's supposed to work?

Answer: The Vacuum should vary with changes in altitude and engine settings. Make sure you disconnect the primary vacuum pump in accordance with section 3.1 prior to performing the flight test.

3. I can't get 3 1/2 inches of vacuum when I perform the flight test. Why not?

Answer: Troubleshoot the System in accordance with section 3.2 of the installation report 08072 or 08074. If no faults are found, replace the vacuum regulator. This should correct the problem.

4. If I perform the alternative method of compliance do I still have to check the system every year?

Answer: The continued airworthiness section of 3.3 of installation report 08072 or 08074 explains what is required for continued airworthiness. (A check should be performed at each annual).

Installation Questions

5. Can I run three instruments with the Standby Vacuum System?

Answer: No. The System is designed to run two instruments. It will not reliably operate three instruments.

6. How long does it take to install the Standby Vacuum System?

Answer: Estimated Installation time is 4-6 hours.

7. My aircraft already has a vacuum warning light from the factory. Do I have to install the one in the Standby Vacuum System Kit?

Answer: Usually, the factory installed warning lights are tapped into the wrong location to work as intended by the STC. You should install the warning light supplied in our kit as well.

Complying with AD 99-24-10 or 05-11-05

8. I have an SVS-III, What do I do?

Answer: Check the Shuttle Valve Serial Number (printed on a black and silver placard affixed to the Shuttle Valve usually underneath the Adel Clamp). Also, check FAA Form 337 in aircraft records for Serial Number.

If the Serial number is 10243 and above, do a paperwork upgrade. If the serial number is 10242 and below, order the SVS-V upgrade kit. Refer to the Alternative Method Of Compliance document for more information.

9. I have an SVS-1A, what do I do?

Answer: Purchase the SVS VI upgrade kit and convert the system. 

10. I have an SVS-1, what do I do?

Answer: This is our older automatic system. There are no longer any replacement parts available.To comply with the AD this system should be upgraded to an SVS-V.    

11. What paperwork must be filled out when complying with the AMOC (Alternative Method Of Compliance for AD 99-24-10)?

Answer: You must make a logbook entry stating "Complied with AD 99-24-10 in accordance with alternative method of compliance. Upgraded system to SVS V".

12. How do I know if I need to replace my shuttle valve?

Answer:  You should replace any shuttle valve with a serial number (on the sticker on the shuttle valve) is lower than 10243.  This can be done by purchasing either the 05158 upgrade kit (for SVS-III systems) or 05174 upgrade kit (for SVS-1A systems). 

13. What must be done when performing the flight test of the Standby Vacuum System?

Answer: Refer to section 3.1 of installation report 08072 or 08074 which can be downloaded from our "SVS Documents" page.


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