Aviation From My Rocking Chair: All That Sucks Is Not Bad

Hello again to all in Chapter 430, and I hope you and your families are healthy and warm. The subject for this month’s column is…(drum roll please) Pneumatic Instrument Systems: Attitude Gyros, Directional Gyros, and Rate of Turn Gyros such as Ball/Bank and Turn Coordinators.

If we wanted to check the FAA requirements for gyro instruments, where would we look? I know I heard everyone say, “FAR 91.205(b)”, and you are all correct. Reading down the list of required gyro instruments for VMC flight either day or night, the total is 0, none, zip, zed, nil. Wow, that should save a lot of money, eh? Well, yes it could, but, for the sake of yourself, family, and friends, please consider at least a venturi-driven Ball/Bank. From personal experience, the horizon really does disappear on a moonless night in the Texas panhandle, or that under-cast may have grown after your 180 degree turn.

Now, let’s complicate things a bit. FAR 91.205(d) covers gyros required for IMC flight. These are: (1) Rate of Turn. (2) Artificial Horizon (3) Directional Gyro. They can be either pneumatically or electrically driven. All aircraft which operate IMC, also require a back-up power source for air-driven gyros, or a back-up electric Rate of Turn indicator. Normally, the Gyro Horizon and the Directional Gyro are vacuum pump driven, and the Ball/Bank or Turn Coordinator are electric. In this way, a failure of either system leaves at least partial panel for emergency navigation.

Also installed in the panel is a Vacuum Gage (or Suction Gage). When the system is operating correctly, the normal range is 4.5 to 5.5 Hg. As you have probably noticed, the Vacuum Gage is usually very small. This becomes a problem, in that pneumatic system failures may happen very suddenly or very slowly, and slow failure is much more dangerous unless you constantly cross-check ALL instruments, including the Vacuum Gage. If gage pressure is low, suspect a dirty air filter, dirty screen, sticking regulator, worn air pump or a system leak. Zero vacuum pressure points to a pump failure, collapsed line or an inoperative gage (I should be so lucky!)

O.K., Rick. Why do I even need to worry about vacuum pump problems? I just bought a new 0-320 with a 2400hr TBO. My answer, dear friends, will possibly shock you. Of the two types of pumps (wet and dry), the “wet” type is longer lasting due to being lubricated by engine oil. Unfortunately, it is also much heavier, and more expensive. Note: not all engines can be equipped with these pumps. The “dry” type pump has a much simpler mounting, and is considerably lighter and cheaper. The problem being that they must be kept free from oil or they will fail very quickly.

The following is the recommended AIRBORNE pump replacement schedule:

(1) Any Model# beginning with 200-212. 1100 hrs.

(2) E211cc or E212cw. 1200 hrs (3) Any Model# Beginning with 215-216. 1200 hrs.

(4) Any Model# Beginning with 220-242. 600 hrs.

(5) Any Model# Beginning with 420-442. 450 hrs.

(6) 832cw or 842. 300 hrs.

If you are surprised by this information, that is a good thing. Occasionally include the Vacuum gage in your scan, cross-check the other instruments, fix any problem as soon as you land, and fear not that “dark and stormy night.” Well, troopers, if you see a shuffling guy hanging around this aviation neighborhood, wave and say “Hi.” It’s probably me. Rick Vaux TC 4130