General Questions

[N170CT]

I would like to ask two questions of our distinguished 170 folks:

1. Why are we instructed to "Switch to single tank operation for cruise operations above 5000 feet"??

2. Why have most of our 170 owners relocated the compass from behind the instrument panel to other locations?? Are there any adapters out there for installing a compass in the original location?

Appreciate any and all ideas/comments.

[GAHorn]

I would like to ask two questions of our distinguished 170 folks:

1. Why are we instructed to "Switch to single tank operation for cruise operations above 5000 feet"??

2. Why have most of our 170 owners relocated the compass from behind the instrument panel to other locations?? Are there any adapters out there for installing a compass in the original location?

Appreciate any and all ideas/comments.

Are you perhaps thinking of AD 72-07-02 which applies to Cessna 172's? This AD does not apply to our 170's.

AD72-07-02 required the installation of a placard that reads:
SWITCH TO SINGLE TANK OPERATION UPON REACHING CRUISE ALTITUDES ABOVE 5000 FEET. Cessna Service letter SE72-7 details the placard change and recommends owners who have experienced vapor problems install a vent kit to eliminate the vapor problem. The reason for the AD was fuel interruptions at altitudes above 5000 feet caused by vapor formation in the fuel lines. Cessna did testing as detailed in SE72-7 and was never able to duplicate the vapor condition being reported.
The formation of vapor was associated with high altitudes, high temperatures, humidity and original paint schemes. The logic in switching to single tank once reaching a cruise altitude above 5000 feet would be to allow any vapor forming in the down feed fuel line to flow up the line and into the tank and out the vent system on the tank not being used. If the engine began to experience power irregularities due to vapor forming in the line of the tank in use, then selecting the opposite tank with no vapor should return the engine to normal operation.
SE69-26 announced the availability of kits to add the vent lines to earlier 172 models. When the vent system was modified the AD no longer applied and the placard was removed. The kit lists for $989. Most 172 owners saved their money and either switched to the single tank operation or ignored the AD instructions, and are still flying happily along without the advice of Cessna lawyers.

My compass is still living happily where Cessna originally put it. Some owners found electrical system changes, and magnetized control systems influenced their compass when they checked full and complete movement of flight controls on the ground. Such manuevers in flight frighten me so I don't do them and my compass seems to work just fine.
Seriously, some owners wanted more modern-looking panels, or center-stacked radios, or otherwise wanted the compass relocated for a variety of reasons, but it's true that some aircraft had large compass deviations due to control wheel placement. I believe it's important to calibrate the compass with the controls in the nuetral position, because that's where things are in flight. I had a letter recently regarding a member who's compass would really swing when he turned his landing light on. But I have to wonder "So what?" That's a time when in/near the pattern and the need for a really accurate compass reading is moot. Short-term compass irregularities are not something to worry about I don't believe. Many airplanes have huge compass deviations when electrical systems are energized such as lights, air conditioning, anti-ice equipment, etc. A Cessna 425 Conquest will turn it's compass a complete 180 degrees with the windshield heat on and a King Air will easily swing it's compass 20-degrees or more whenever the cabin fan operates (a continuous event in order to condition the bleed-air pressurization.) A placard to the effect is all that's required.