Q: Hi Steve,
I am planning to buy a nice Piper PA-31-310 Navajo. I’ve got my eye on one that’s based locally and I really like this airplane. It does burn some fuel, but boy is it a good performer.
During the pre-purchase inspection the mechanic told me that he thinks I should upgrade the air conditioner to the R-134 type . I asked why, since the AC seemed to be blowing lots of cold air during the test flight.
He said that the system was old and that he suspected it had been recharged just prior to the test flight to cover up the fact that it was old. He pointed out a couple of hoses that do look pretty tired.
I know that the old R-12 refrigerant has been outlawed by the EPA and that the new stuff is R-134a. Will R-134a work in the R-12 system?
How about charging the system with some of the other stuff that’s advertised as an R-12 replacement? If not, what other options do I have?
—Staying Cool
A: Dear Cool,
The first thing I would do is test your present system (vacuum testing is the best method) to see if it’s tight and leak-free. If it is, then there are still places you can buy R-12; check eBay. You need to know that the use of R-12 was banned due to its detrimental effects on the ozone layer, so please think that over and if you do decide to go ahead, do everything you can to prevent the release of R-12 into the atmosphere.
A better solution is to charge your system with one of the R-12 replacements. You can find the list of approved R-12 replacements online at the EPA website. (See Resources for the site information. —Ed.)
This list also describes the components of the replacements. Some products such as Freezone RB 476 consist of a mix of hydrochloroflurocarbons (HCFCs) such as R-134a and HCFC 142b.
Let’s discuss why charging a R-12 system with R-134 won’t work. The molecules in the R-134 are much smaller than the R-12 molecules—the new refrigerant will probably leak out of all hoses that were okay for R-12. The mineral oil used to lubricate the compressor in the R-12 system reacts with the polyalkylene glycol (PAG) or ester based synthetic oils used in R-134 systems; if all the mineral oil isn’t washed out of the system components prior to charging with R-134, this reaction will compromise the lubrication and can lead to compressor failure.
R-134a, which is also an HCFC, isn’t as effective at moving heat as the R-12 coolant therefore systems designed for R-134a will usually have larger condensers and more powerful compressors. But older systems can be effective with R-134a.
You have the choice of recharging your system with R-12 (if you can find it and you qualify for purchase under the EPA’s restrictions), recharging it with one of the approved replacements, or upgrading your system with components designed for R-134a.
The simplest next step is to simply recharge your system. That’s what your mechanic believes the seller did prior to demoing the airplane. This choice is only a good one if the system doesn’t leak.
Air conditioning systems do require regular maintenance. Preventive maintenance tasks include checking and maintaining lubricant levels, maintaining coolant charges and replacement of seals, O-rings and hoses. Check the aircraft logs to determine if the system has received regular maintenance. The service manual for the Navajo contains detailed instructions for servicing, maintenance, testing and troubleshooting system problems.
If you choose to charge the system with one of the approved replacements, you’ll need to drain the existing oil from the compressor, remove and replace the receiver/dryer (dehydrator), and purge the system to remove any additional oil and contaminants. Then the correct amount of new PAG or ester oil should be added.
Finally, the system is checked for leaks. The most common way it to connect the system to a vacuum pump capable of pulling at least 28 inches of vacuum before closing the system and watching the manifold or servicing gauges over a period of time, say 30 minutes, to see if the system leaks.
If it doesn’t leak, then charge the refrigerant levels in accordance with the manufacturer’s instructions.
The last option is to buy a complete retrofit kit from ConversionAirKits.com. These kits include a new compressor, a new receiver/dryer, new hoses, new valves and all the oil, coolant, and hardware needed for a like-new R-134a specific installation.
The ConversionAir kits are tailored for each airplane type and come with everything needed to “modernize” your existing system. Installing a ConversionAir kit is the best long-term solution.
Happy flying.
Q: Steve,
I own a Comanche and my fuel gauges are almost worse than no gauges. Rather than throw big bucks at getting these fixed—the technology is at least 50 years old—I want a simple dipstick. Do you have the dimensions to make a dipstick for the 30 gallon main tanks on my Comanche?
—Stick Man
A: Dear Stick,
You’ve come to the right place. I have measurements for the exact dipstick you’re looking for. Research shows that every Comanche—the 180, the 250, the 260 and the 400—is equipped with a main fuel tank that has a capacity of between 28 and 30 gallons. The center of gravity of all these main tanks is 90 inches aft of the datum.
I checked these measurements after replacing one of my 30 gallon fuel bladders by dipping the fuel level during the initial fill. The figures provide a guide for fabricating a dipstick that will allow you to get a close estimate of the fuel level in your Comanche main tanks. I hold the stick against the outboard rim of the filler neck when I dip the tank.
Here are the numbers:
1 7/8 inches from bottom = 5 gallons
3 1/8 inches from bottom = 10 gallons
4 7/16 inches from bottom = 15 gallons
5 ¾ inches from bottom = 20 gallons
7 inches from bottom = 25 gallons
8 1/8 inches from bottom = 28 gallons
After I have a good reading of the fuel levels, I simply divide the fuel quantity by a conservative estimate of my fuel consumption—I use 11 gph for my 180 hp engine—and add in my safety margin to get my safe flying time from takeoff to landing. The dipstick/clock method of fuel management is dependable.
The safety margin I use depends on the type of flight (IFR or VFR) and the weather along the route and at the destination. For instance, let’s imagine the fuel quantity after I dipped my tanks is slightly more than 40 gallons. Based on my fuel consumption/hr standard, I have enough fuel to fly for 3.5-plus hours.
If the weather is forecast to be benign (no strong winds aloft and CAVU at my destination), I feel comfortable with a 30-minute reserve. If the flight will be under IFR or crappy VFR conditions, I need to adjust my flight time to preserve my one-hour safety margin.
I once heard well-known flight instructor, writer and aviation humorist Rod Machado tell his audience that two keys to becoming an old pilot are don’t run out of gas and don’t run into anything. The dipstick/clock method of fuel management is a low-tech tool that works every time.
Happy flying.
Steve Ells has been an A&P/IA for 39 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to editor@www.piperflyer.com.
RESOURCES
“Choosing and Using Alternative Refrigerants”
epa.gov/ozone/snap/refrigerants/macssubs.html
ConversionAir Kits
ConversionAirKits.com
eBay
ebay.com
