The Kerrville engineers who designed 36G were on their game. They thought of everything including VAC system failures when they engineered the M20K 252. 36G came out of the factory with a standby Electric VAC pump mounted in the AFT battery compartment as well as they replaced the standard VAC driven Directional Gyro (DG) that is installed in most single-engine prop planes with a much more expensive Electric Horizontal Situation Indicator (HSI). HSI's are much better than a simple DG since they help pilots orientate themselves visually to the course they are trying to intercept. The system is also Electric, so it doesn't depend on a VAC system to operate. They also have a huge Gyro instead of the small one mounted in the DG. The bigger gyro is more reliable, sealed from the outside atmosphere and mounted in a more stable location in the rear of the plane. HSI's were generally only installed in turbine airplanes in 1988. Adding one to a Mooney was a major step forward from a safety perspective. The combination of an electric HSI and also having a standby VAC pump has greatly reduced the likelihood that 36G or her Mooney cousins will enter the dreaded graveyard spiral. The years of flying Mooney's have clearly proven that the extra expense has saved lives...
Mooney's are notorious for their reliability, speed and safety because serious thought went into every detail. Not many prop planes have both an HSI and a standby VAC pump even in 2007, which is likely why Mooney's have significantly less IFR accidents than any other single-engine airplane! Based on the AOPA Mooney Safety Review, Mooney's that are flown by IFR pilots have an average of 1.89 IMC accidents per 100,000 hours flown compared to 4.97 of similar aircraft. This is a significantly relevant difference, which I personally credit to the fact that a VAC failure is less likely to kill you in a Mooney than most any other comparable airplane. Way to go Mooney!
It is true enough that I could certainly get away with installing a dry pump and waiting until it fails to install another one, but why take the chance? I have the resources, I have ample evidence that dry pumps are failure-prone, and I am not the type of person to take unnecessary risks -- Period! The decision is clear in my mind, I hope it is in yours. Let's install a Wet pump...
Preparing for a Wet Pump Installation
Installing a wet pump is not all without some disadvantages. The most notable is the fact that a small amount of engine oil is used to lubricate and seal the camber so it will produce a vacuum. What happens is that the discharge side of the pump sprays a very fine mist of oil into the engine compartment. This is certainly safe, but annoying if you are trying to keep the engine compartment clean. It is not a lot of oil, but even a small drop can spread everywhere. This single limitation is why the Dry VAC pump was created in the first place, but overcoming this problem is simple.
There is certainly no harm in a bit of oil spray on the engine, but there are options to keep your engine clean as well as greatly reduce engine oil consumption.AirWolf, of course, has a product named the "AirSeps" that they licensed years ago from a company called Walker Engineering. Walker created the AirSeps years ago for general aviation aircraft that was the standard Oil/Air separator commonly used in aircraft with a Wet VAC pump. When Wet pumps were being replaced with dry, Walker got out of the aircraft business and focused on diesel and large equipment that uses similar, but much larger, versions of the AirSep. AirWolf bought the rights to license the AirStep and has sold it directly to owners for years.
The AirWolf AirSeps is not only used for Wet Pumps, it turns out to be superior at catching crankcase blow-by and pushing it back into the engine. They take the overflow output of the VAC pump to pressurize the AirSeps, which overcomes engine crankcase pressure. This forces overflow oil back into the engine. It also greatly reduces water vapor that develops in the engine helping mitigate against rust.
We installed an AirSeps at the last annual in preparation for the Wet pump and to help keep the engine in tip-top shape. There are other Air/Oil separators on the market including the M20 that is commonly seen in most aviation magazines. In my personal opinion, the M20 is too small and uses gravety to push oil back into the engine. The AirSeps has a material advantage since the VAC pressure overcomes the crankcase pressure. This forces the oil back into the engine instead onto the belly of the plane. It also has quite a bit larger cannister so it can hold a lot more oil before overflowing.
I recommend if you are going to install a Wet pump, install an AirSeps while you are at it. The systems only cost a few hundred dollars and it will easily pay for itself when you don't have to spend hours under the plane cleaning oil off of the belly.
Wet Pump Installation
The installation of a Wet pump is basically exactly the same as a dry pump. Virtually every engine ever made to be installed in an airplane is already feeding oil to the pump location. The dry pumps just cover the hole while the wet pumps let the oil flow into its chaise.
The first task is to inspect the installation of the current VAC pump. It is important to take note of inlet and outlet lines and how it mounts onto the plane.
After the hoses are labeled, carefully remove the hoses and then the 4 nuts holding the pump to the engine.
After removing the pump, take an inspection mirror and carefully inspect the engine mount. AirWolf also recommends removing the top plugs form the engine and then crank it over a few times. If the oil is feeding properly, you will see it spit out of the oil feed hole. You can see the oil feed hole and residue after the test in this picture at the bottom right.
The next step in the process is to carefully clean the engine VAC PUMP mounting plate. This is not something to be taken casually. the oil pump will leak like a sieve if it is not properly cleaned. It is also important to inspect the rubber gasket around the shaft for leaks. If you detect any, replace it.
The next step in the process is to fill the center gear connection to the pump with a quality Aviation Grade grease. This will squeeze between the new pump and the gear to mitigate against rust and keep the gears from binding.
The one final step that I do is to make sure the oil feed hole is clean of any debris before installing the pump. The easiest way to do this is to use some safety wire and carefully clean the hole. You do not want to force this. Slow and careful is the trick.
The final step is to mount the pump onto the engine. First install the gasket, then using the 4-8 sided nuts provided with the pump kit mount the unit on the engine. The nuts that come with the AirWolf Wet pump are not the standard lock nut. The unit is larger and has a very tight tolerance around the mounting holes. A standard wrench will work as long as you have one with the 8 positions. You will also need a torque wrench adapter to do the final torque. Craftsman, Snap-on, etc. sell these attachments.
After mounting the unit, you need to connect the hoses. I recommend replacing them. They are a standard 5/8" tube that you can get from Aircraft Spruce. Just make sure that whatever you buy is certified for a vacuum system. You don't want a hose that will collapse.
Speaking of replacing hoses, you also want to replace the vacuum system filters. I like the clear-view filter above. You can actually see the filter and compare it with the cart on the outside. This turns the annual filter replacement into an on-condition replacement. This filter gives you an instant reading of the cleanness of your VAC system. It is OEM equipment on Monney's now, but they have STC's available for many airplanes. This is also available from Aircraft Spruce.
Wrap up
All that is left is to do a through run-up of the engine. It needs to be brought up to temperature and validate that the pump is pumping properly. If you get zero reading on your VAC gage, you probably have the hoses reversed or the inline air filter installed in the wrong direction.
I would also suggest that you fly around a few VFR hours to make sure that everything is working properly. If you see fluctuations on the VAC gage, you see excessive oil in the cowl, etc., then there is a problem somewhere. The AirWolf manual has a very good step-by-step procedure on troubleshooting if you run into any problems. They also offer free technical assistance.
This concludes the Wet Pump installation. It is a very easy, few hour job. It does not take a lot of skill or any specialized tooling. This is, however, not an item that an owner can do without supervision. An A&P will have to sign this work off before a certificated airplane can be returned to service.
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