PREVENTIVE MAINTENANCE - Part 2
May 2013
Previously, ShopTalk tackled preventive maintenance on your Mooney from firewall-forward. This month’s ShopTalk will address airframe and firewall-aft areas on Mooney aircraft. Some subjects to discuss are: Landing gear & wheel well, cabin interior, battery, exterior & wings.
When inspecting the airframe one has to take into account the hours and calendar years when it comes to aging aircraft problems. Rubber products tend to lose their flexibility as they age, such as landing gear shock discs. Let’s say they have a manufacture date of 2003. The airframe may have a few hundred hours of flight but the rubber disks are ten years old. These discs need to expand and contract to absorb energy when landing and taxiing preventing damage to wing spars and fuel tanks.
On short body Mooneys these discs are typically good for 10 to 15 years on the main gear and longer on the nose (if not often flooded with solvent when the engine is washed). Long body planes are heavier and tend to need main gear discs more often. To determine if the discs are still flexible, take the weight off the wheels (airplane on jacks) for a half-hour or so to let them expand. If the disks can be easily turned then they are flat and need to be replaced.
Another area subject to deterioration is rat socks that attach to the inside of the wheel well and main landing gear rod. These fabric or leather socks are similar to the witch's boot on a sports car manual gear shift lever. They keep debris and carbon monoxide out of the wing box area and cabin. Verify their condition and if a tight seal isn't provided, repair or replace them.
Tires are another area where poor maintenance can ruin your day if not corrected in a timely fashion. A typical example of this is having low air pressure in your tires. Side loads when you land and turn off the runway roll the tire under the wheel, ripping out the valve stem and causing a flat. Picture youself stuck blocking the taxiway intersection. At best, this is it embarrassing but additional damage to the airplane is possible. Insure your tires are properly inflated before you fly.
Dry rot and cracks on tire side walls can put you in a simlar circumstance as mentioned above. Ask your mechanic how many cracks in the tires are too many and replace them before they blow out. When you are replacing tires use a Michelin Airstop inner tube. When first certified by the FAA, inner tubes were natural rubber which tend to loose air over time. Michelin's Airstop tube is a synthetic rubber compound that holds air better then natural rubber; you will not have to refill the tires as often. Doing this is just another way to keep from getting stranded on a taxiway.
While you are under your airplane inspecting the tires, check out the brake pads. Replace the pads when they are two-thirds worn down. This prevents the piston in the brake caliper from being pushed out so far that the O-ring on the piston starts to leak. It will also get you longer life out of brake discs by not grinding on the rivets that secure brake pads to backing plates.
When calipers begin to leak it is usually time to rebuild them. When changing pads, clean and lube the brake caliper pins and bleed all air from the system so the caliper will float free when there is no brake pressure. This will extend the life of the linings.
A common problem seen during an annual is brake master cylinders leaking. If there is hydraulic fluid dripping from the master cylinder then have it rebuilt. Back bleed the brake and you won’t have that cylinder blowing out and you having a flat brake pedal right after landing. Locate the brake reservoir and maintain its level for proper brake (and flaps on 1960s planes) operation. NEVER ADD AUTOMOTIVE BRAKE FLUID! Use only MIL5606 hydraulic fluid. Automotive fluid will contaminate the system resulting in many expensive shop hours flushing out the clogged systems.
Always check the landing gear down-lock tensions at each annual using proper Mooney tools and torque settings for your airplane. If you have an electric gear airplane and the electric motor portion has over 3,500 hours on it then get it overhauled or replaced with a new or overhauled unit. I recommend replacing the no-back spring in the landing gear transmission every 3,500 hours if the plane has never been used as a trainer. If it does have trainer history and the no-back spring has not been replaced recently, then replace it ASAP.
When the plane is on jacks make sure all gear doors fit together properly and the gear is retracting the correct amount into the wheel well. A partially open gear door will flutter and wear the hinge out prematurely. A partially open gear door on one main gear will cause asymmetrical drag and induce a yaw in flight.
Always grease the steering horn every 100 hours and at every annual. If there is excessive play in the horn then pull it out and shim it or have it replaced. These simple tasks with the steering horn should keep your Mooney on the runway and out of the ditch next to the runway. While you have the grease gun out, grease all retraction pivot points on each gear.
Everyone thinks a annual is all about packing wheel bearings. However, the wheels turn under load only intermittently. Unless the airplane operates often on a dirt runway or taxiway, 200 hours or when you change a tire is plenty of greasing for normal use aircraft. I used to have a customer that went to Baja and land on a dirt runway right along the beach. This is not a normal use aircraft and the wheel bearings had to be clean and greased every time the plane was in for maintenance. So look at where you land and then tailor wheel bearing greasing to match your usage.
Oil pressure gages on pre-1977 aircraft have a flexible oil hose. Like all rubber products, this hose will deteriorate and leak hot oil into the cabin. Check its date tag and if older than 15 years replace it with a new hose. Some pre-1977 aircraft have a fuel flow/pressure flexible hose in the cabin (others have a metal line). If this hose is over 15 years old then replace it also. Heater and defroster hoses on the pre-1977 aircraft are typically rotten and falling apart, so replace these as needed as a properly defrosted windshield is pretty important on cold days.
While under the instrument panel, look at the flexible hoses that go to your vacuum gyros and suction gage. Some 1979 and newer planes came from the factory with soft surgical tubing installed and these hoses were notorious for failing causing a vacuum leak. Most have failed by now but there are still a few planes out there with these crummy hoses installed. If you have these old hoses replace them with proper low pressure aircraft hose material.
The pilot's instrument panel is shock mounted on rubber mounts which sag and break with age. Take a careful look at them and if one needs replacing then replace them all. They usually last 15 to 20 years.
I see quite a few airplanes that are missing an elevator trim wheel chain cover. Insure this little plastic cover is properly secured in place as it’s possible for a seat belt to get caught in the chain if the cover is missing. A jammed elevator trim chain could ruin your day while landing.
On the floor behind the elevator trim wheel on post 1978 aircraft is the emergency gear extension handle and cover. Make sure that the cover for the handle is properly locked and no one has stepped on it. If this cover is not properly secured the landing gear will not properly retract and the gear circuit breaker will pop. This could cause pilot anxiety on takeoff and most passengers don’t want to witness this.
Check cabin and instrument panel interior lighting every now and then. Having these lights working properly makes life easier especially if night flying is an occasional experience.
Retractable step aircraft have a rubber PC (Positive Control, by Brittain) cup that tends to rot out and cause the step to not retract properly. The PC system utilizes four control cups. Two in the tail cone that attach to the rudder push-pull tube and one at each aileron bell crank out in the wings. If these leak, rolls and turns under PC operation become uncoordinated.
Having intact cabin door seals and wind lace make for a quiet and warmer cabin in flight. I have found a door seal that actually works on late model Mooney aircraft. AvTec makes a two-row door seal that keeps cold air out. If you need a door seal this is the way to go. However, don't use it on pre-1978 aircraft that have an external round door handle. Door seals on these airplanes are already too tight. The interior door handle is poorly engineered and is notorious breaking off leaving the pilot and passengers locked inside. Savvy owners of these early Mooneys carry vice grip pliers in their planes just for the above problem.
Aircraft batteries have come a long way in the last 20 years. When Concord first introduced their sealed battery it was not much better than a tried and true Gill flooded battery. I even remember Sears sold a Die Hard battery for aircraft a long time ago. Concord now produces a very good sealed battery and it is not uncommon to see one 5 or 6 years old still working fine. Most flooded batteries won’t make it 4 years without needing replacement, some as soon as 2 years.
A lot of factors effect battery life. The charging system on your plane is the first thing to monitor. You will need a voltmeter for this and older planes only came with an ammeter. Some GPS units have a voltmeter built in. The basic thing a battery needs for long life is regular activity. With a full electrical load (all lights, pitot heat on, etc.) and low RPM (1200 - 1500), voltage should be 12.9/28.0 volts. In cruise with light electrical load buss voltage should be no higher than 13.9/28.4 volts. If your airplane is not within these parameters then you need to have the regulator adjusted.
With flooded batteries you need to check electrolyte levels at least every 6 months for alternator systems and every 90 days for generator systems. With hot weather and high charging voltage, inspect more often. If necessary, add only enough distilled water to cover the battery plates.
If your plane is sitting for more than 30 days you may want to install an outlet for a battery maintainer and plug in your plane so the battery won’t go dead. Some airplanes have a clock that runs when the master switch is off, and this will run down your battery.
If you elect to install an outlet for a maintainer then make sure it is fused properly and get a quality maintainer not just a charger. No, you can’t charge the battery through the APU connector. If the aircraft is parked where there is light, a solar type charger can provide the trickle charge. If the cigar lighter socket is hot with the master switch off, it may be used as a socket for a maintainer.
When cleaning your windows always use a plexiglass approved product along with a soft untreated cotton rag. Don’t use a circular motion as this will eventually put swirl marks on the windows. Use an up and down motion to clean bugs off and if the plane is covered with dirt, rinse the dirt off first. Dirt under the cleaning rag acts like sandpaper on the windscreen.
One item often overlooked on the wings are fuel cap O-rings that seal out wash and rain water that gets trapped in the fuel fill cavity. If an O-ring looks cracked and most do, then replace it with proper fuel tolerant O-ring and grease/lube the cap and new O-ring.
As has been discussed here, you as the owner/pilot can do many of the items listed in this article. Don't forget to make an entry in the airframe logbook of what maintenance was accomplished. Date and sign the entry with your name and pilot's certificate number.
As always if you have a question about this article feel free to contact me at my aircraft repair shop at 307-789-6866 or via e mail. Until the next ShopTalk, enjoy flying your Mooney.
