Pre “J” Mooneys and M20K Mooneys that have the Rocket conversion have one thing in common. Most all use the “shower of sparks” starting system.
The “shower of sparks” system is fairly simple, but widely misunderstood. Most folks think there must be a “genie in a box” that makes our engine start by magic. I’ll try and explain how it all works and why and also give some easy tips on how to check and see if your system is working properly.
I receive many calls from owners and mechanics that are experiencing “shower of sparks” (SOS) starting problems and asking for help in diagnosing their problems.
Why We Need the “Shower of Sparks” System
Our Mooney engines operate on the four-cycle principle: intake, compression, power and exhaust. On the intake cycle, air and fuel are drawn into the cylinder. The fuel/air mixture is compressed on the compression stroke. Near the top of the compression stroke, the mixture is ignited forcing the piston down. The power stroke and the hot gases are expelled on the exhaust stroke, and once completed, it all starts again and is repeated hundreds of times each minute.
Unlike automobile engines, aircraft engines have fixed ignition timing. Normally the magneto is installed to fire the spark plug at 25 degrees before the piston gets to the top of the compression stroke. This timing is for the best operation of the engine after it is running. The problem, with the timing set to 25 “top dead center” or “TDC”, the engine would try to run backwards. So, to get the engine running in the right direction, we must delay or “retard” the time of ignition. The best time for ignition to occur in starting is just as the piston is at the top of the compression stroke, TDC, or just a little past it. In this case we must delay the spark for 25 degrees of rotation.
Shower of Sparks Component Parts
The “Shower of Sparks” system is composed of a magneto with two sets of points installed, a vibrator switch, ignition switch and the aircraft battery.
The magneto has two sets of contact points. One set is adjusted to open 25 degrees before TDC used for normal operation. The other set; the “retard points” are adjusted to open 25 degrees later at TDC for starting.
The ignition switch has 6 positions. “Off” which grounds both magnetos and makes them both inoperative. “Right” allows the right magneto to operate and makes the left magneto inoperative. “Left” allows the left magneto to operate but makes the right magneto inoperative. “Both” allows both left & right magneto to operate.
The other two positions are “turning the key just to the right of the both” position activates the shower of sparks vibrator, and “pushing in” on the ignition key at this position activates the engine starter.
How it Works
When starting our Mooneys the ignition switch is turned to the far right. The starter vibrator is activated in this position and can be heard as a buzzing sound. The ignition key is pushed inward to activate the starter. At the same time, the ignition switch internally grounds the right magneto so it will not operate. The right magneto is set at the same 25 degrees as the left for normal operation, so we do not want it to fire and cause the engine to “Kick Back”. So during the starting process, only the left magneto is used for starting. The ignition switch also selects the “retard points” and disables the normal points in the left magneto.
The vibrator switch is now supplying interrupted aircraft battery power to the left magneto coil thru the retard points. When the piston reaches TDC on the compression stroke, the retard points open allowing the magneto coil to charge and discharge as rapidly as the vibrator points can open and close. The result is a continuous spark that resembles a lightning bolt igniting the fuel/air mixture. This event continues for a few degrees of rotation on each cylinder until the engine starts and the switch is released to the both position. In the both position, the right magneto is enabled as well as the left magneto’s main set of points.
Symptoms of a failing SOS system
1. Engine kicks back during starting
2. Hard starting
3. Engine starts when the key is released to the both position
Troubleshooting the Shower of Sparks System
1. Check for a buzzing sound as the ignition switch is turned past the both position but before pushing IN on the key. If a buzzing sound is heard, most likely the vibrator is alright, but it can still be defective.
2. With the cowling removed, disconnect the starter relay from the starter so that the starter cannot be engaged.
3. Fig 5 shows the starter relay. Remove the small wire at top of the relay to disable the starter
4. Remove the spark plug leads and top plugs of all cylinders. Remove the bottom spark plug leads.
5. Rotate the prop, by hand, until the number one cylinder is at Top Dead Center, TDC on the compression stroke.
6. Make certain that the starter has been disabled and cannot be activated by the ignition switch. Some early Mooneys (M20B and M20C) have a starter disengage switch located beside the starter vibrator switch. This switch was installed so that the starter relay could be disengaged to allow hand propping. NOTE: Whenever hand propping a Mooney or any aircraft with Shower of Sparks system, be sure and disconnect the starter relay. The SOS must be activated during the hand propping and you don’t want the starter to engage while someone is near the prop.
7. Hold the #1 spark plug lead by the insulation and place the spring at the end of the harness lead, very near the cylinder. Have another party in the plane turn and hold the key to the far right past “both”. The vibrator should be buzzing and a constant arc of electricity, “shower of sparks” should be seen between the tip of the lead spring and the engine cylinder. Sometimes it may be necessary to move the prop back and forth a few degrees to find the exact spot where the “retard points” will open.
Note: This step varies a little depending on the manufacturer of the magneto harness, but we will assume we are working with a 4 cylinder Lycoming, Bendix magneto with a Bendix harness. In this configuration the magneto will fire the top left sparkplugs, #1 and #3, and the bottom right sparkplugs #2 and #4. Other aftermarket harness manufactures have the harness fire the opposite bottom left and upper right.
8. When the “shower of sparks” is visible, I rotate the engine through its firing order (1, 3, 2, 4) to see that each cylinder is firing properly.
9. If no spark is seen, it is most likely the retard points in the left magneto. The magneto will have to be removed and the retard points inspected, reset or replaced. If the retard points in the magneto are good, the problem will be the vibrator or ignition switch.
10. Most starter vibrators in Mooneys are located on the cabin firewall just to the right of the compass center post. If your Mooney is original and still has the forward access panels, the switch is easily found by removing the co-pilot side panel. If your Mooney has been modified with a 201 style windshield, the vibrator will be more difficult to remove unless it was relocated by the modifier.
11. As our Mooneys age I am seeing more defective ignition switches. After 40 years, many are just worn out. A few clues are: keys that come out in any position; engines that will not start; and when the key is released; and the engine starts.
Hopefully this article has taken the “genie” and magic out of the “shower of sparks” system and will help you diagnose its problems. As always we are available to help you with your Mooney questions.
Don Maxwell email@example.com
MAPA Log, Volume 31, Number 32 (January 2008)
We’ve received a request for a “HOW TO” on replacing brake linings. Although not an approved task for owners under the preventive maintenance provision of Part 43, the task is fairly simple but will require a sign off by an A&P.
1. Flat blade screwdriver
2. Diagonal side cutters
3. 1/4″ combination wrench
4. 7/16″ combo wrench or socket and ratchet.
5. Pin punch and small shop hammer
6. Brake lining rivet tool
7. Brake Linings
9. 5606 Brake fluid
10. 8 oz pistol grip oil can
11. 1/4″ clear vinyl tubing
12. Coffee can
13. “C” clamp
14. 5/32 Cleco and pliers
15. Work bench and vise
16. .032 safety wire and pliers
Before 1967, our Mooney’s used Cleveland brake assembly part # 30-5. This assembly used a three hole lining current part # 066-11100.
After 1967 and for a few of the 1966 Executive models, Mooney switched to the Cleveland assembly # 30-56. This assembly used a two hole lining current part # 066-10500. Both assemblies use rivets part # 105-00200.
It is not necessary to jack up the aircraft or remove a wheel to replace the linings.
1. First I remove the safety wire, if installed, on the two 1/4″ bolts that attach the brake cylinder to the back plate.
2. Next I place the blade of a screwdriver between the pressure plate and the brake disc. Fig 4 Prying the pressure plate towards the brake cylinder will move the piston inward and allow clearance for the installation of the new linings.
3. Remove the two bolts and remove the pressure plate.
4. The brake cylinder can now be removed from the torque plate by sliding outward.
5. Remove the pressure plate by sliding it off of the anchor bolts.
6. Place “C” clamp over piston and brake cylinder to keep piston from coming out of cylinder.
7. Take the pressure plate and back plate to your work bench. Using the punch and hammer, remove the rivets holding the old linings. You can also use a drill but with caution. Too large a bit and the hole in the plates can be drilled oversize and cause replacement of the plates. These items are very expensive.
8. Clean oil and residue from plates.
9. Place the rivet tool in a vise. I prefer to use the screw type tool. Using the hammer type tool can damage the three hole 066-11100 lining which is very thin.
10. I use a 5/32 Cleco to secure the lining to the pressure plate. Insert a rivet and squeeze. Remove the Cleco and install the remaining rivets in both plates.
11. Returning to the plane, inspect the anchor bolts for rust, wear and nicks. Polish with fine emery or scotch Brite pad. Check bore of torque plate(18) and clean as necessary. The anchor bolts must slide freely in the torque plate.
12. Remove “C” clamp and install pressure plate over anchor bolts.
13. Install brake cylinder by inserting anchor bolts into torque plate.
14. Install back plate on brake cylinder using the two 1/4″ bolts and torque to 90 inch pounds. Check to see that the assembly is loose and floating in the torque plate. Safety wire bolts together using .032 safety wire.
15. Install a length of clear vinyl tubing to the vent on the hydraulic reservoir and the other end in the small coffee can. Fig7
16. Fill the oil can with 5606 aircraft brake fluid. Automotive fluid will NOT work. Attach a small length of the clear vinyl tubing to the oil can by sliding it over the stem. Fig 8 Pump the can until the tubing is full of fluid with no bubbles.
17. At the bottom of the brake cylinder is a bleeder valve. Loosen the bleeder with the 1/4″ wrench one full turn. Attach the loose end of the oil can tubing over the bleeder valve. Pump about one half the contents of the oil can thru the brake system being careful not to allow bubbles to enter the clear tubing. Remove the tubing and tighten the bleeder valve.
18. Repeat procedure for opposite side and remove tubing from brake reservoir.
19. Check reservoir for proper fluid level and adjust as necessary.
20. Check brake operation before starting engine. There may be a low brake pedal on the first application.
Later models aircraft with inner gear doors will require the loosening of the inner gear doors but not removal. Some model have a single bolt at the center axle and a brace at the forward gear leg. Others have two bolts at the axle center that are safety wired together. Removing these center bolts will allow the gear door to move away from the brake cylinder. Be aware that each of these bolts have washers as spacers between the axle and gear door. These must be replaced as installed to provide clearance between the gear door and the tire.
Dual Puck Brakes
Dual puck brakes are installed or are optional on the “K” model and up to the current models. These brakes are constructed like two of the single puck units molded as one unit. They use the same 066-105 lining just twice as many per side. Relining is done in the same manner as the single puck units.
SB M20-190 & AD 75-23-04
Applies to early 201 and pre “J” Mooney’s equipped with Dukes Electric gear actuators. Recent inspections per this Ad indicate that either the inspections are not being done or being done improperly. In my opinion, the Dukes actuator is barely adequate for the job and must have regular inspection to be dependable.
AD 75-23-04 refers to compliance with the SB M20-190. The SB, dated 1-16-75, is divided into three parts. Part 1 requires that every 200 hrs the actuator be removed from the aircraft, partially disassembled, and inspected for gear wear. Removal of the actuator may be a little intimidating the first time a mechanic removes one, and in some cases, I have found that the SB has not been fully complied with. In other cases the actuator may have been removed but the most important part of the inspection is not being completed. The SB requires that all of the old grease be removed and that the housing be flushed clean with solvent and air dried. The reason for this procedure is that the gears wear in the middle and by looking at the end no wear is apparent. Fig. 1 is a photo of a worn ring gear as it would look as viewed from the end. Fig. 2 is of the same ring gear viewed from the side showing the wear. The gear appears normal from the end, but is nearly worn half way thru in the side view. Fig. 3 is of the ring gear, the worm gear it runs against, and a quarter to give you an idea of the size of these parts. As you can see, it is very important that these inspections be performed as required.
Part 2 of the SB requires that the actuator be greased each 100 hours. To grease the actuator the middle belly panel must be removed. Remove the top end cap bolt per Fig. 4, and apply applicable grease, with a hand grease gun thru the lubrication fitting until it runs out of the top bolt hole. If the bolt is not removed the grease seal on the worm gear shaft will be damaged.
Part 3 of the service bulletin refers to the change in washer sizes used for shims on the ball end of the actuator.
Mooney SB M20-189A allowed for the substitution of the ITT actuator to replace the Dukes actuator. The ITT actuator is almost a carbon copy of a Dukes actuator and if anything, I believe, not as good. My concern is that both actuators have the same gears and that the ITT actuator is not addressed in the AD or SB M20-190. I inspect the ITT just as I would a Dukes, and have found several near failures that have never been inspected.
If you have an early 201 or pre “J” Mooney with electric gear and the entry in your AD log for 75-23-04 states not applicable or not installed, or ITT installed, I would encourage you to have the gears inspected as soon as possible.
Emergency Gear Extension
I want to address what I think is a misconception by many about emergency gear extensions.
The only time that an emergency gear extension will be successful is if the electric motor fails. The emergency gear extension system drives thru the gear actuator. If there is a failure of any component of the actuator, ring gear, worm gear, drive coupling, the emergency system is inoperative and you’re in for a big let down.
Inner Gear Doors
I have flown several pre “J” manual gear Mooney’s that have had the inner gear doors added by STC. These gear doors make the manual gear much more difficult to retract and, I think, dangerous to lower. Though probably not a mechanical problem on manual gear models, I believe the added load to a marginal retraction system such as the Dukes & ITT can only accelerate the gear wear.
Mooney Service Instruction M20-112 was released March 8, 2008. This service bulletin has long been awaited by Service Centers and owners of pre “J” model Mooney’s with electric gear actuators.
SI M20-112 can downloaded here.
Mooney SI M20-112 will offer the 40:1 as a retro fit for the Dukes 4196 and ITT actuators. The Dukes 4196 actuator will require Mooney Kit SI20-112-001. This kit will include the Dukes Actuator repair kit and 40:1 gear set Mooney Pn. 940164-501.
The ITT actuator will require Mooney Kit SI20-12-003. This kit will include the ITT Actuator repair kit and 40:1 gear set, Mooney Pn. 940164-503. The gears in both kits are identical. The difference in part numbers is due to the type of bearings required for each actuator. Necessary bearings and seals are included in these kits. The original 20:1 gears sets are still available for these actuators; however, I would encourage you to replace your gears with these improved sets as soon as possible.
Electric gear has been an option for Mooney airplanes since 1965 and standard beginning with the 1969 models. Early electric gear Mooney’s used the Dukes Pn. 4196-00-1(X) actuator. In 1975, Mooney offered the ITT Pn. LA11C2110, 2114 or 2115 as an alternate actuator in SB-M20-189A. Both of these actuators were used thru 1977 except on the M20J which used a Dukes Actuator Pn. 1057-00-(X) that incorporated a 40:1 gear set.
The Dukes 4196 actuator and the ITT actuator both use the same 20:1 ratio gears. Both actuators have a similar external appearance but are easily identified. The Dukes 4196 actuator has 6 screws that attach the end cap to the jackscrew housing. It also has a grease fitting at the bottom of the gear housing. The ITT actuator has 4 screws that attach the end cap to the housing and no grease fitting. Internally, the Dukes 4196 actuator uses needle bearing. The ITT actuator uses bushings. These 20:1 actuators cycle the landing gear very quickly and the gears wear rapidly.
In 1975, the FAA issued AD-75-23-04 against all Mooney’s equipped with the DUKES 4196 actuator. The AD calls for the greasing of the actuator each 100 hrs and the removal and inspection of the actuator gears each 200 hrs.
The AD 75-23-04 does not reference the ITT actuator even though both actuators, Dukes 4196 & ITT use the same gears. We treat both actuators as though the AD applies to the ITT and have found many to have significant wear.
In 1977, the new M20J, 201, was introduced. The “J” model used a Dukes actuator Pn. 1057-00-(X). This actuator had a larger motor and the actuator used a 40:1 ratio gear set. AD 75-23-04 does NOT apply to this Dukes actuator. The gear operated at a slower rate with less wear and tear on the gears and other component parts and allowed the installation of lower gear doors.
The Dukes and ITT actuators used in Pre-J Mooney’s are very small, approximately 1/4 the size of actuators used in current model Mooney’s. An example of how lightweight these actuators are is their application in various Cessna aircraft as a flap actuator. The ITT & Dukes actuators are no longer in production and replacement parts are very difficult to obtain. Mooney offers an upgrade kit to install the newer style actuator but the cost is over $13,000.00 for the kit.
AD 75-23-04 requires the removal and inspection of the actuator gears each 200 hours of operation. Over the years, the compliance with this AD has taken its toll on many of these actuators due to their light weight construction. The 40:1 gear kits will reduce the inspection requirements and extend the life of these actuators.
It is unclear, at this time, if compliance with the new 40:1 ratio gear kits eliminates the AD all together or just extends the mandatory inspection time to those recommended for the 1977 “J” model with the Dukes 1057 actuator. Recommend time for the Dukes 1057 actuator is 500 hours for the initial inspection and 200 hrs thereafter.
The pricing for the original M20-190-001 20:1 gear sets are $489.08. The pricing for the SI20-112-001 and 003 40:1 gear kits are $800.00 and $818.00.
Initial setup of the 40:1 gear sets will require complete compliance with the service instructions included in the kits by a mechanic experienced with these type actuators. Most of the older, experienced Mooney Service Centers can provide the gears and install them for you. Have your mechanic remove the actuator and send to the MSC of your choice. As always, we are available for your tech support needs.
Please visit our web site www.donmaxwell.com for this and other maintenance articles previously published in the MAPA Log.
E-mail firstname.lastname@example.org or 903-643-9902.
MAPA Log (2007)
1. Crosley. The Crosley automotive engine was a liquid cooled, 4-cylinder, in-line engine which would develop 25 hp. In 1948 eleven Mites were completed. The crankshaft on the seventh Crosley powered airplane failed immediately after delivery. Al Mooney switched to Lycoming and all Mites with Crosley engines were retrofitted. Mooney delivered sixty-six M-18L Mites in 1949 with Lycoming engines.
2. M-18C. In 1953 there were ten M-18C Mites and twenty-two M-18LA Mites completed known as the “We Scotsman.”
3. All of the above. In 1958 Engineering Research Company (ERCO) sold the Ercoupe to Forney Aircraft in Ft. Collins, CO. Air Couipe of Carlsbad, NM purchased the Ercoupe from Forney in 1960. During 1963 Air Coupe sold the design rights and tooling to Alon Company of Wichita, KS, and Mooney acquired the Alon Aircoupe in 1967.
4. C-145 Continental. The M-20 prototype was powered by a Continental 145 hp C-145-2H engine and the first flight took place on September 3, 1953.
5. True. In January 1960 Ralph Harmon joined Mooney as the vice president of Engineering and later was vice president of Engineering and Manufacturing. During his last two years with Mooney (1969-1970) he was president of Mooney.
6. a cowl chin. In 1964 the M-20E was introduced with a 200 hp IO-360-A1A Lycoming engine. It was the first Mooney with the fuel-injected engine. 366 M-20E’s were sold the first year.
7. In 1968, the M-20E was not produced at all.
8. a fixed gear. In 1963 the Master M-20D was introduced. The company made the landing gear non-retractable on the M-20C as well as a fixed-pitch propeller. Only 161 Masters were produced between 1963 and 1966.
9. Eagle Bill Taylor made the first test flights on May 17, 1947.
10. 1969. In 1969 the M-20C (Ranger), M20E (Chaparral), M-20F (Executive 21) and M-20G Statesman all had electrically operated landing gear and flaps as standard equipment.
11. All of the above. In 1968 the M-20C Mark 21 was renamed the “Ranger.” The price was reduced to $16,600. In consideration, the cowl flaps were fixed, the entrance step did not retract, and the dorsal fin was removed.
12. 10 inch longer cabin. In 1966 Mooney introduced the M-20F (Executive) which had a 1 foot longer fuselage and 10 inch cabin.
13. an all metal airframe. Beginning in 1961 (M-20B) all models were of all-metal construction.
14. ”The fastest gear in the west”. Promoting the M20C Ranger in early advertising featured a genuine Texas Ranger and claimed to be the “fastest gear in the west”.
15. Wichita, KS. Mooney Aircraft Inc. (the second company to bear Mooney’s name) was form on June 18, 1948 in Wichita, KS, by Al and Art Mooney, C. G. Yankey, and W. L. McMahon.
16. 1964. Refinements in the 1965 Mooney line included new rectangular rear windows that replaced the contoured windows in previous models and headroom increased with new contoured roof and headliner.
* “Those Remarkable Mooneys” by Larry A. Ball, 1998.
1. The original Mooney Mite (M-18) was powered by which automobile engine?
D. Model “T”
2. Which model Mooney was known as the “Wee Scotsman”?
3. What was the Mooney Cadet (M10) originally?
D. All of the above
4. The original M20 was designed for what engine?
A. T10-541-AIA Lycoming
B. IO-360-AISA Lycoming
C. C-145 Continental
D. 0-200 Continental
5. Ralph Harmon was lead of the engineering team that designed the Beech Aircraft Bonanza and converted the M20 wood wing design to metal.
6. The difference in the M20C and the M20E is apparent by
A. a dorsal fin
B. wing root fairings
C. a cowl chin
D. tinted windows
7. In 1968, the M20E was
A. the most produced model
B. equipped with electric flaps
C. not produced at all
D. available with fuel bladders
8. The M20D Master was
A. a fixed gear
B. turbo charged
C. 6 place
D. fixed flaps
9. The first Mooney Test Pilot was?
A. Bill Wheat
B. Joel Smith
C. Bill Taylor
D. Scott Crossfield
10. Electric gear and flaps became standard equipment in which year model?
11. The 1968 M20C differed from previous models by what distinguishing features?
A. One piece windshield
B. No dorsal fin
C. Fixed cabin step
D. All of the above
12. The M20F (Executive) 21 introduced
A. the Continental TSIO-360 engine
C. throw over control yoke
D. 10 inch longer cabin
13. The M20B was the first Mooney to have
A. fuel injection
B. hydraulic flaps
C. all metal airframe
D. P.C. (Positive Control)
14. Early advertising billed the M20C Ranger as
A. “The plane Al built”
B. “Bullet Fast”
C. “The fastest gear in the west”
15. Mooney Airplane Company was originally located in
A. St. Louis, MO
B. Oklahoma City, OK
C. Wichita, KS
D. Dayton, OH
16. The Mooney “round window ERA” ended in what year?
MAPA Log Volume 30, Number 11
Answers to test questions
Fluctuating ammeters are a common complaint we receive from many Mooney owners, normally on a weekly basis. Callers tend to be frustrated after spending hundreds of dollars replacing alternators, voltage regulators, ammeters and wiring and the problem continues to exist. During daylight flights, a fluctuating ammeter is just an annoyance. However, a night flight is a whole different story. I once had a caller describe his problem as, “. . . it’s like attending a séance.” As the ammeter needle dances back and forth, the panel and instrument lights pulse and other engine gauges try to keep time with the ammeter. Alone at night, IFR, this phenomenon can be very discomforting.
Fluctuating ammeter needles are very common on all Mooney’s starting with the first M20J’s. In nearly all cases, the problem is caused by corrosion on the terminals of the master switch. An easy diagnosis is, while in flight, to apply pressure to the connectors on the back of the master switch with your finger. You will not receive a shock. The fluctuation of the ammeter should stop.
The master switch is a double pole, single throw switch. This simply means it is two separate switches in one case, and that both switches are either on or off at the same time. One side of the master switch controls the master electrical relay at the battery, thus supplying power to the aircraft buss. The other side of the master switch completes, or opens, the circuit between the alternator field and the voltage regulator. This switch arrangement has been used throughout Mooney production with both alternators and generators. The difference between earlier pre “J” models and later Mooneys is the actual master switch itself. Earlier models, (Figure 1) used a toggle type switch with screw on connector, (Figure 2) to attach the wiring to the switch, while later models use a rocker type switch, (Figure 3) with slip-on type connector, (Figure 4) to attach the wiring.
The voltage regulator can be compared to a control tower at the airport. It directs the alternator on what to do and when. The communication between the voltage regulator and the alternator is directed thru the field wire. If the connection between the two devices is good and corrosion free, the two communicate freely. There is no delay from the alternator in response to the voltage regulator request. The result is a smooth and steady ammeter movement. Corrosion on the slip-on terminals of later model Mooney’s can cause resistance, or delay, in the communication between the
It’s kind of like flying into Love Field with all the kids in the back seat talking at the same time. voltage regulator and alternator. The delay between the request of the voltage regulator and the response of the alternator is what causes the ammeter fluctuation.
The tower says “expedite,” you miss the call due to the fuss in the back, the tower calls again, now you start to expedite, and then they say “hold your position.” Anyway, hold, stop, hold, stop that’s what the ammeter is doing.
The good news is the fix is simple and inexpensive. Simply clean the terminal on the back of the master switch. The switch is easily accessible on most models. Newer models have the master switch located above the ignition switch and are more difficult to reach. Sliding the connector back and forth on the terminal is all that is required in most situations. Extreme situations may require the master switch to be replaced due to corrosion inside the switch.
As usual, feel free to contact me with your technical questions at (903) 643-9902. or email email@example.com.
This article, and other previously published articles in the MAPA Log, are available on our web site at www.donmaxwell.com.
Another helpful web site for diagnosing electrical charging problems is www.zeftronics.com.
MAPA Log, Volume 28, Number 3 (March 2005)
Every week I receive several phone calls and e-mails regarding fuel tank leaks, with questions such as:
“I smell fuel when I first get in the plane and then the smell goes away”;
“I have a screw on the panel at the cabin door that leaks when the tank is full”;
“I have fuel stains in the wheel well”;
“ I have a panel under the wing that leaks”; and
“ How much will it cost to have my tanks resealed?”
In most cases, the leak can be repaired and a total “strip and reseal” is not needed. We repair at least one fuel tank per week.
Fuel Smells in the Cabin
Your plane has been closed up for a period of time. You open the cabin door and smell fuel, and within a few minutes the smell is gone.
The source of the fuel smell can be a fuel tank leak, a leaking fuel selector valve, or leaking fuel gauge sending unit. The leading edge of our Mooney wing is hollow. A fuel leak in the forward or outboard tanks will leak into the leading edge of the wing. The leading edge of the wing is open to the cabin at the forward edge of the seat track on each side of the cabin. Fuel fumes or smells enter the cabin at this point. Holes are drilled in the lower leading edge of the wing at each rib and at the fuselage. These holes are to prevent fuel from accumulating at the ribs and from entering the cabin. Fig. 1 is an example of the weep hole at the fuselage showing fuel leakage stains.
Fuel selector valve leaks are evident by stains around the fuel selector stem. In most cases these leaks are repaired by disassembling the selector and replacing the O-rings.
Mooney M20 B thru E models have fuel gauge sending units located inside the cabin, just in front of the rear seats on each side of the cabin. Models F thru S have sending units in the same location, as well as, having an outboard sending unit in each wing. These sending units have cork or neoprene gaskets. Leaks in these areas will be detected by blue stains on the tank walls below the sending units. In most cases, tightening the screws that attach the sending unit will stop these leaks.
A leak found at a screw in an access panel is usually caused by a cracked nut plate. Fig. 2. The nut plate in the fuel tank has a plastic cap to keep the fuel from leaking around the screw threads. These caps, over time, become cracked due to age or possibly someone has replaced the screw with one that is too long. To repair the leak, remove the screw. You do not have to drain the tank. Apply a fuel resistant sealant, such as Permatex #3, to the screw threads and replace the screw in the panel.
Fuel Tank Construction
A Mooney fuel tank is a metal box constructed out of several individual pieces. Once assembled, each seam is sealed with layers of a fuel resistant sealant. If this sealant is damaged by a hard landing, or by age, fuel can leak between the sealant and the tank walls, migrating to a point where it can exit. This point of exit may be a rivet or seam between wing skins.
I have come across logbook entries where the same rivet has been replaced several times trying to stop a leak. Using our method, we have located the source of these leaks in other parts of the tank.
I did not invent this procedure, nor do I remember who told me; however, this is how our service center finds leaks in Mooney fuel tanks.
1. The first rule to remember in chasing a fuel leak is: The source of the leak is never where the leak appears on the out side of the tank.
2. Remove the fuel from the tank.
3. Turn the fuel selector off.
4. Remove the top fuel tank access panels on the wing that has the leak. The panels are removed by first removing the screws.
5. Once the screws are removed, we use masking tape to tape around the access panel and the middle of the panel for protection of the paint. The panel is sealed with a sealant. Do not beat on the panels. An elephant could stand on the panel and it would not come loose.
6. Next, we use a thin putty knife, that has been sharpened on one side, to slide in between the seam of the access panel and the wing. Using a nonmetallic hammer, gently work the putty knife between the skin and panel. It may take several times around the panel with the blade and hammer, each round a little deeper under the skin, until the panel releases.
7. Once the panels are removed we place mirrors in the bottom of the tank so that we will be able to see the upper seams of the tank. Do not cover the stringers in the bottom of the tank. Many leaks are in this area.
8. Next we apply liquid hand soap, thicker the better, to all the seams in the tank.
9. We now cover all the removed access holes in the wing with Plexiglas. Fig. 3. The Plexiglas covers the entire hole including screw holes. The Plexiglas is held in place by masking or duct tape around the out side border forming a seal. The Plexiglas can be cut from an old windshield or purchased at most hardware stores. New glass is preferred as you will be looking thru the glass to find our leak source.
10. Next we take a standard shop vacuum cleaner. We attach the hose from the vacuum cleaner to the vent on the fuel tank. And no, this will not collapse your tank. Fig. 4
11. Turn on the vacuum and, using a flashlight, look thru the Plexiglas panel and look for bubbles in the soap. The mirrors should be positioned so that you can see the entire interior of the tank.
12. Bubbles indicate the source of the leak as air is drawn into the tank thru the leak.
13. Once the leak, or leaks, are found, Fig .5, the area will need to be thoroughly rinsed with water, dried, cleaned and repairs made using procedures in the Mooney Service Manual..
14. Two types of sealant are recommended in the manual. PRC and Flame Master brands. We use Flame Master CS3204B-2 for repairs in the tank and topped with CS-3600 for a protective coating and CS3330B-2 for access panel sealant. Fig 6. We use sealants in Semkits that contain both sealant & activators. They are mixed together in a small caulking tube. Once mixed together, we transfer the sealant to a cup and apply with small acid brushes. The sealant will apply easily if thinned with a small amount of MEK.
15. Replace the access panels using CS3330B-2. I recommend at least 48 hours before fuel is added to allow for proper curing of the sealant. Temperature is key to a successful repair. Avoid cold weather & high humidity if possible.
Most leaks are easily found and repaired using this method. I share this procedure with you so that you and your mechanic can locate and repair your Mooney’s fuel leak. We welcome you to visit our service center at any time.
Please visit our Web Site at www.donmaxwell.com for this and other articles published in the Mooney Log.
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MAPA Log (February 2005)
Mooney Service Instruction M20-39
One item I always suggest to the new owner of a pre “J” Mooney is a pair of vise grips and that they be kept in the cabin. Why? Well, our pre “J’s” have a cabin door handle that is an aluminum casting. This casting is attached to the door latch mechanism by a small steel roll pin. Over the years, and many cycles, the casting becomes worn and will let the roll pin fall out. When this happens, unless you can find the pin and reinstall it, you are locked in the cabin. Hopefully there are people around that will let you out. If not, you may have to attempt the dreaded hot start and fly to another airport to get someone to open the door.
So, you have two choices: keep the vise grips handy, or install the new style door handle. Mooney has provided the new door handle and necessary materials in a service kit. The kit is Mooney Service Instruction SI 20-039-000. Current cost of the kit is $212.59. Labor to install is 3 to 5 hours. Check your cabin door handle. Does it feel snug? Do you feel any “play” or lost motion in operating the door handle? If so, I suggest you upgrade to a newer style handle or keep the vise grips handy. Check with your Mooney Service Center to order.
To view and print: SI M20-39
MAPA Log, Volume 26 , Number 2 (February 2003)