Manual Gear Mooneys

May 2003

The February issue of MAPA Log featured an excellent article titled Belly-in. Written by Tom Rouch of Top Gun Aviation, it focused on landing gear failures in electric gear Mooneys; especially ones equipped with Duke gearboxes. He also touched on the subject of manual gear Mooneys and really brought the point of how not to pick up a Mooney after a gear collapse. Thank you, Tom.

This month’s ShopTalk will explain how the wear and tear on a single part on those manual gear Mooneys can contribute to a gear collapse, for which the pilot is usually blamed. Well, for the pilots of those older Mooneys who have had a gear collapse or for those about to, the Fates have set you up. Conventional wisdom (i.e. following “The Book”) in this case, will not make you safer. The obvious truth is that most of our airplanes are aging and are not being replaced with new ones. Parts are wearing out that the manufacture never expected to still be in use after some thirty or more years.

To operate manual gear Mooneys, owners must comply with Airworthiness Directive 73-21-01. This AD has multiple sections, one of which requires that at each annual inspection the landing gear rigging and down-lock preload torque must be checked. Special tools (one for the nose and one for the main gear) are required to determine just how much torque it will take to transition the over-center links from the locked to the unlocked position when the Johnson bar is fully engaged into the down-lock block mounted under the instrument panel.

AD 73-21-01 refers to Mooney Service Bulletin M20-35A for the details of this inspection. However, following SB M20-35A alone is inadequate in preventing gear collapse and may even contribute to the problem! One would think that as long as the nose gear preload is 150 inch-pounds and the main gear preloads are 275 inch-pounds (these are the minimum values) then the gear would be secure. On an airframe with less than two thousand-hours, that is probably an accurate statement. However, most manual gear Mooneys have three thousand to five thousand hours and parts are showing wear. Because this service bulletin directs you to the gear system linkages located in the gear wells and belly of the airplane, it is easy to overlook the Johnson bar and especially the down-lock block itself. The extra force it takes to engage the Johnson bar into the down-lock block is actually the down-lock preload on all three landing gear. You can see that the stainless steel Johnson bar is constantly putting pressure on the aluminum down-lock block when the gear is down and locked.

After a thousand or more gear cycles, the stainless steel Johnson bar will look unworn. However, an inspection of the bottom side of the aluminum down-lock block will reveal a trench where the Johnson bar slides into place and that the hole in the block is becoming egg-shaped. This wear on the hole in the down-lock block is towards the backside of the hole, because of the movement and pressure on the Johnson bar. As this wear continues, two things begin to happen. First the down-lock preloads become less simply because the Johnson bar is not as far forward into the hole as it would be if it was a new down-lock block. The second is the Johnson bar does not as easily engage the down-lock pin located in the down-lock block. This is the pin you press with your thumb to release the Johnson bar and retract the landing gear. As the block wears, a small lip develops inside the bore of the down-lock block. Eventually, the lip grows large enough to inhibit the handle of the Johnson bar from easily moving upward enough to engage the locking pin.

During the annual inspection, your favorite well-intentioned airplane mechanic complies with Airworthiness Directive 73-21-01, as he is required to. He discovers that your down-lock preloads are a little on the low side. Being a good Mooney mechanic he refers back to Service Bulletin M20-35A and adjusts the landing gear preloads to the Service Bulletin requirements by lengthening the gear rods in the main wheel wells and shortening the gear rods in the nose wheel well. Now the force on the aluminum block to hold the gear down and locked is back to about its original value but the wear inside the hole accelerates because the inner surface is no longer flat. Additionally, the force required by the pilot to move the bar forward to where the handle will clear the worn lip is now higher (especially in older Mooneys) because of the recent adjustments. Through the years, as the hole becomes more egg-shaped and the lip becomes more pronounced, it becomes almost impossible to push the Johnson bar far enough forward to engage the original hole in the down-lock block. The geometry inside the egg-shaped hole has changed and is compromising the functionality of the locking pin.

The gear down light on the dash operates by one microswitch located on the front side of the down-lock block. This micro switch has a little metal lever that extends through a hole in the down-lock block. The Johnson bar when inserted into the block pushes up on the microswitch lever about the time the lock pin clears the ridge on the handle. Note that the microswitch operation is not a direct function of the lock pin. It’s not unusual to see the green light illuminate before the lock pin is completely engaged. Therefore, do not consider it a “Gear down and locked” light. The only part of the landing gear system functioning as a down lock is the small pin dropping over the ridge of the handle; a movement of about 0.1 of an inch!

Here is the typical scenario: The pilot says, “I had a green light and the bar was engaged into the down-lock block on the instrument panel. But as I set the aircraft down on the runway and as soon as I [slowed down/applied the brakes] the Johnson bar just flew back between the seats”. Of course, your propeller is probably going to hit the ground if the engine is still running and it’s not uncommon to bend the landing gear tubes inside the belly. The nose gear doors get ground down along with the belly skins. Then some Bubba in a tow truck comes out and tries to lift or drag your airplane off the runway and he hasn’t read Tom Rouch’s article about how to do it adding more damage to the plane. What I’m getting at here is you don’t have to be a low time manual gear Mooney pilot to have this happen to you. This is a mechanical problem caused by old age wear and tear. Maybe I have missed it but I have not seen in any Mooney publication an inspection procedure for the manual gear down-lock block. That’s why your mechanic does not look at it. So you, the aircraft owner, by reading this article are becoming educated.

The next time you go flying, bring along a mirror, a flashlight and another pilot. Get to a safe altitude and let the other pilot fly the airplane and keep the airspeed below the gear extend speed. Do not allow him/her to be distracted by the inspection procedure. Better yet, perform this inspection with the airplane on jacks. From now on, perform this inspection at each annual. Retract the landing gear and illuminate the hole in the down-lock block with the flashlight. Using the mirror, inspect the condition of the block; specifically these items:

1. Is the hole round?

2. Is there a lip inside on the backside of the hole?

3. Is the lip more than a sixteen of an inch?

4. When the gear is extended, does the lock pin easily engage? (Have the pilot extend the gear while you observe the thumb button move that final 0.1-inch.) The pilot should not have to resort to hitting the Johnson bar forward (a common technique) to get this to occur. Repeat this test a few times.

The remainder of this inspection should only be done with the airplane on jacks.

5. From the pilot’s seat, extend the gear. Observe if the handle hangs up on the worn lip preventing the pin to drop over the handle ridge. Repeat this test several times. If the handle hangs up just once, consider that to have been a gear collapse.

6. Determine when, in the locking process, the gear down light illuminates. The microswitch lever can be adjusted (by carefully bending) so the switch is made just as the pin locks.

7. Pull down on the Johnson bar handle. The locking pin should prevent the handle from moving down and the gear retracting. If the handle does release (without depressing the thumb button), consider that to have been a gear collapse. This is an excellent procedure to use in flight.

Examine the photo of the down-lock block accompanying this article. That block was removed from an M20E with about twenty-eight hundred hours total time on it and is completely unserviceable. To change the down-lock block, the aircraft must be on jacks and the special gear adjusting tools from Mooney along with a torque wrench must be used. After the block is installed, the gear down-lock tensions must be set. Normally they will increase because a proper sized hole causes the Johnson bar to be further forward. I suggest setting the manual gear Mooneys to near the low side of the tensions; typically 150 inch-pounds for the nose gear to stay locked even when porpoising down the runway. 275 inch-pounds for the main gear are more than enough when landing in a crosswind or trying to fold the landing gear with a sharp turnoff. If the tensions are set at the high end recommended by SB M20-35A, it will be more difficult to lock the gear down and will not provide any more sense of security as far as the gear not collapsing.

At today’s prices, the typical gear-up landing or gear collapse including a propeller strike will cost between $25,000 and $35,000 to repair properly but that’s a ShopTalk article for a later date. A previous article on this subject is available in the January 2001 ShopTalk. The cost of a new down-lock block is now over $600; almost double the price quoted in January 2001!

As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.