Q&A: Landing Gear Troubles and a Hydraulic Fluid Leak in an Early 210

Written by Steve Ells
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February 2015-

Dear Steve,
My 1964 Cessna 210 during a recent annual was noted not to have the landing gear horn sound, so the wire was replaced.
Two flights ago during taxiing when I lifted the nose, the horn started; when I relaxed, it stopped and I had no problem.
During taxi on very my last flight, the horn was okay, but then on base turn the horn started to sound—even though I had not retracted or touched the gear handle during the short flight.
I pushed the gear handle down, but it would not go to neutral and the green light would not light, so I went around and came back for a low approach pass and requested that the tower check if there was anything visually wrong. The tower told me that the gear was down and the gear doors were open, which I did notice by looking at the under-the-wing mirrors.
After this go-around, the radio sounds started to weaken and then I had total electric failure.

Thank Almighty God, I landed safely and taxied to my mechanic. He charged the almost-new Concorde extra-crank battery and put the plane on stand for many gear cycles (which went fine), but now the gear horn sounds as soon as you turn the master switch on—and it stays on until you push the green light knob. Then the green light shines and the horn sound stops.
My mechanic checked all the wiring, switches and solenoids and they are intact, he said, according to the repair manual. This plane has a generator which charges fine according to the mechanic's test.
He noted a hydraulic fluid leak, which I think might have contributed to the gear door not closing in combination with low voltage from the battery and low hydraulic fluid.
But how do we solve this wiring enigma? And what is the normal recommendation regarding the rather common hydraulic fluid leak in Cessna 210s? (I had a turbo 210 and it had the same chronic problem.)
I would greatly appreciate your advice and the advice of any Cessna 210 owners and mechanics.
Thank you in advance,

—Gear Horn Cessna

Dear Gear Horn Cessna,
As I read your letter I note the following problems:
1) Gear horn sounds when you turn on the master and at other times. If you push the green gear light, the horn stops.
2) The gear-down cycle isn't being completed, since a) the doors aren't closing; and b) the gear handle is not returning to neutral.
Your 1964 210 is a 210D model. This was the first year that the wing flaps were controlled electrically rather than hydraulically. The hydraulic pump for your aircraft is mounted on the accessory case of the engine, and the hydraulic system filter along with the system filler are located on the firewall. The power pack is in the center console below the instrument panel.
Your system has a gear up/gear down handle and an emergency hand pump.

When the engine is running, the hydraulic pump is also rotating, causing fluid to continuously circulate through the pump, the power pack and the tubing between the two.
If "gear up" (or "gear down") is not selected, the system is at rest; although the pump may be moving fluid, it is being returned to the reservoir in the power pack. The pump is at rest when the landing gear is fully up and locked with the doors closed and when the gear is fully down and locked with the doors closed. Here's how the system works:

Gear-Down Sequence
1. Gear handle is moved to select "gear down." The gear handle is held down by the handle release spring in the power pack. Moving the gear handle to the down position also moves the gear handle up/down switch to the closed position in the down limit switch circuit.
A spool-type valve inside the power pack is connected to the gear handle. Gear handle movement repositions the spool valve, cutting off the unloaded return path and directing pressurized fluid to the nosegear door, strut door and main gear door actuators. The doors open; fluid pressure also actuates the main gear-down lock release cylinders. The down locks are pulled back, out of the way.
Pressure builds in the system (it doesn't take much pressure to open the doors). The door actuators are held open by static pressure. When the pressure reaches 750 psi, a priority valve in the power pack is unseated, releasing hydraulic pressure to the nose and main gear-up lock cylinders and then on to the nose and main gear hydraulic actuator cylinders. Fluid on the non-pressure side of the actuators is vented back to the reservoir through the power pack. When the nose and main gear legs are fully down, the gear-down switches close. This completes the circuit from the 10 amp circuit breaker on the bus, through the three gear-down switches, through the down side of the gear handle up/down switch and energizes the door solenoid. The door solenoid moves the solenoid valve (in the power pack) which then directs pressurized fluid to the nose, strut and main gear door actuators. The doors close.
The door solenoid is spring loaded to the doors-open position so that if electrical power is lost, fluid will go to the doors-open cylinders.
Pressure again builds up as door actuators reach their limit of travel. After it reaches a value between 750 and 1,250 psi, the time delay valve (in the power pack) opens and fluid activates the handle release valve, allowing it to return to neutral. The handle should return to neutral within seven to nine seconds after the gear is up or down and locked and the doors are closed. Once the handle is released to neutral, the pump is unloaded.

Landing Gear Wiring
The landing gear warning circuits consists of a path from the "Ldg Gear" circuit breaker through the left, right and nosegear down switches to pin B on the power pack plug. From pin B, the circuit continues to the green gear safe light receptacle through the light to the gear lug on the dual warning unit.
If the throttle switch is closed (more than 14 inches of throttle position) then the horn won't sound. If the throttle switch is open, the horn will sound. If the throttle switch isn't grounded properly, that will also cause the horn to sound.

Conclusion
First, I believe the reason the gear doors didn't close and the handle didn't return to neutral during the short test flight was due to the low system voltage. Low voltage didn't provide enough electrical energy in the gear door solenoid to overcome the spring force that is always present against the landing gear door valve; therefore, no fluid was sent to the doors-closed side of the actuators.
Since no fluid was sent to those cylinders, there was no way for the fluid pressure to build enough to upset the time delay valve and direct fluid to the handle release.
I think the system will function as it should when the system voltage is normal.
Second, I believe your problems with the gear horn are related to the throttle switch. You said the wire was replaced. If it was a wire at the throttle switch, check to make sure the switch is properly grounded at the "c" common lug, and that the switch is secure in the bracket and that the bracket is solid.
Lastly, always repair all hydraulic leaks. Most actuator leaks are easy to solve: just open the cylinders and install new O-rings (and backup rings) as needed.
For years I was running a generator and it worked pretty well. Recently I decided to upgrade and replaced my generator with a Plane-Power alternator conversion kit. It's a good kit and I'm very satisfied.
If you stay with the generator on your
210D, I recommend getting a new regulator from Zeftronics.

Happy flying.

Know your FAR/AIM and check with your mechanic before starting any work.

Steve Ells has been an A&P/IA for 43 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 lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to .

Resources
Generator-to-alternator conversion kit
Plane-Power, Ltd.
Plane-Power.com

New regulator for an older style generator
Zeftronics
zeftronics.com