Electric Landing Gear Circuit Breaker ('67 M20F)

[BWatts]

While flying today my electric gear wouldn't go down. After some trouble shooting we found that one of the breakers near the gear hand crank was popped. We tried to reset, wouldn't stay in. Eventually got it to stay seated and got the gear down electrically without having to hand crank. Plane is at the shop now being evaluated. 

 

Anyone experienced this before? What should I expect as far as fixes?

 

Thanks

[Prior owner]

Your gear wouldn’t move down at all, or

did it move down and you had an unsafe gear light?

[BWatts]

It didn't move at all... gear indicator window on the floor showed it was still up... Also no green light. 

 

After messing with the breaker a lot, we got it down... both visual indicator and light. 

Edited March 19, 2021 by BWatts

[carusoam]

The common cause of this occurrence....
 

It happens when a backseater accidentally kicks the E-gear latch on the floor... engaging the e-extension mechanism...

The manual and the electric power have a debate... and the tripped CB is the usual result...

 

Not sure how the mechanical system is engaged on the M20F...

But make sure both systems are working properly...

Best regards,

-a-

[RLCarter]

Not sure I would have continued attempting to reset a circuit breaker, no since in potentially causing an electrical fire.

[carusoam]

There is technical guidance for number of attempts to reset a CB...

Trying once... you have proved something is wrong... and current is flowing at a very high rate... enough to set off the CB...

Trying more than that is not recommended... the CB limits the current to stay at a safe level...

Having somebody push the button disables this safety mechanism... things start to heat up with each attempt at resetting...

 

Once on the ground... check the CB... it is possible that is just worn and no longer able to carry its rated load... they are mechanical devices subject to wear...  

Often we see worn CBs that get used a lot... and carry a heavy load... like standard landing lights...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Raymond J1]

In the maintenance manual, the breaker of the power circuit is 25 A while that of the control circuit is 5A. On my F, I had the unpleasant surprise to see a power breaker increased to 50 A...

 

[RLCarter]

12 minutes ago, Raymond J said:

In the maintenance manual, the breaker of the power circuit is 25 A while that of the control circuit is 5A. On my F, I had the unpleasant surprise to see a power breaker increased to 50 A...

Guess someone thought it would be TWICE the protection.........

[carusoam]

That will keep the CB from activating again...   

-a-

[ArtVandelay]

25,50 whatever it takes.

[MBDiagMan]

7 hours ago, Raymond J said:

In the maintenance manual, the breaker of the power circuit is 25 A while that of the control circuit is 5A. On my F, I had the unpleasant surprise to see a power breaker increased to 50 A...

 

Yeah.  DONT DO THAT!

[MBDiagMan]

10 hours ago, BWatts said:

While flying today my electric gear wouldn't go down. After some trouble shooting we found that one of the breakers near the gear hand crank was popped. We tried to reset, wouldn't stay in. Eventually got it to stay seated and got the gear down electrically without having to hand crank. Plane is at the shop now being evaluated. 

 

Anyone experienced this before? What should I expect as far as fixes?

 

Thanks

I went through this recently and the problem was a locked brake in the actuator.  Once it’s on Jack’s, fashion a way to measure current to the actuator.  I bought an inexpensive DC Amp clamp.  Actuate the gear multiple times while monitoring current.  If it immediately exceeds the 25 Amp current limit, look for a short in the wiring or more likely the actuator itself.

To quote Carusoam “PP only, not an A&P.”  Although my education is Electrical Engineering.

[Raymond J1]

The DUKES engine having a nominal of 40 A at 4200 rpm, can someone explain to me the choice of Mooney for a limitation to 25 A ?

 

[47U]

1 hour ago, Raymond J said:

The DUKES engine having a nominal of 40 A at 4200 rpm, can someone explain to me the choice of Mooney for a limitation to 25 A ?

Just a guess, since I have manual gear...

Mooney engineers oversized the actuator for longevity and reliability.  They determined that 25 amps provided enough power to run the gear and anything more than that could damage components should something jam up or perhaps a failed limit switch.  Limiting the amps allows use of smaller power wiring, saving weight.

There’s real engineers on MS that can give real answers.  I’d like to hear what they think.    

[Raymond J1]

43 minutes ago, 47U said:

Just a guess, since I have manual gear...

Mooney engineers oversized the actuator for longevity and reliability.  They determined that 25 amps provided enough power to run the gear and anything more than that could damage components should something jam up or perhaps a failed limit switch.  Limiting the amps allows use of smaller power wiring, saving weight.

There’s real engineers on MS that can give real answers.  I’d like to hear what they think.    

I thought like you at first.

Then looking at the wiring diagrams and the evolutions between 1964 and 1967, i.e. when the Dukes mounting was an option to be added to the standard harness, the limit switches, the relay coils, the leds and even the retraction safety switch are protected in 5 A. Only the power circuit (which supplies the current to the motor) is protected in 25 A. The wiring on all wires is in 18 AWG...

If we were a purist, we would have to replace the wires of the "safety switch" to increase their section and make them compatible with the rest.

At the refurbishment of the Dukes engine, I steamed the winding, which reduced the operating current by 5 A (it was at a peak of 23 A at the end of the ascent), When we isolate each train (dismantling the connecting rods), it is the front train that consumes the most current...

Edited March 19, 2021 by Raymond J

[Ragsf15e]

No way that thing spins 4200rpm putting up our gear though.  I thought the original gears were fast... if that motor was pulling 40amps, I probably would’ve shoved the gear right through the wing during retraction!

[carusoam]

What regulates the motor rpm then?

12V, 40amps, 4600 rpm...

 

If the motor is going bad... it may have really high amps at 0 rpm... each time it starts, it probably spikes for a microsecond...

+1 for measuring the amps while the plane is up on jacks...

PP thoughts only, not a mechanic...

Best regards,

-a-

[N201MKTurbo]

With a DC brush motor the speed is controlled by the voltage times the motor constant. The current is controlled by the torque load on the motor. 
 

4200 RPM seems reasonable. If it takes 100 cranks of the emergency system to swing the gear down and the crank gives you 2 turns of the  motor for 1 turn of the crank, then it takes 200 turns of the motor to swing the gear. 200 / 4200 * 60 = 2.8 seconds to swing the gear. 

[MBDiagMan]

14 hours ago, Raymond J said:

The DUKES engine having a nominal of 40 A at 4200 rpm, can someone explain to me the choice of Mooney for a limitation to 25 A ?

 

That means 40A Max.  The current draw will depend upon the load it’s under.  Apparently in the Mooney landing gear it is under a load that should not exceed 25 Amps.

[Shiny moose]

Its not as simple as this but CB sizing is based on wire sizing, cover material, location among other factors. Placing a larger amp CB without considering the wire is just wrong

[TTaylor]

On 3/18/2021 at 8:31 PM, BWatts said:

While flying today my electric gear wouldn't go down. After some trouble shooting we found that one of the breakers near the gear hand crank was popped. We tried to reset, wouldn't stay in. Eventually got it to stay seated and got the gear down electrically without having to hand crank. Plane is at the shop now being evaluated. 

 

Anyone experienced this before? What should I expect as far as fixes?

 

Thanks

1. Bad circuit breaker

2. Motor needs to be rebuilt 

3. Gears are worn and need to be replaced

4. The gears need to be cleaned and lubed

Most likely the electric motor needs to be rebuilt 

 

 

[EricJ]

6 hours ago, MBDiagMan said:

That means 40A Max.  The current draw will depend upon the load it’s under.  Apparently in the Mooney landing gear it is under a load that should not exceed 25 Amps.

^This.    The landing gear system is engineered to require a certain amount of torque from the motor, and probably allowing for tolerance and aging the circuit breaker was sized at 25A because beyond that something must be very wrong.   It's possible the motor might be in the region of causing a lot of damage to whatever got stuck if allowed to draw more current.

This just means the motor has a high de-rating factor, which should be good for reliability and longevity as well.    The wiring is probably also sized for 25A safe load.   I would not put a larger breaker in there.

[MikeOH]

CBs are sized to protect WIRING.  The tacit assumption is that something must have gone wrong with the LOAD.

[Ragsf15e]

11 hours ago, N201MKTurbo said:

With a DC brush motor the speed is controlled by the voltage times the motor constant. The current is controlled by the torque load on the motor. 
 

4200 RPM seems reasonable. If it takes 100 cranks of the emergency system to swing the gear down and the crank gives you 2 turns of the  motor for 1 turn of the crank, then it takes 200 turns of the motor to swing the gear. 200 / 4200 * 60 = 2.8 seconds to swing the gear. 

I’m sure you could be right, but my 68 F emergency crank is like 40 revolutions to get the gear down.  Later models have the different system.

[Vance Harral]

13 hours ago, N201MKTurbo said:

4200 RPM seems reasonable. If it takes 100 cranks of the emergency system to swing the gear down and the crank gives you 2 turns of the  motor for 1 turn of the crank, then it takes 200 turns of the motor to swing the gear. 200 / 4200 * 60 = 2.8 seconds to swing the gear. 

I'm guessing 4200 RPM is the unloaded speed of the motor, but I don't think it turns anywhere near that fast under load.

Your math is about right except there is no gearing between the emergency crank cable and the motor shaft in the M20F vintage of airplanes.  The emergency engage just slides the crank cable spline directly into the motor shaft, it's a 1:1 hookup.  The motor shaft is geared to the drive mechanism by a worm gear arrangement, which originally had a 20:1 ratio.  J model actuators and "upgraded" early-model actuators have 40:1 gears.  We've had both in our airplane, so I'm pretty familiar.  Prior to the 40:1 upgrade, gear transit time was about 3 seconds, and emergency extension required about 70 cranks from full up to full down.  After installing the 40:1 gears, gear transit time is about 6 seconds and emergency extension requires about 140 cranks from full up to full down.

Doing the math for 40:1 gears, 6 seconds is 0.1 minutes to make those 140 turns.  That's about 1400 RPM at the motor shaft.