Help...Gear Won't Come Up

[Ernie Martin]

GMAs et al, can you help me?

Today, on my '73 non-turbo 337G, coming out of a Bahamian island, the gear won't come up. Green gear light stays on, there's no evidence (sound, drag) that the gear is trying to come up and then I notice that the red undervoltage lights come on and stay on after I put the gear handle back down. So I recycle (avionics switch off, alternators off, master off, wait a couple of seconds, then master on, alternators on, avionics switch on). After recycle, the electrical looks OK (no red lights, normal 27+ voltage). I try to put the gear back up and I get the same results (red lights, etc.). After another recycle, I give up (I'm alone, trying to fly the plane, and concerned that if on one of the tries the doors stay open, then the drag won't let me get to Miami with the fuel I've got). I confirm that the doors are closed and fly back to Miami with the gear down.

What should we look for? I already checked the dipstick on the powerpack and found it OK. My A&P knows Skymasters like the back of his hand, but he knows the earlier version with engine driven hydraulic pump(s). It doesn't seem to be a fuse, because the powerpack seems to be drawing power, perhaps excess power. What besides the powerpack (which was overhauled in '99)?

Would appreciate any help anyone can give me.

Ernie

[Bob Cook]

Ernie

First of all try pumping the doors down manually and when fully extended turn the power on and see if the doors come back up.

IF they do .......great..

Now then check the squat switch and make sure it goes to the gear up position when you put weight on the tail and let the oleo extend. make sure your undercarriage is four fingers dialated <G>.

It probably is due to either a broken wire on the nose gear squat switch.

Last but not least make sure you have a good battery connection. that motor draws a lot of power and if you do not have battery capacity to assist the alternators, the alternators alone is not enough power to get the gear moving. This could have caused the undervoltage light to fire and would probably trip the breaker as well.

Let me know the results and i will try and help you from here.

btw there is little drag with the gear down. just fly it back and don't bother with trying to play with it. Remember you need lots of strokes with the manual pump if you are going to try it.



Bob

[Ernie Martin]

Bob:

Thanks for the quick reply. I've edited the earlier message to clarify that the doors stayed closed throughout and that I then flew to Miami with the gear down. I also made clear that the undervoltage lights REMAINED ON even after I moved the switch back to the down position.

I will pursue your suggestions, but now that I've clarified the original message, do they still fit the problem? For instance:

Squat switch: if this was the cause, we wouldn't get the undervoltage conditions. It seems the powerpack is trying, given the voltage drop.

Breaker: this never tripped. Once I reset (to get back to a normal electrical condition), I would try it again and have a repeat of before, so the powerpack seems to come on again.

That leaves the bad battery connections. Both engines started strong, without any indication of weakening power, and I would guess that a starter draws more than the powerpack. When I first considered this suggestion, I thought that it could not be, because the undervoltage lights REMAINED ON even after I moved the switch back to the down position and you'd figure that things would go back to normal once I took the powerpack load off. But of course you may be right, if the undervoltage was large enough that both alternators tripped off. And this (both alternators tripping off) is probably what happened.

I'll check all, especially the battery connections, but if you have any other thoughts after these clarifications, please post them.

Thanks.
Ernie

[Bob Cook]

Ernie

The starters go to the battery direct thru their own solenoids. The gear powerpack goes thru the master solenoid (which could be bad). Totally different path.

My tests are still valid to determine if the pump operates and the high pressure switch opens when the system reaches full pressure. This occurs when the doors close. The squat switch prevents gear retration on the ground,therefore, if it doesn't work the gear stays down. It energizes a relay that is mounted in front of the pump on later models.

When doing the static test by pumping doors down by hand then turning on master sw the doors should come up briskly, pump stops in about 3-4 seconds. The max load on the pump would occur at max pressure when the doors are closed and the pump is building up pressure.

I have a gut feel that the master solenoid is at the heart of the problem. if it has not been changed in last five years it should be changed. they are about 25 dollars. i would change them all including starter solenoids.

I always listen for the pump and the sounds going up and down. The doors should retract and full gear up in 11 seconds. If longer, time to check things out.

Regarding lights. to reset the alternators etc. the ***master*** must be turned off and turned back on. If the alternators are not putting out then it is time to hit the emergency "start" switch which will put a current into the field. I think there is enough residual magnatism that this start is not really needed. Remember MASTER OFF to reset lights and overvolt buss disconnect. It is a failsafe circuit that requires full disconnect before reset.

fyi

bob


fyi

bob

[WebMaster]

Ernie, had the same problem with my 69 D. I think the squat switch is the first place to check. There is a solenoid under the panel, which prevents the gear lever from going up. If the gear handle will go up, then it's a power pack problem, but I'd check the squat switch first. Also, when you are doing that, check front strut inflation. The gear has to extend enough to have the squate switch make contact with a striker plate. If it is making contact, make sure it has power going to it, and from it.

[Jerry De Santis]

Ernie, Larry is right on target! It's most likely the squat switch.
The front nose struct must extent after you flit off for the squat switch to make contact. Simple solution, charge the nose gear.
Cheers
Jerry

[Ernie Martin]

OK, guys, thanks a bunch for your ideas. We'll get on it tomorrow. I'm still doubtful it's the squat switch, given that I wouldn't keep getting the undervoltage condition. It seems the powerpack is trying, as seen by the voltage drop. If it was the squat switch, I'd put the gear lever up and get nothing. Instead, the two alternators drop off, suggesting that the powerpack got the signal to try to raise the gear. But, we'll see.

Ernie

[SkyKing]

Ernie,

I trust you have already acquired "The Bibles' for your particular airplane, i.e., the Cessna Service & Parts Manuals for this 1973 "G" model!

Now, your 1973 normally aspirated "G" model Service Manual may have different numbered sections than the one for the P-model series (1973-1980), but in our P-model's Section 5, "Landing Gear, Wheels, Brakes And Hydraulic System", you will find a "Troubleshooting " chart to go thru and eliminate each possible cause. Since the power pack is running but nothing is happening, yes, you could have a defective squat switch or the oleo is not extending all the way for it to properly close, but I'll just bet that's not the case as these are fairly trouble free.

If you'll recall earlier threads that GMAs went through on the gear, on the later power packs (1973 and later) you have TWO solenoid operated valves... the one next to the pilot's right leg is the DOOR Solenoid and the one next to the co-pilot's left leg is the GEAR Solenoid. You can check that the DOOR solenoid is operating properly when you turn on the master... you should hear a familiar 'clunk' as it energizes.

Now, in flight, if the power pack is running and under strain, and the doors are not opening... and you should be able to discern a difference in the sound of the air slipstream if they were opening... then it's possible you may have a door solenoid valve jammed or stuck in the 'DOOR CLOSE' position. Does yours have the later solenoids -- as recommended by Cessna for changeout --that have the removable tops so the plungers can be cleaned?

Now, if the doors are opening but no gear movement and the power pack is under strain, it could very well be that you have a gear solenoid valve jammed or stuck in the 'GEAR DOWN' position.

Finally, IF the doors open but the gear does not cycle, THEN you may indeed have a faulty squat switch or perhaps the nose strut is stiff and not extending all the way to close it, but I doubt this is the case.

Try this: Block the main wheels so it won't roll, jack the nose up using the jackpoint just aft of the nose gear wheel well area and jack the nose off the ground so that the nose wheel is not touching the ground... an inch should do. Then, with an external power source applied to the ground service plug receptacle to supply 28vdc (use a spare battery and a charger hooked in parallel), reach in through the pilot side window and move the gear handle into the up position. You should have power pack initiation, doors then opening and the nose gear should begin moving up. You can stop the process at anytime through the cycle by pulling the landing gear circuit breaker... be ready for that. Since the mains are on the ground with weight on the wheels, nothing should happen with them, and since you're not going to allow the cycle to continue, the power pack won't be placed under addtional strain.

Now, if when you move the gear handle to the up position and nothing happens, cycle the gear handle switch a couple of times to see if you can get it to initiate... and if you get the doors open but the powerpack starts straining and the nose gear doesn't start moving, reach in and pull the landing gear breaker for a moment so the powerpack stops operating, then engage the breaker again. By doing this a couple of times you're allowing the gear solenoid to get a zap of electrical current and if the gear solenoid is stuck, this might break it loose.

But... if you haven't cleaned the door or gear solenoids, and they most likely haven't been touched because most shops won't take the time to remove all the plastic around the lower pedestal, you really should check these first and make sure they're clean and without any dried 5605 fluid keeping the plungers from working properly.

Hope this helps. BTW, remember that landing gear systems from 1973 and on are completely different than the earlier variety.

SkyKing

[Bob Cook]

skyking

regardless, why is his electrical system quitting because of overcurrent yet the breaker for the gear pump is not. This is an electrical problem most likely.

bob

[Dale Campbell]

I have a 337H with the same power pack. I had a problem last year when I took off & tried to raise gear. I moved handle to up position, the gear horn went off, so I put handle back down. I tried 3 more times, with same problem. My problem was the olio did not extend, so the squat switch did not transfer. If you had the problem with squat switch the horn would have sounded. To have the high power draw you have a problem in the pump circuit some where, or the pump motor has a problem. Only my guess.

[Ernie Martin]

Skyking:

Thanks for your detailed reply. After checking the other things you, Bob and others suggested (like squat switch, bad master solenoids, dirty gear solenoids, etc.), I'm going to try your idea of trying to raise the gear on the ground with just the nose jacked up (your text is copied at the bottom of this message). I'm a little concerned of doing damage to the plane. Couple of questions:

1. Assuming that I've got the gear circuit breaker properly identified and I've confirmed that I can pull it out instantly, will it stop the process? In case the breaker is mislabeled, isn't it easier to just turn off the master?

2. After I pull the breaker (or turn the master off) and everything has come to a stop, what is the best procedure to put things back? Do I do it electrically by turning master switch off, putting the gear lever down, re-inserting the breaker and then turning master switch on OR better to pump the gear up manually?

3. If, for whatever reason, pulling out the breaker (or turning the master off) doesn't stop the process quickly enough, are you suggesting that the weight of the aircraft is too large for the mechanism and that the gear won't retract and damage the aircraft?

Thanks a lot for your help.
Ernie

[YOUR IDEA: Block the main wheels so it won't roll, jack the nose up using the jackpoint just aft of the nose gear wheel well area and jack the nose off the ground so that the nose wheel is not touching the ground... an inch should do. Then, with an external power source applied to the ground service plug receptacle to supply 28vdc (use a spare battery and a charger hooked in parallel), reach in through the pilot side window and move the gear handle into the up position. You should have power pack initiation, doors then opening and the nose gear should begin moving up. You can stop the process at anytime through the cycle by pulling the landing gear circuit breaker... be ready for that. Since the mains are on the ground with weight on the wheels, nothing should happen with them, and since you're not going to allow the cycle to continue, the power pack won't be placed under addtional strain.]

[GMAs]

First off Bob.. you need to check your schematic again... as the starters and all the rest of the ships power goes thru the master contactor... yep it does... the starters don't get their power by a different means... they get it from the master and then send it to the starter contactors... if you want to try something take the masters switch and turn it off... and see if you can still start the engines... smile... now the only thing that still is connected when the master is off... is the clock... no more ... no less...

ON TO ERNIES PROBLEM…..

Ok Ernie... you gave them all the symptoms... for a heavy electrical load... but, no circuit breakers popped out... that means that its a inductive load...

First order is to get the service manual out and turn to the page in the back which shows the schematic of the wiring... (we make the mechanics do this too...but, I usually make them sit at the table and go through all the possibilities and then make a list of the symptoms and items that may cause it...) from that we can get a good picture in our mind of what the system is doing.. from the way it should to the way it isn't...

[here lets take a example... if the gear pump was shorted what would the symptoms be... Ok we put on one side of the page pump shorted... and on the other we put all the possibilities of how it would effect the system... high current, lights dim, no sound, etc.. etc.. see it is logical stuff that can help find the problem]....

SQUAT SWITCH…..

So... the squat switch is one area to start with...lets put a safety jack under the nose jack point and leave the gear on the ground… turn the master on ,gear pump ckt breaker in..., and put the gear handle in the up position… the gear doors should open and then stop.. and the pump make a groaning sound… turn off the master… or you can leave the handle in the down position.. and with the master off.. hand pump down the gear doors… and stop… and then visually inspect all the wires and the oleo strut for full compression of the squat switch as you lift the nose off the ground with the jack… on the jack point… and the strut extends the switch should be in full contact… BE VERY CAREFUL ABOUT THE MASTER SWITCH AND THE GEAR MOTOR CKT BREAKER... WHICH SHOULD BE SAFETIED..(PULLED AS PER THE SAFETY WARING) AS WE DON'T WANT TO GET INTO THE GEAR COMING UP OR THE DOORS CLOSING WHEN WE ARE IN THE NOSE GEAR WELL AREA... The pump and master should be off for this test…. With the gear up handle in position…. If in fact that squat switch was not functioning... either by the fact that the wires at the knuckles or bend points are open.. then you would have had the pump motor for the gear running under load.. the gear doors would have opened up... which you didn't say they did... The lights won't change until something moves….

THE TEST POINTS… for the squat switch is located on the pilots side up in the forward area of the nose gear well… it has a little rubber boot over the terminals… here is where you want to attach your volt ohm meter… after you remove one of the wires going to the squat switch…so as to isolate it from the rest of the circuit. Test and then replace being careful not to over torque the nuts on the terminal strip… you want it tight though and make sure to replace the boot over the studs when you are done… so it doesn't interfere with the wheel or other parts…


... however, I don't think this is your problem

CONTACTORS….

what we want to do here is go across the devices'... and check for voltage... with the system master switch on… you should not get more than 1 volt.. if you do.. check it out further... and replace the item. first from the battery... touch the test lead to the stud of the battery positive side.. then to the cable attachment point on the master contactor... then across the master then to the pump contactor...yep it has one inside on top of the wheel well... then to the pump... going across these items will tell you if you have a problem inside it... now if you have a current meter...clamp.. you might want to put that on and see what the current draw of the motor is...

…. However, I don't think this is your problem either… but, a look see first thing… as from here things get expensive….

GEAR PUMP…..

Next you didn't say the pump was running... you said it was quiet... I mean you should have heard it come up on line and start to load up.. but, wait we have one other clue...

The under voltage light was coming on... when you put the gear handle in the up position... which indicates that the alternators were putting out before and now they are not able to meet the demand... so something is pulling the buss power down.. and if it gets so far down... your not going to be able to make the ckt breakers go pop... because they are not handling enough voltage/current... besides if it still is within the limits of the breaker.. it will not open up...

So lets take a look a the main item the pump... yep.. the pump motor has brushes in it too... and if they get too worn.. guess what they do... high resistance short ……ahhhh...


As skyking says.. the solenoid's are the next item that I look at... but, we can check all of these things real quickly by hand pumping with the master on.. from inside the plane... if the gear doors open on the cycle... and the gear gives resistance to the pump after they are open... good... now in the up position with the plane jacked up all the way around... you should be able open the doors and start to move the gear…by handpump.. again the ckt bkr for the pump motor should be pulled for this test… listen for the selos to click.. and then feel them if they get warm... your working... if they haven't been cleaned... which is more a gear down problem... you need to do that too... while your in their... (every 2 years routine maintenance at annual time)


But,.... from what you said so far.. I would think the pump motor is bad... not a cheep item either.. but, you don't have to get the whole power pack... the pump motor will unbolt right out of the plane... by its self... we had one before that had a bad segment in the motor.. and it just wouldn't keep running... and you don't want to let it recover and then think things are ok.. they are not.. and you need to find the problem and have the bad part replace... G.M> GMAs

[Ernie Martin]

Thanks, GMAs, expect to check all this out tomorrow.

Ernie

[Bob Cook]

RE solenoids

The battery goes to ALL three solenoids, 2 starters and master solenoid. The solenoid coil is energized thru the master solenoid. Yes, the starters won't start, but the current for the starter is NOT going thru the master solenoid. The solenoids are in parrellel not serial... beg to differ with you.

Secondly, if the master solenoid has a high resistance, the battery (capacitor in this case) is not adding to the power source for the gear motor. In this case the alternators trip out on overload and no gear....

At least this would be the case for the "P" series that I am familiar with.

These contactors (solenoids) have been a source of trouble for years.

at least this MHO..

bob

[skymaster]

howdy: i usually get a couple guys to pull the aircraft carefully on its tail. I run nose gear up and down. be careful not to drop. knock on wood solved most glitch.occacionaly, after take off it needs a couple hand pumps to put some pressure on the switches. especially if its been sitting. just got back from rtc training. ins company is back on the hook. J