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#1: Here's a description of Rob Berger's gear retraction actuator
problem with G-YAKA, a 1983 airframe.
The problem first manifests itself as a inability to maintain main system accumulator pressure (i.e. 50 bar) with the gear selector in either the 'up' or 'down' position. The neutral was ok because the gear actuators are not pressurized; however, if the gear is already up it may unlock with the selector in this position. On the ground with the selector in the 'down' position, air may also be heard escaping from the exhaust ports in the gear selector itself.
On another Yak 50 I was told that one gear leg was slow to retract (not the case with 'KA') This particular aircraft had, I would suggest advanced deterioration of the rams/actuators, the pilot eventually ended up with only partial gear extension on one side! (After emergency blow down). It was only after using hi 'g' maneuvering that full extension was achieved, very lucky!
Cause: Upon strip down of the gear actuator cylinders it became evident that the cause was severe corrosion of the actuating piston/ram assembly (made in one piece), the rubber seal were largely ok, as were the cylinder bores (hard chromed). However, the retaining circlips were very badly corroded as was the piston/seal holder/ram (made from a soft alloy steel). One circlip was so corroded (i.e. not tags left, just rust!) it was close to migrating and becoming loose in the cylinder, this could have easily caused the actuator to jam!
Another good indication of piston/ram corrosion is brown/dark red liquid (rust in suspension with water/air system lubricant) exhausting/dripping from the exhaust ports in the cockpit gear selector, this is a warning and should not be ignored.
Although I have managed to obtain a pair of used replacement rams, this is not a permanent fix. The rams are becoming very difficult to get hold of new and I suggest that manufacture maybe the only option, I'm looking into this right now, perhaps making the ram in two pieces thus using a high quality stainless steel for the piston/seal holder fixing the problem of corrosion for good.
In the meantime it would be my suggestion that other Yak 50's of a similar age (i.e. most of them in the western world) have their gear actuator internal components for corrosion. Landing a 50 with only one gear extended would, I guess, be most unpleasant! (It would probably turn over!).
A reply from Keith Elcomb: A reluctance for the gear to lock down or retract correctly when the main air pressure falls to 18-20 atmos is an indication of excessive friction within the ram, as is a failure for the internal or external locks to actuate. A Russian engineer expert told me that full operational status should remain down to 15 atmos, when carrying out static cycling tests.
Rusty liquid dripping from the gear selector valve would be a most serious indication, in my opinion, of advanced internal ram corrosion. Water within the ram can freeze solid in the winter (or indeed in any low temperature situation -altitude?) and could pose a very dangerous situation.
Correct jacking of the aircraft to allow the gear to be manually pushed through its operating arc will usually establish if there is any roughness, or actual seizure of the internal mechanisms. It is vital that any individual working on the gear should establish that the system is totally free of compressed air, that the main and emergency valves are shut and that the aircraft is securely jacked.
Serious injury or death could result to any person getting caught in the mechanism when air pressure is applied to the system, if the gear is hanging loose and/or the gear valve is set in a position that applies air to move the gear leg in either direction. This mechanism is a serious finger/hand guillotine! I personally favor totally de-pressurising the system and removing the 'gear retract' air supply pipe.
Manually 'swinging' the gear will then tend to pump out any water/oil and corrosion products. Judge the degree of corrosion that has occurred between the last service interval (when this test was carried out) and the current test by examining the fluid for rusty water, rust particles or actual metallic fragments. At this point, fill the ram through the open union with air tool oil, sucking the oil into the ram by moving the gear leg. Flush out as best as possible, once again examining the fluid that will be squirted out of the connection. The only way to truly determine the state of the internals is a complete manual strip after the rams have been removed. This is a difficult and tedious task, taking up to 10+ hours to remove and dismantle both rams, but one that may have to be considered.
Full and free travel of the gear, with no feeling of roughness or strange sounds, coupled with a freedom from air leaks and completion of satisfactory cycling tests should be a reasonable indication that the rams are likely to remain functional for some time. Fortunately, the internal rubber seal assembly seems to be reasonably robust. However, removal and stripping for detailed examination is the only real insurance and I am afraid that owners will have to be prepared to meet this cost if they really want to know what is happening internally. Careful attention to opening the water accumulator drain valve at the end of a sortie (shutting it after use!) and introduction of compressor oil to the air system at 50 hour service intervals should help to get the maximum life from the ram units. What is the expected average life? Sorry - but I really have no idea. There are so many variations regarding aircraft usage, location, service history etc.
Secure jacking is especially vital as any subsequent testing or cycling of the gear under air pressure subjects the aircraft to very considerable shock loads that could easily dislodge inadequate jacking arrangements. All the Yaks have extremely robust pneumatic gear systems that can frighten anyone observing gear cycling for the first time! Please, please be extremely careful.
Pilot Bill Perrins handled his incident in the Yak 50, after the starboard gear leg failed to deploy completely due to low air pressure in the main reservoir, coupled with a stiff ram mechanism, with very professional coolness and his ability to recognize and deal with the situation.
There really is no substitute for a good understanding of the emergency gear extension procedure and all the other tricks in the trade to get the gear down and locked with those 'greens' showing.
Another Reply from Tom Johnson: Dec. 2004
#2
Replacement landing gear rams/actuators: The plan is to make
the barrel out of anodized aluminum with a hard chromed piston. All the
internals & seals will be off the shelf American parts that you can get from
you local industrial hydraulic shop. Our intent is to produce a high quality
part that will be easy to support in the field. It uses all US dimensions &
SAE hardware/fittings.
My main reason for contacting you directly is your comment about how hard it is
to remove the gear rams/actuators. I can assure you I have found an easy way to
pull them in and out. I am guessing you attempted to pull the front mounting
bolt & cotter pin by going in from the bottom with thin wall open end
wrenches and an extremely thin pair of needle nose pliers? Use my procedure and
you can have them out in 1 hour total.
Pull the rectangular panel immediately behind the gear pivot & both round
panels on each side of it. Reach inside the round holes, and go forward toward
the leading edge. you will find a lightening hole in the rib on each side of the
gear. This is how you will access the front bolt. You simply need to align the
bolt with the lighting hole by slightly retracting it, just enough to break the
down locks loose and the top of the gear will align with these holes, in fact
one half of the down lock will form a pocket for your 12mm sockets to rest in,
there is enough room inside the wing above these round access panels to go in
with a full sized part of dikes, or needle nose pliers to remove the cotter pin,
and then use two ratchets to remove the nut & bolt. Next push the gear back
forward by hand to the down and locked position, even with the bolt removed this
will recompress the piston inside the ram making the total length of the ram as
short as possible, alternatively you could introduce air to the check valve
(front) end of the ram and the piston will retract its self.
Next remove the access panel at the front of the gear well & pull the rear
ram mounting bolt at the uplock, then simply pull the ram aft until the check
valve hits at the bottom on the round hole it passes though simply turn the ram
90 degrees (back end toward the ground) and the top of the ram will come out and
allow you to pull the check valve though the hole. NOTE: Depending on if your
airplane still has the cloth seal around this rear hole, or if the top of the
ram will not exit the hole due to the clevis that threads in to it being to
long. You will need to remove the clevis that screws into the end of the piston.
Simply push the ram back forward towards its normal position, until the clevis,
and its lock nut and safety wire is easy to get to (there is room to spin the
ram even with the check valve on it). FIRST to make reinstallation simpler,
count the number on threads from the lock nut to the end of the threads, write
it down for later use, as when you go to reinstall the ram, and need to rig the
clevis you can simply reinstall it to the same depth by screwing it in until you
have the same number of thread exposed, assuming it was rigged correctly before)
NOW use the rectangular inspection cover to reach in and cut the safety wire,
back the 19mm nut off (be sure to catch the little locking post/insert that
rides in the slot of the clevis-it's the thing that the 19mm nut was
safety-wired to). And then unscrew the clevis from the piston, doing this WILL
make the ram short enough to remove as I described above.
When you go to put the ram back in, insert it though the hole in the gear well
you removed it from, reinstall the clevis & safety. Now remember how we got
access to that darn front bolt? We had to unlock the gear from it's down lock.
How are we going to do this without an actuator connected to it? Look up through
the rectangular inspection hole just above the top of the down lock and beyond
it. You will see a saddle, or U shaped assembly (the "U" will be
facing down) insert a round 1 inch pipe above the down lock until the end sets
in this saddle or "U" (the pump handle from your wing jack is perfect)
then pull down, this will unlock the gear and it will usually hang just perfect
to align the bolt hole in the uplock; where the ram/actuator attaches, with the
lightening holes in the ribs (the same ones you used to take it out) This is a
very wordy way of telling you how to do it, but take just minutes to do and you
can use full size tools, use both hands & still use the rectangular hole to
look through to see what you are doing!
All of the above assumes you have the airplane jacked up, some important notes
on that procedure: Jack point inserts are 14mm, 1.5 thread pitch. Recent
experience tells me that you can find 14mm bolts at Home Depot but they DO NOT
have 1.5 thread pitch, they usually only have 1.25 or 1.75. so plan ahead. (I am
also working on manufacturing REAL jack pads for the 50, interested?) once you
have the jack pads/bolts installed I would recommend lifting the tail to
level by hand. One person CAN do it. Climb under the rear fuse put you back
under a reinforced area and, stand up. Then you can jack it straight up, rather
than putting the jacks under the wing, and lifting at an angle. As you know the
gear on the Yak work extremely fast, and with great force. It WILL knock the
airplane off the jacks at the very least, and kill you if you get caught by it.
The only way to detune this "slamming" of the gear is to reduce the
amount of pressure supplied to the rams. Bleed your bottles down to no more than
20 K/cm. In fact for your safety you should empty the emergency bottle by
removing the forward line from the check valve on BOTH gear rams, and then open
the red emergency gear handle. The air will blow out the open lines, also
probably a good cleaning procedure for the valve and lines considering the last
time you used them was? It has been stated that the gear should operate normally
down to 15 K/cm, and in fact if it won't. That is an indication of binding in
the system or advanced corrosion/crap in the rams. If you bleed your system down
and you gear won't work right, DO NOT recharge you bottle. First pull the normal
up and down hose connections from each end of the ram/actuator. In side this
fitting you will find a small disc with a tiny hole in the middle. Its a flow
restrictor, actually slows the gear cycle down (if you think they slam up and
down now, leave these little guys out. The gear would probably retract though
the wing!!). With these restrictors out the remaining bottle pressure will cycle
your gear, ONLY do this if they are so sticky they will not move with the low
bottle pressure. I suppose you know you must do ALL this with the bottle shut
off. My suggestion would be to select the gear position you want, then slowly
crack the bottle open, hissing just enough in to the system to move the gear the
desired amount. Close the bottle, gear stops where it is.
Dean Courtney deancourtney696@hotmail.com <mailto:deancourtney696@hotmail.com>
205-405-0701
#3 Tail Wheel Reversal Problem: If you still have the tail wheel oleo
shock installed, PLEASE don't leave your airplane in the hanger with the tail
wheel turned around backwards (its normal position after being backed into the
hanger). We did, returned a week later and our tail oleo had went flat. No big
deal facing forward, but it bends things facing backward. Not exterior sheet
metal. It crushes the bottom of the fuse bulkhead just behind the hinge point.
So when you open the panel to refill that oleo, you get to do that, plus a
weekends worth of sheet metal work! We are installing the mod from Richard Goode
that replaces that damn oleo with a spring and rubber donut, AND from now on,
always spinning our tail-wheel to where it belongs.
Cheers,
Dean Courtney deancourtney696@hotmail.com <mailto:deancourtney696@hotmail.com>
205-405-0701
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