Screwjack Wear

This article appeared in the September 2002 issue of the Falco Builders Letter.

In September, John Harns was forced to make a wheels-up landing in his Falco. It did very little damage to the airplane, but it curled the tips of the propeller and caused some abrasion to the nose gear trunnion and right wing tip. At the time of the accident, he was shooting a practice ILS, dropped the gear but did not get a green light, and the landing gear circuit breaker popped.

John tried to turn the emergency crank, but it would not budge. He made three fly-bys at the airport, and the tower said the landing gear was down, but a pilot flew in formation with John and reported that the nose gear was in a trail position and not fully extended. The main gear looked like it was fully extended.

John put it down on the airport at midfield. He shut the engine off before touchdown but could not stop the propeller from windmilling. The right gear collapsed, then the nose gear and then the left main. The airplane slid for about 300 feet and rode on the hubcaps of the main wheels. The cowling was scraped, and the nose gear door was torn off. They picked the Falco up with a military bomb jack and managed to poke a couple of holes in the wing and horizontal tail in the process.

The main gear screwjacks were badly bent. The nose gear screwjack appears to have frozen, and after the accident the screwjack could be pushed through the screwjack sleeve with hand pressure. John took the gearbox out, and it was all operating normally. At the time of the accident, the Falco had 1300 hours. John also mentioned that the nose gear drag struts have a lot of play, and he plans to add sleeves to the bolt holes, but he doesn't think that this had any role in the accident.

John sent us the screwjacks and screwjack sleeves. It's apparent on inspection that the bronze inserts in the screwjack sleeves are badly worn. The main gear screwjacks have about .020" of end-play, and they are very loose and free to wobble side-to-side. John said that he never had any chatter in his system. It appears that the nose gear screwjack sleeve experienced much greater wear, so that on gear extension, the threads climbed and seized, then on landing the screwjack completely pushed through and sheared off the remaining threads.

When we screwed one of John's screwjacks into a new screwjack sleeve, the end-play completely disappeared, thus it appears that the wear is confined to the bronze inserts of the screwjack sleeves.

The screwjacks use a 1/2-10 acme thread, class 2G, which will result in end-play that is barely noticeable when new. The male threads have a flat crown of about .040-.050", thus an end-play of .020" would indicate that the threads are about half-way worn through. A visual inspection of the female threads is difficult and misleading, because even when new, it is difficult to see a flat crown on the threads, thus we recommend measuring the end-play and the side-to-side play or wobble. To measure the end-play, hold a ruler against the screwjack sleeve, and move the screwjack to the limits, and note these on the ruler.

Because this could happen again, we will be issuing a service bulletin requiring inspection and replacement of the screwjack sleeves on evidence of excessive wear, and also recommending maintenance procedures which will reduce the wear.

Before we do this, we would like to get a base of data on the wear that you are seeing in the field. Please measure the end-play of your screwjacks and report this to us, along with the number of hours on the airplane/screwjacks.

In the case of John's Falco, the airplane had been operated from a hard-surface runway, and only rarely from a dirt or grass airstrip. However, it is often dry and dusty out west (John lives in Idaho) and an accumulation of grit on the screwjacks would contribute to wear.

Until we have some statistics, our recommendation is to immediately inspect the screwjacks for end-play if the airplane has 500 hours or more, and to inspect for end-play and looseness of the screwjacks at each annual inspection. If the screwjacks have .020" of end-play, then we recommend replacing the screwjack sleeves.

To maintain the screwjacks, we recommend cleaning and lubing the screwjacks at each annual. It is highly probable that much of the wear is caused by dirt and grit becoming embedded in the screwjack grease. Alfred Scott


As it happened, GOCAD was in bits for its annual when the FBL arrived a couple of days ago.

We carried out the inspection and the play we have in our screwjacks is negligible -- nowhere near the 0.020" that concerns you.

We have always kept them well lubricated -- probably a couple of times within each 50 hour -- especially the nose gear screwjack because it is more exposed and in a harsher environment than the others.

Our total airframe time is 309 hours. We did have a slight altercation with the runway early on -- at 46 hours we replaced the nose gear screw jack, but not the sleeve. Apart from that all the parts that relate to this current problem are as first fitted.

Best regards,
Clive Garrard


John's story with the screwjacks reminded me of the trouble I had on my very early flights in 1995. -- popped circuit breakers, massive wobbling of the acme thread. We traced it to a batch of undersized rods- - remember. My shipping ticket for the kit was Feb 1990.

Stephen Friend


I have noticed "chatter" in my nosegear screwjack for several years (5 or 6) and assumed that it was "dry". Since that began I have made it a habit of greasing it 3 or 4 times a year which seems to have solved the problem. The "chatter"or vibration was most noticeable in the cabin when I lowered the gear. I haven't had a chance to check the end-play. Will let you know.

Bill Russell says he has also noticed the chatter when he lowers the gear.

The end play on my screwjack is negligible. The side to side play is about 1/4 ". My Falco has about 450 hours and for about three years I had it hangared at an airport with a grass runway. As I mentioned earlier I grease mine about three times a year. Perhaps greasing it at every oil change (I change mine about every 25 hours) would take care of the problem.

Half-as-wise Rives


Aircraft Serial # 44
Total time 624 hrs

After a good cleaning of grease from the screwjacks, end play is as follows:

Nose: 0.024"
Left: 0.006"
Right: 0.005"

Also, with 10" of screwjack sticking out from the sleeve, the tip wobbles:

Nose: 1/2"
Left & Right: 3/16"

Tristano Caracciolo

Tristano Caracciolo now owns Gerry Walker's old Falco.


There is another dimension (!) to the screw jack problem. Increasing whip in the screw jack assembly effects the operation of the down-limit switch if this is mounted on the screw jack sleeve.

I noticed a week or so ago that the main gear doors were not quite shut on the ground. On my aircraft these are normally shut when the gear is down. On investigation I found that increasing whip in the jack had allowed the fitting on the sleeve to come closer to the limit switch so that the spring on the switch had contacted the edge of the fitting rather than the actuating plate itself, prematurely tripping it. Fortunately I had rigged the system so that the springs are normally half compressed when the switch trips, hence the gear was locked down in spite of the premature activation.

One could imagine that the actual sleeve itself might contact the switch tripping it before gear lock. The sleeve itself is travelling down as well as forward at that point in the cycle.

Obviously the problem is non-existant if the the switch is in it's originally designed position. Placing the switch and activating plate as far forward as possible, and eliminating the lip I have on the activating fitting will help. So to would a longer bronze nut in the screw jack.

I operate from a dusty gravel strip, and have around 400 hours on the airframe. Screw-jack end play is arond 0.08". I lubricate every 25 - 30 hours, and lately have taken to using motorcycle chain lubricant which gets the little black ball bearings on the spot without the thick grease that I find traps dirt and particles of gravel causing popping of the circuit breaker on retraction.

Ian Ferguson


I finally got around to jacking up my bird and measuring the linear play in the jackscrews. I used a dial gauge with a magnetic base mounted on the nosegear jackscrew sleeve to get an accurate measurement of the amount of play. The total linear play for the nosegear jackscrew relative to the sleeve is 14 thousandths of an inch so I've got some margin yet. I couldn't get the dial gauge on the main gear jackscrews, but my calibrated fingertip indicated that the play was much less than for the nosegear -- perhaps 5 thousandths. This is reasonable since the main gear jackscrews are pretty well protected from the elements, while the nosegear jackscrew isn't. Also, out of curiousity, I added up my landings and estimated my test retractions (at annual, etc.) -- my plane's landing gear has been cycled about 500 times.

I figure I'll have to replace the nosegear jackscrew sleeve at about 900-1000 hours tach time (I'm at 602 hours now), but I'll add an item to my annual inspection checklist just to be safe. I'll also make a tool to let me measure the play in the main gear jackscrews.

Jim Petty


I checked the screwjack end-play on my Falco by the procedure you suggested in the Sept. 02 Builders Letter.

The main gear screwjacks both have end-play of .010 (approx)

The nose gear end-play measures .025 to .030. Apparently I'll need to replace the nose gear screwjack sleeve. Please ship me a replacement ASAP. Good flying weather is getting scarce in Michigan.

Total Tach Time on N1593R is 511 hrs.

Happy Hollidays,

Dick Reichenback
(989) 684-9822


Jim Quinn reports that on his Falco, the nose gear screwjack end-play is about 3/4 of the thread length. He has replaced the nose gear screwjack. The main gear screwjacks are very tight.

Total time on the Falco is about 1400 hours.

  Neville Langrick replaced all three screwjack sleeves on his Falco. His airplane has about 700 hours on it, but with an average flight of perhaps a half-hour, the airplane probably has 1500-2000 gear cycles. In inspecting the screwjacks, the main gear screwjacks had about .020" of end play and the nose gear screwjack was well outside the limits. Indeed, the nose gear screwjack sleeve appeared to be on the verge of failure. Prior to changing the screwjacks, Neville had noticed a problem with gear retraction and he had found that the gear would not retract unless he had a certain amount of airspeed, and that the required airspeed was increasing over time. At the time, he thought that this was a problem with the pitot pressure switch, but it is now obvious that the real problem was that the screwjack sleeve was so worn that the screwjack was starting to slip within the screwjack sleeve. In other words, it was jumping the threads.  

On measuring the nose gear screwjack, I have approximately .01" play forward and aft with a small amount of lateral play. The aircraft has made 53 flights which equated to 1066 cycles plus an unrecorded number of cycles for maintenance, possibly a total of some 1100 cycles. The number of cycles of the undercarriage gear is a better indication of wear that the number of hours flown. The majority of my flights have been from hard runways, and I have always cleaned the screwjacks at every annual before regreasing.

Stuart Gane


I finished my annual condition inspection yesterday. In the process, I checked the screwjack wear and found it to be nil. N72GR has 360 hours of operating on pavement only.

Glyn Russell


G-OCAD is undergoing its Annual at the moment, so we've taken the opportunity to remove the screwjacks from the aircraft. They have been thoroughly cleaned and we've done some measurements on them with the following results: Port and Starboard Main Gear 0.003" and the Nose Gear 0.007".

All these are well within your suggested 0.020" - I thought you might like the information.

The aircraft continues to give excellent service, flies beautifully and still attracts lots of positive comments - one from an RAF Tornado GR3 driver!

Clive Garrard