Nose Gear Side Load Event

I commented to the controller coming into St. Louis Spirit, recreation flying today is kind of dumb to which he responded, dumb or interesting? Yea, some of each.

It's just that there are so few decent days to fly this time of the year, one tends to test the elements when he shouldn't.

The problem is this very deep low pressure system over the Rockies creating these tightly packed isobars across the mid west and that means wind from the south, lots of it, and low level turbulence, lots of it too. The chop at 140 knots was totally uncomfortable so it was slow up to the blue line and ride it out.

The missions was important I suppose, I needed a new can of mixed nuts from Sam's for our usual news watching happy hour around here. A couple friends from our informal geezer club provided transportation for lunch also.

The landing at KSUS was ok even with the ATIS wind gusting from 11 to 28 knots across the runway at 90 degrees, felt good about that, touching down on one wheel with the right wing quite low. No problem, lucked out one more time. With 5000'+ of runway, the machine can be finessed on with no concern about running out of concrete.

Two hours later coming into our 2100' x 25' tree lined strip with the same wind condition was another matter. It took quite a bit of attention just to keep the machine reasonably level on final at 70 knots and in the flare it almost pranged on as the turbulence did not let up one bit. As it turned out, the touch down was not all that bad with some deft stick movement the last second or so, but something was vibrating severely. I thought I blew a tire. As the beast slowed the vibration stopped completely and I thought wow, what was that all about? All was smooth and there was no flat tire.

After stowing the airplane in the hangar, I decided to take a walk out to the runway and see if there was anything to see. What I found was a bit disconcerting. The nose gear had gone into a wild side to side oscillation leaving rubber marks swerving left to right where it was doing this weird gyration. It was not a typical NG vibration, it was the NG pivoting from left to right about 30 degrees, the tire marks are very clear and it did this for maybe 6 cycles. I think what happened was the NG contacted the surface in a slight crab and the NG did not like that one bit. It was not the usual full aft stick landing due to the gusty conditions. Not a good situation. At Spirit it was a full aft stick landing but there was plenty of time and runway to work it on.

I will inspect the NG thoroughly and next time keep the hangar door closed when such a day presents itself. 2100' with trees is not enough runway with such wind conditions.

Live and learn, it goes on forever.

David,

I had a similar thing happen one time in very windy conditions. I think what happened to me was that the nose wheel touched the runway surface an extra time during the lift off in the very gusty conditions, just enough to kick it off center so it was no longer aligned for straight taxiing. Then when I landed it kicked pretty hard to get back into alignment and oscillated several times. No damage was done. I cannot be sure exactly what happened but this is what I decided it must have been. It has not happened since

Randy C

Randy said:

David,

I had a similar thing happen one time in very windy conditions. I think what happened to me was that the nose wheel touched the runway surface an extra time during the lift off in the very gusty conditions, just enough to kick it off center so it was no longer aligned for straight taxiing. Then when I landed it kicked pretty hard to get back into alignment and oscillated several times. No damage was done. I cannot be sure exactly what happened but this is what I decided it must have been. It has not happened since

Randy C

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That makes good sense. The NG breakout force was recently reset and it was stiff, it could have come off the concrete a bit cocked at KSUS.

Thanks for the report, glad it has not happened since.

I had the same thing happen in my RV-7A when landing in a very strong crosswind. I had never had any signs of NG shimmy up to that point. I discovered cracked NG wheelpant brackets after this occurred. So - was the shimmy the source of the cracked brackets, or were the cracked brackets the source of the shimmy? Either way it was a bit disconcerting - the shimmy was very strong.

Mark Olson N407V RV-7A N16XV F1-EVO Rocket

Tire pressure?

I once witnessed my neighbor's RV-6A exhibit similar behavior. I'll be honest, it scared the heck out of me. I forced him to come out to the runway and look at the skid marks.

What was the ambient air temperature, and could the (expected) low temps have caused your tire pressure to fall below optimum?

Nose gear breakout and tire pressure are the first two things I'd look at.

So, wondering .. was this close to failure or not?

Most all of the discussion on failure in past has been on vertical movement, but obviously there was a good bit of horizontal movement in this event.

What should be looked at, or asked about? Any reason to suspect something has been driven further than it should be and should be suspect for further service? What do Raiz or others that have looked hard have to say?

Apparent good news is that others have experienced similar events and have not had problems since. However, my background says just because you pulled the trigger on the gun 4 times and nothing happened, you cannot assume you can continue to do it.

Not trying to start a firestorm....Just wondering,... let's keep each other safe...

The entire NG structure from the fork up to the firewall appears normal. There are no visible cracks in the gear strut support at the mount, nor in the fork weldments. The fairing attach brackets are the newer type that slide aft without removing the axle bolt and they too are OK. The only cracks found are with the front NG cover and they are pre existing, they were there before the event.

Tire pressure measured at 34 psi. The tire was serviced to 45 psi about 6 weeks ago but the OAT was much warmer then. The side load break out force on the axle bolt is 24 pounds.

We know from video the NG strut is quite flexible fore and aft. I believe it is a design feature of the system, if it were more rigid than it is it would break under severe load, instead it flexes. We know this is true in the fore and aft mode, no doubt it is just as flexible from side to side.

What probably is going on when landing in a severe, gusty cross wind, the NG contacts the surface not aligned with the direction of flight and a side load induces the side to side oscillation as it cocks the wheel off in the opposite direction which soon goes through center and comes out the other side aimed the wrong way, and then it gets cocked off the other way and so on and so forth until it dampens itself and the cycles stop.

The first landing that day was as it should be - NG held off as long as possible and when it did touch down it is aligned with the direction of flight. There was plenty of time and runway to work it out.

The second landing was not so due to minimum stopping distance on a down hill, short wet runway. There was not enough time to conquer the cross wind completely, which sometimes is a tail wind, the instinct being to get it on and stopped ASAP. The aircraft probably touched down in a slight crab and that's what induced the side to side oscillation. Everything was happening rather quickly, but I do recall hauling full aft stick when the vibration was occurring and that's when it stopped. That landing was not the usual full aft stick because the speed was a bit higher than usual and even at 70 knots, control was a challenge. The stall speed of this machine is about 51 knots with 20 flaps, 40 did not seem prudent considering the conditions.

This all points to limitations. Every pilot and airplane has limitations and I was really pushing the envelope on that flight. Not smart at all, the mission was not that important. I have been flying this beast for some 7 years with hundreds of landings and this was the first such event. Obviously the envelope was exceeded and that's something we need to think about. The NG system is adequate for what we do but it has its limitations and we are learning more about that all the time.

PS. If the moderators feel the information here is pertinent to safe flight, perhaps the thread could be moved to the safety section and re titled something like NG side load event.

Thanks for the details and inspection report

Details on tire pressure and also stick movement relieving oscillation are good to know. Glad to hear nothing appears out of the ordinary with a serious look.

Sounds like that drift across the runway is what set this up. I'm sure someone can do the math as to amount of drift, nose touchdown speed, and tire friction on various surfaces, that will get into oscillation and possibly even to failure. (those taildraggers like grass for some ODD reason that make the landingings easier/nicer....)

Meanwhile, it is a good reminder that while touching down, there are in fact three dimensions we need to be careful with the nose gear.

Thanks for the post

David-aviator said:

...<SNIP>
PS. If the moderators feel the information here is pertinent to safe flight, perhaps the thread could be moved to the safety section and re titled something like NG side load event...<SNIP>

Click to expand...

DONE!!!

NG OK after side load induced oscillation

It's been cold as a well drillers butt around here but I finally got everything back together today and went for a flight to see if the NG was OK - it is.

Even did one cross wind landing, managed to keep the thing fairly straight and it was perfectly normal.

The key to keeping things smooth and the NG happy is don't land in a crab.

Another thing to Check?

I recently was putting my 6A back in the hanger and Richard King (Excellent builder in Houston area) noticed a "creaking" noise as I used the tow bar on the nose wheel. A closer check with the cowling off looked OK until I put the tow bar back on and, without rolling the nose wheel I turned the nose wheel back and forth to reproduce the creaking sound. It was coming from the area where the gear leg attaches to the Engine mount. Placing a finger on the end of the gear leg, touching it and the engine mount at the same time with the pad of an index finger, a very slight amount of movement was detected.

The AN5-20 Bolt in the upper gusset on the engine mount had, ever so slightly, egged out the hole in the gusset thus allowing movement. I suspect that this is a weak area on RVs (including mine). Some very good friends (Doug and Jerry {two genius builders}) brought in a tapered reamer and the AN5-20 bolt has been replaced with an AN386-3-12A TAPER PIN.

I strongly suspect that the egging out is the result of an imprecise original mfg of the hole and bolt fit that deteriorates further with turns that carry a bit too much speed off the runway. I always land with the nose wheel off the ground until it comes down naturally but at that point, if there is a taxiway close some braking and turning took place on the occasions when "I can make that turn now" entered my mind. Now I think, "I will start turning after this plane has come to almost a complete halt!" Slow turns in the "A" model is a MUST IMO to prevent the above problem.

I wonder if a similar amount of play may have contributed to the horizontal wobble described in this thread? I also wonder if some of the flip overs may not have flipped on the first wobble or so in some of the unexplained accidents?