DA40 (LYCOMING, NON SPEED KIT) NOSE WHEEL SHIMMY.

[antony]

Hi,

After seven years and about 900 landings my DA40 occasionally, develops a serious shimmy on landing and a rather less obtrusive one on takeoff.

There is no pattern to this, the last, yesterday, was with a 15 knot cross wind - solo. The previous was calm conditions with full load - landings were normal. These incidents can be months apart.

Very infrequently, the symptoms that correspond to a "corked" nose wheel (IE, ball out of centre) occur; disappearing by the next flight; but as yet, never followed by a shimmy on landing.

It will go into the workshop to be checked, but if anyone has information or an opinion of cause/cure, I'd be grateful.

Thanks

Antony

[mdieter]

I got a lot of vibration on lake off / landing due to an out of balance tire. It wasn't noticeable to the eye, but I guess it was warn enough to shake the whole airplane.

[Chris B]

Hi Antony -

As Mark suggests, a bad tire could cause these issues.

But IMO the first thing to check is that the tension on the nose wheel mount is within specs. When that gets loose, the nose wheel will shimmy, as specifically mentioned the AMM:

Steering friction prevents nose wheel shimmy.

I know this from personal experience.

The tricky part is that after extensive experiments with my aircraft, the optimal tension (to prevent "corking") is right at the lower limit of the spec (see section 32-20-00, pages 209-210) in the AMM. But too loose, and the wheel shimmies.

Note that the AMM specifies applying "corrosion protection to the nut, stud and washers only." The AMM specifies "CRC corrosion shell," but here in the US my IA recommends using LPS-3 which is apparently very similar. This will help keep the critical parts from binding.

Chris

[Steve]

What Chris says is right on. Mine never shimmy'd much until last year. Checked the tension several times, always within specs. Finally changed the nose tire, and it stopped. I guess either tire imbalance, tension too loose or tight, or both can cause it. It is definitely a pain.

I was able to minimize it when it was occurring by holding the nose wheel off as long as possible. If runway contact was made below about 40 Kts, no shimmy.

Steve

[Charles K]

I will further concur - loose nose wheel causes noticeable shimmy after lift-off.

[antony]

Thanks everyone for your help.

The aircraft is being checked by the Diamond Agent - I'll report back after a couple of flights - may be some time, WX is awful here.

Unlikely to be out of balance, the occurrences are too random with nothing felt or heard after lift off, the tire seems OK, but is the original - only the mains have been replaced so, it must be suspect.

"Speed related" made me think. A commonality seems to be light or cross winds IE higher ground speeds/longer runs.

The cause, as suggested will probably be the tension, maybe aided and abetted by the tire.

antony

[johnu]

I had the same thing. turned out to be the tire. I had it changed and it went away.

[Northeast 458]

Just a thought about the nosewheel shimmy you report.
I have had this sometimes on my DA40D; reported it and had
the nosewheel adjusted as others report, all found within MM limits, too.
When it re-occurred a few months later, I checked the mainwheel tyre
pressures and found a small deflation on one side only (can't remember which).
I re-inflated both tyres to the same pressure, and the nosewheel shimmy was cured.
(Even with twin nosewheel gear and these tyre pressures checked, it's been known
to happen on 737-200's too)!

If no-one's tipped you off yet about tyre re-inflation, there's one small trick that
can help access the valve... even with the speed fairing in place. No power on the aircraft,
release park brake on hard and level ground, then push or pull aircraft until the valve
is visible in the access hatch; chock aircraft and set park brake.
Using a short length of nylon rope (1/4 inch or 5mm diameter will do), looped at one end
to form a hole about 3"/7.5cm long: push the rope loop up between speed fairing
and tyre wall to catch the stem of the valve; when ready to attach your tyre pump hose,
pull down on the rope end so the valve is easier to access in the middle of the access hole.
Repeat procedure for other side after re-inflation. Remember to ensure aircraft immobilised
after each re-positioning !!

Hope it helps

Northeast 458 (owner since 2004, DA40D)

[gmills]

I support the tire balance if all other settings are correct. Seen this on many Cessna's too.

[antony]

Damper tightened, tires checked / reset (down a couple of pounds, but it had been below freezing for the previous 8 hours or so). All now seems well - no problems on T/O or landing in calm or cross wind conditions.

Looking back, I suspect the problem had been creeping up on me - ground running seems so much more stable. Imagination perhaps?

Thanks again for all your support and suggestions; it did help knowing the problem wasn't an isolated one.

antony