When is a hole not a drain?

[PA1195]

Here's some pics from a Pre-War Taylorcraft right main landing gear. It failed last winter locally when slow taxiing on skis. Previously no unusual bumps or bangs were noted by the pilot (not me). It appears the MLG diagonal tie strut first separated mid-span followed by outward gear deflection until it reached the limit of the bungee safety cable. Further bending occurred which partially bent and broke the two upper "A" frame tubing. The wing tip never hit and with help the pilot was able to lift the wing and tie the right and left gear together with rope.

Below are pics of Taylorcraft SB 78-001 which recommends inspecting a previously drilled drain hole in the lower MLG strut. If no hole is present then one can be drilled in either of two locations.

However for this gear there was a drilled hole in the location specified in the SB but it went into solid steel and never breached the hollow portion of the strut. Also note the internal corrosion in the strut at the break, as well as at one at the base of one of the "A" tubes. The other is also corroded.

I determined the depth of the hollow strut with a stiff wire then noted that location plus the drilled hole that matched Taylorcraft's specs. Obviously sticking a wire into the hole to inspect for corrosion is useless. I cleaned the hole with brake cleaner and a probe to confirm the blind cavity. Drilling a hole 1/2" further from the gear bolt would have hit the cavity, and so would making one spec'd as the alternative in the SB.

Edit: Note the SB 78-001 was published while the Ferris' were in business. Their gear and maybe others were built to allow a drain from the spec'd hole. Confirming a drain exists by inserting a long wire, applying air pressure, or spraying oil through a small extension might be a good idea.

Gary

[Scott]

Re: When is a hole not a drain?

Interesting Gary
1/2" from the C/L of the fitting is clearly not going to get you into the tube.

I wonder how many more drain holes like this are giving people a false sense of security. I agree confirming the hole actually goes into the strut is critical.

Just for clarity, the SB and the CE both specify only the trailing edge location for the hole if one needs to be drilled. You can't miss in that location!
S

[PA1195]

Re: When is a hole not a drain?

The strut's internal dimensions may vary by time of manufacture and Ferris folks just happen to pick some that passed through ok.

It's not unreasonable to do a one time inspection of both gear legs without an AD being issued. Probe the drain hole to confirm passage to the interior of the strut. Note any corrosion dislodged. If airworthy apply tubing oil or an approved anti-corrosion spray. Make a log entry referencing this non-mandatory SAIB below.

Gary

[PA1195]

Re: When is a hole not a drain?

As a note.....the partial tie strut segment in the pics above is slightly bent longitudinally. Like curved when a straight edge applied over the length indicates a curve away from the fuselage. It may have experienced a side load or compression event(s) by person or persons unknown that led to later failure when weakened under normal tension.

In a compression load the tubing element stretches the build material on the out-curved convex side and compresses the same lateral material on the compressed or concave side. Either or both may cause a delayed failure after repeated events. Take a piece of wire and bend it until it fails as proof of this concept.

For example: http://web.aeromech.usyd.edu.au/AERO...be_failure.pdf

Gary

[N96337]

Re: When is a hole not a drain?

I know my gear legs sounded like one of those Australian sticks with all the beads in them, when I took them off the airplane. I figured on building a jig and changing to at least 1 1/4" axles when I rebuild them. (since Dave Rude doesn't have his jig anymore.)
John

Go with the 1-1/2" axles, more options and they fit right over the 1-1/4"

[PA1195]

Re: When is a hole not a drain?

The owner of the broke gear above said today he could blow air into the left side undamaged gear's tie strut drain hole and it came out near the bungee sleeve at the top of the strut. Same for oil he sprayed into the drain hole. Hole was where the factory SB said it should be on the strap. Bad gear had a blind hole in solid metal.

I guess the right gear above that broke and had no drain got rust and separated. Being open at the top lets in air and moisture eventually.

Another source of corrosion is that big hole at the top of the outboard axle flange the brake stuff fastens to. On the gear above the lower gear legs that weld there with the lower strut tab were rusty inside looking through that hole (~5/16""). It might be a torque stop hole for the brake backing plate but not sure.

Get 'em looked at and tuned up.

Gary