Inspection of Flap Hinges, SB M20-186 and M20-186A

While complying with AD 98-24-11, aileron links, I have had several Mooney’s thru the shop with severe outboard flap hinge corrosion. This corrosion I addressed by two Mooney Service Bulletins, M20-186 and M20-186a.

SB M20-186

This service bulletin was issued on 12-5-72, and affects all Mooney M20C, D, E, F, G. Aircraft. The Service Bulletin was introduced due to reports of corrosion at the outboard flap hinges on aircraft operated around sea coast area’s. Originally this corrosion was easily over looked due to the hinge fairing covering a major portion of the hinge. Today these hinges are more difficult to inspect due to the new style of full coverage flap hinge fairing.

Figure 1

Figure 2

The flap hinge in (figure 1) is representative of three aircraft that have been through our shop in recent months. As you can see the hinge appears to be laminated and swollen against the fairing. A classic example of inter-granular corrosion. (Figure 2) is a comparison of the removed hinge and a new one. The corroded hinge has exploded to over twice its original width.

Figure 3

Figure 4

To inspect your aircraft for this corrosion, lower the flaps, and inspect all exposed portions of hinges on the flap and wing for evidence of a blistered or scaly appearance. (Figure 3) Using a thin blade knife or equivalent, probe between the flap hinge and flap hinge fairing wherever possible. If the newer style full coverage hinge covers are installed , you will need to remove these. If hidden corrosion is present a powdery or flaky residue will appear. (Figure 4) Removal of the flap and replacement of moderate to extensive corroded parts is required.

SB M20-186A

In January ,1975, 186A was issued affecting all M20C’s SN 2923 thru 20-1149, M20D SN 253 and on, M20E SN 587 thru 21-1163, M20F all thru 22-1208, M20G all.

In addition to aircraft operating in sea coast area’s, it was discovered that the flap up stop bolt at wing station 147.25, (figure 5) located at the outboard flap hinge, is a modified AN3C bolt which is a stainless steel bolt. Under certain environmental conditions, this can cause an undesirable galvanic couple with the aluminum flap hinge and can increase the probability of corrosion.

Each aircraft owner should make a personal inspection of their aircraft’s flap hinge’s for this corrosion. The SB refers to aircraft from coastal areas, but the aircraft I refer to in these photo’s were not from coastal areas and had been well maintained otherwise. I feel that these are not isolated cases and several more will be found. If corrosion is found you may want to obtain copies of these SB’s, from Mooney, through their web page, for your mechanic. The SB calls for the removal of the two AN3C up stop bolts and replace them with NAS 428-3-7, whether corrosion is found or not. None of the aircraft that I have seen with this corrosion had the stainless bolts installed.

Don Maxwell

MAPA Log, Volume 24, Number 4 (April 2001)