Can You Repair An Aircraft Flap

But because flaps are a secondary flying control doesn't mean they can be ignored. Just a pocket-size bit out of rig and they can cost several knots of cruise speed or roll the plane plenty that the wing attach cams tin't overcome it. They too endure abuse from the ability pulses off the propeller wash, specially when extended. When extended, they transfer a huge leverage to the aft wing spar via the flap rails. All this stress is spring to present itself somewhere, and some of it shows upward as cracks in the flap trailing edge around the rivets. This is a well known problem, simply in that location are varying opinions about the timing of when it becomes an airworthiness issue requiring some maintenance action.

Major repair or reskinning of aircraft flying controls is a job not usually attempted by the generalist mechanic. There is good reason for this also; most flying controls accept twists and bends that more often than not require the control exist set in a jigging fixture during the repair. Few shops in the field have proper fixtures for this type of precision work. However, the typical flap on the strutted winged 100 series Cessnas is about equally square and direct every bit a surface can go. No special fixtures are needed to keep everything in line.

Back up Basis

Our subject airplane is a restart (produced subsequently 1996) 172 with multiple cracks in the left flap abaft edge. These were discovered during a routine annual inspection, and then it's up to the Authorized Inspector to make up one's mind if this impairment requires some action to be returned to service as airworthy. Every bit with whatever repair, the first finish for maintenance guidance is the applicable service transmission.

The 172R and 172S serial Maintenance Manual makes no mention of cracks in the flap skins nor does the Structural Repair Transmission that'south written for the restart 172 & 182 shipping. Since the airframes, specifically the flaps, are the same as the legacy aircraft, we tin look to their maintenance documentation for guidance. The legacy single engine Cessna never had a structural repair transmission, so we'll start looking through the applicative service manuals from most recent to older until we discover useable repair reference data.

The legacy maintenance manuals aren't set upward in the Air Send Association 100 Affiliate System (ATA 100), so we'll be looking in the maintenance transmission sections for the information. The 172 Series Service Manual 1977 thru 1987 "Section 18 Structural Repair" has no give-and-take of the flap skin cracks. The next three older maintenance manuals exercise accept specific guidance on the flap peel damage and repair, and they all say the same thing.

172 Skyhawk Series Maintenance Manual 1969 through 1976 has data in "Section xviii Structural Repair". The 1963 through 1968 100 Serial Maintenance Manual has help in "Section nineteen Structural Repair". The get-go reference to the flap event in the 1953 through 1962 100 Series Maintenance Transmission is in "Section vii Flap Control Organisation" paragraph 7-xvi Repair of Flap. This and then refers the reader to "Section nineteen Structural Repair" in the aforementioned manual.

According to all of these older legacy manuals, the primary limiting items that require flap peel replacement are the post-obit:

any pare crack longer than ii inches
a crack that does not originate from the trailing border or a trailing edge rivet
cracks in more than six abaft edge rivet locations

Based on these criteria, our flap needs some sort of repair to go on safely in service.

The Repair

As with any repair, we need some guidance for our scheme. I could splice a new section of skin at a rib to replace the area with the cracks. That would leave a nonstandard seam right in the middle of the flap. Structurally that would exist fine, only it would be very visible to anyone doing a casual walk effectually.

A splice also requires a more than involved log entry with references to the approved or accepted data used to design the repair. A skin replacement only requires a log entry stating a new skin was installed. It'south much cleaner looking, both on the plane and in the records. The best part is that a consummate flap skin replacement really takes less labor and the aforementioned parts cost to achieve.

There are several sources for the corrugated flap skins, and all have their good points and not and then adept points. Ordering the new skin from Cessna gives one a certain feeling that it will fit exactly as information technology should and the part volition arrive fully coated with an epoxy primer. Sourcing from Cessna will hateful you're going to pay a bit more than up front end, only in that location will exist less effort needed to become the part in proper shape for installation.

There is as well the case of the abaft edge reinforcement strip. The one on our cracked flap is in perfectly good condition. However, it is largely the crusade of the cracking trouble in the get-go place. The original reinforcement is a apartment aluminum strip with a round bead that helps stiffen the trailing edge of the flap. The problem is that the strip is apartment, and the upper and lower skins come together at an bending. The flat strip forces the skins to curve flat at every rivet pinching the skins, resulting in stress risers and eventual cracks.

The new trailing edge stiffener in identify on the lower skin with the old style stiffener merely to the right.

While the erstwhile style reinforcement strip is exposed, this is a perfect time to supplant it with a new extrusion that has the same wedge angle as the skins. At that place are several versions of this border available, but we selected the unit from McFarlane. It is a solid aluminum wedge machined so that the skins lay into an inset, making it very piece of cake to fit properly. At about $60.00, this is a very worthwhile upgrade. The extra rigidity is a big help when riveting to keep a nice direct line on the trailing border.

The repair starts with laying the flap down on a big flat surface where it can be secured top side up. You lot'll notice the flap lies flat all the way around, and that's the shape we want to maintain during the repair. The top peel and trailing edge reinforcement strip is drilled off using standard airframe maintenance practices. While the flap innards are exposed, it'southward a good time to make clean everything and prime the within for future corrosion protection.

The old skin tin be laid on top of the new skin every bit a hole drilling pattern. We often use some flat weights to hold the parts flat on our forest elevation table for this functioning. As the holes are drilled in the new skin, temporary fasteners called Clecos are installed to continue the parts in proper alignment. Nosotros too drill by the new pare into the wooden table elevation, so the Clecos really anchor everything to a stable base.

All cracked upwardly. The old pare laying on acme of the new skin after pattern drilling the rivet holes.

The new pare tin can now exist removed from the pattern and installed on the flap frame. We install Clecos through the new skin into every second hole in the flap spar. Standard solid rivets are installed in the open holes, then the Clecos are removed to rivet these holes.

The flap assembly tin now be slid to the edge of the table with only enough exposed to allow access for riveting the reinforcement strip. Keeping the minimum abaft edge exposed volition help keep a squeamish directly edge while riveting. A few former aircraft batteries can be set on tiptop of the flap to hold everything in position. Batteries offer a nice flat surface of non-scarring plastic that won't misshape the skins. Of course, aircraft batteries should exist used since using automotive units might give someone the incorrect impression about one's loyalties.

Some technicians would choose to set the trailing edge rivets with a squeezer. We have found the hand squeezer is much more difficult to control for minimal waving of the trailing edge. One tech can easily set all these rivets with a pneumatic rivet gun and bucking bar and should get a very straight trailing edge.

The abaft edge hanging out with Clecos installed and gear up for riveting

Don't Exist Scared

Cracked flap skins is i of those items we see ofttimes ignored due to a perception that the repair might be complicated or expensive. These cracks can be end drilled, and the assembly remain in service to a point. Once the line is crossed into the realm of unairworthiness, a repair must be made. This is ane case where an airframe repair by installing the consummate new part is affordable, in aviation terms, and the finished production is often meliorate than the original.

Co-ordinate to FAR 43 Appendix A paragraph A (b)(one)(2) & (xxii) this pare replacement is a major repair. Similarly, making any splice repair to the flap is a major repair per paragraphs (xxiii) & (xxiv). From a records history perspective, it makes adept sense that if yous're going to have permanent repair documents recorded with the FAA, they might equally well show yous fabricated useful improvements in the procedure.