'52 & earlier cowl latch secondary safety devices

[Bruce Fenstermacher]

This is an effort to add and or link to threads discussing what I consider to be an extremely important secondary safety device for '48 through '52 non-pressure type engine cowl latches. If you have one of these 170s you need to insure that the cowl latches have some sort of secondary device that will prevent them from popping open in flight. Even if the latch arms and retainer clips where new I would still not trust them. Many many of these cowls have some type of device, but they didn't come from the factory with them and there are examples out there without any still today. There were at least three aircraft at convention this year with no safeties. And the owners are not aware of the issues old timers have known about for years. An accident waiting to happen.

I personally think that having a secondary device on at least the front two cowl latches (all four is better) is as important as any other safety wire or device on the airplane. My own current 170 has a replacement cowl after the latches opened in flight and the aircraft was force landed, ground looping and ripping the landing gear from the fuselage. All for a cowl latch.

There are basically two methods of safety devises seen on the line. A pin that is held in front of the lower part of the latch which does not allow it to extend. And a pin, screw or Dzus type fastener which holds the bottom latch securely in place. A third method is to replace the entire latch with the Dip Davis latch mod which is STC'd.

Here are two common methods of installing a safety pin on front of the bottom latch. The difference mainly being what holds the pin. One method I like is using AN502-10-8 Phillister head screws that replace two of the rivets that hold the latch to the cowl. The screw heads are drilled but the hole must be opened up to accept a pin.

Phillister screw latch safety.jpg

The screws are 10/32 and slightly larger in diameter than the stock rivet hole. I use a 10/32 tap to tap out the hole without drilling it out and insert the screw. Though it would not be necessary for the purpose of the safety I then install a AN363-1032 nut on the back because this screw/nut is replacing a rivet. Because the hole in the Phillister screw would place it to close to the skin of the cowl I also spaced the screws out with some aluminum washers I had. I'd use stainless if I didn't have the aluminum washers.

The factory spacing of these rivets is just a bit far away to use a AN 415 or AN416 pin in my opinion and so I make my own pins out of .060 stainless TIG welding wire. (make extras). My first cowl did have AN502-416 Phillister screws with have 1/4-28 thread. They did not replace original rivets and were drilled a bit closer together. The hole had to be tapped as a nut could not be placed on the back because the holes where to close to the latch for a nut. I was able to use AN-415 pin with this setup.

A very popular method to make a pin holder is to fashion a simple bracket out of .040 aluminum. The bracket must fit precisely between the the outer latch shell and the latch. The bracket is then riveted to the shell and an AN-415 or 416 pin is used.

Photo on 9-2-13 at 1.02 PM #2.jpg

In this example the holes for the pin are wallowed out which happens over years.

Photo on 9-2-13 at 1.02 PM.jpg

Photo on 9-2-13 at 1.01 PM.jpg

Other methods I've seen I will try to get pictures of involve a screw through the latch and into a nut place attached to the back of the latch shell. This method was somewhat tedious requiring a screw driver to remove. Two variants of that would be to drill the same holes opposite each other in the latch arm and the latch shell. Using a 10/32 screw about 3/4 in long through the shell and locked in place with a nut. The latch arm hole will need to be elongated to swing over the screw and a pin in then inserted into a hole drilled in the screw end. Experimentation is required to get the best lenght of the screw. A variation of this screw method would be to use a AN392 clevis pin the correct length instead of the screw and nut. The pin would be placed into the latch shell hole and held in place with a tight o-ring or a small cotter pin in a hole drilled into the pin. This pin would not be tight and could then be lined up in the latch arm hole and a AN415 or 416 pin used through it.

Here is a link to a thread showing Cam Locks installed: http://www.cessna170.org/forums/viewtop ... 24&t=10799
Here is a link to a thread showing Dzus fastners used: http://www.cessna170.org/forums/viewtop ... cowl+latch
Dip Davis latch replacement STC info: http://www.cowllatchstc.com/information.html

What ever you use, have at least the front latches safetied by some method.

[GAHorn]

And here is a pic of a completed Dip Davis system, which is what I'd have if I owned an earlier airplane:

Dip Davis.jpg

They can be purchased at: http://www.cowllatchstc.com/information.html