Backup Instruments – Bumps In The Road

For the few hours spent at the hangar yesterday don’t feel there was much forward progress. It turned out to be one of those work sessions with an abundance of issues to resolve. The first bump in the road was discovering the vertical speed indicator (VSI) interfered with the muffin fan mounted onto the instrument panel shelf. The casting for the static port fitting hit the upper edge of the muffin fan as did the gear box for the adjusting screw. This was a self inflicted gotcha … in that, when making the layout for the backup instruments and taking measurements, I failed to take into account the fact the front panel is canted at a slight angle. This makes the back of the instruments lower than the front creating the interference issue.

The casting the red plug is seated in is where the static air line fitting needs to be attached. The casting and gear box on the back of the VSI instrument was hitting the top edge of the muffin fan’s body.

I looked at possibly taking a small notch out of the fan but quickly dismissed that idea because even if it created clearance for the static port casting, there would be no clearance for the larger fitting that needs to be screwed into the static port to connect the static air line onto the instrument. There is the option of gaining the necessary clearance by moving the fan forward a little … which is a possibility, but not something I want to do at the moment.  After quite a bit of brainstorming I wondered if the altimeter would fit into that location. Early on this idea was initially dismissed because the body of the altimeter is a little deeper than the VSI so I figured it would hit the fan. Then it occurred to me the body of the VSI is not hitting the fan, it’s the casting and gear box extending from the lower portion of the instrument that is hitting the fan.  Although the body of the altimeter is a little longer than the VSI, the static port casting is much higher and at the center line of the instrument … seemingly it clears the fan.

The VSI is on the left and the altimeter is on the right. One can see the static port on the VSI is towards the bottom of the instrument … where as the static port on the altimeter is higher and centered on the instrument. This extra height should allow the port to clear the side of the box fan.

There will not be room for a typical compression fitting so will likely need to use a low profile 90 degree fitting directly at the altimeter to clear the fan. Problem solved (I think).

During the previous work session, the mounting holes for the backup instruments were marked and center punched but fortunately not drilled. During the process of making measurements for flip-flopping the altimeter and VSI instruments, it became apparent the adjusting knob for the altimeter is not in the same position as the adjusting screw for the VSI (which is where a mounting screw would normally be). The adjusting knob’s shaft on the UMA altimeter IS NOT in the same location where a mounting screw would be … it is much closer to the edge of the 2 1/4" instrument hole and, as such, will require a notch cut out for the adjusting knob’s shaft … I almost did not catch that prior to drilling the holes.

Using the drill press to drill the mounting holes for the backup instruments in the right instrument panel to a final size of #27. All the holes were drilled except for the bottom left hole where the altimeter will be mounted … it will require a notch for the altimeter adjustment knob’s shaft.

All the #27 holes were drilled into the right instrument panel for the backup instruments. The making of the notch for the altimeter and associated photos will be in a subsequent post.

The next item that created quite a hassle was clamping the modified throttle mounting panel onto the bottom of the instrument panel base for drilling. The center panel section was temporarily attached so the modified throttle mounting bracket could be centered on the instrument panel base for drilling. This turned out to be yet another bump in the road in that I tried using every clamp in the shop and just did not have any with a throat long enough to get on the throttle bracket.  The big issue being the two vertical F-00054-L&R stack angles prevented the clamps from getting onto the throttle bracket. Sure, the center panel could have been removed … but I wanted it in place so the left to right positioning could be determined along with the throttle bracket’s forward and aft positioning so the throttle bracket will end up in about the same place it would have been had I not made a separate mounting bracket for it. Finally, an eloquent solution to the problem presented itself … sandwich the throttle bracket under the instrument panel. I used a long strip of 2" wide 1/6" thick aluminum to sandwich the throttle bracket onto the bottom of the instrument panel using clamps. This method worked quite well.

The modified throttle bracket is sandwiched under a long strip of 2" x 1/6" thick aluminum to hold it in position for drilling. The throttle bracket is in its final position and ready for drilling so #10 hardware can be used to attach it onto the instrument panel base… looking closely at the photo, one can see the piece of aluminum used to sandwich the bracket onto the bottom of the instrument panel base.