Well done in what had to be a pretty scary situation. Kudos to you sir, and thank you for posting your story here for all of us to learn.

This post refers to Piper Navajo aircraft, it should be noted that the Beech aircraft too use a flap transmission, drive cables and actuators. They have been very durable components on Beech aircraft though Beech does put these components on a replacement schedule few adhere to. Failures are rare but a split flap condition can occur on Beech airplanes as well.

My cousin had a split flap years ago in a Maul. His only out was to land on the side of a hill with one wing much lower than the other. He is a good enough pilot that he pulled it off and there was no damage to the plane. As the plane slowed down he had to turn the nose toward the top of the hill to keep the downwind wing from digging into the ground and causing a ground loop.

Some questions for the knowledgeable:

Is this a risk on all Navajos or just early year models?
How much does restricting flap travel to 25 degrees affect performance?
Also, did I understand correctly that there are not flap pre-select indents-you have to hold the flap handle until you get to the desired setting?

I am sure this will open up a lively debate...

Based on my research today, the flap transmissions are different on the serial numbers made 1978 and before (roughly...you can see serial numbers affected in the SB). The flap system was redesigned (system and transmission) in later serial numbers. Therefore I dont see this as a risk in the later models.

Restricting to 25 degrees, not much. A bit slower and more drag on landing. I go in to a 3600' strip regularily and have landed with 40, 25, 20 and 15. Could even do a no flapper there. 40 is somewhat "easier", but not that big of deal.

The early models did not have a preselector. The switch was in "down", until you moved it back to "off". Later serial numbers had a detent at 15. You could select 15-40 in the white arc, but no detents. There are approximate indicators on the shroud next to the flap preselector. There is also a gauge that shows flap angle.

A split flap situation can also happen in the Bonanzas and there is nothing that can be done about it if it is the right flap that extends. The cause for that condition is a disconnected left flap, usually caused by a sheared left flap drive shaft. Since the flap limit switches are on the left flap and that flap is still in the full up position the up limit switch is open, blocking any power to the flap motor to retract the extended right flap.

One solution to the problem is installation of a second up limit switch on the right flap that is in parallel with the normal left up limit switch. The timing of the second switch needs to be such that it opens and will shut the motor off slightly before the left switch opens. That allows the system to operate normally when there are no broken parts of the left flap system but would allow the retraction of the right flap in a split flap condition.

Regards, Lew Gage

That sounds like a very bad situation. Does the Bonanza have sufficient aileron authority to overcome the split flap? I think Baron flaps are the same too?

That sounds like a very bad situation. Does the Bonanza have sufficient aileron authority to overcome the split flap? I think Baron flaps are the same too?

The pilot that was flying for me full time, up until a few months ago, had a full asymmetrical flap condition on a Cessna 310. This was about 3 weeks ago. It is a Robertson STOL equipped T310R. It wasn't a plane we were involved with, and I don't know the failure mode. Right flaps fully down, left all up. Said it took full power opposite engine, full aileron, rudder. But he managed it to the runway, no damage except the interior.

The AD was in response to the gears in the transmission failing (the far part in your picture). The part that failed was the near part (just a sliver of which is in your picture). What I am urging you to consider is that if the jack screw binds in the "threaded" tube (it is actually ball bearings in the interface) and snaps, or if it get a fatigue crack and snaps, or if there is enough wear or a crack in either the shaft or more likely the the tube so the ball bearings can fall out...You will potentially have a full split flap situation on your hands from which you will be unable to recover until you get the other flap back up. The C310R that was able to land with full split flaps that someone referenced has been deemed impossible by test pilots and now myself in the Navajo.

I'm not saying the transmission is likely to fail. I'm just letting you know that if it does fail at greater than 25 degrees you will lose control. The transmissions have been "upgraded" per the AD on the earlier models (my airplane included) from a 20:1 gearing to a 40:1, as well as having a 100 hr inspection requirement. Mine still failed so apparently it can happen. I know my mechanic was doing a proper inspection because he failed my left flap transmission and it was replaced with an overhauled one on the last annual 50 hours ago. The right one passed the inspection per the AD and service manual. However, nowhere does it tell you to check the threaded interface. I think the only way to inspect that would be to remove the actuator and put axial and bending loads on the shaft to see how much play is in the threaded connection. If there is play, then there is wear and the ball bearings may not be secure. (This is just me thinking how I am going to check for myself in the future, I am not an expert and have not had any input yet from the manufacturer or otherwise.)

Again, my advice is limit flap use to 25 degrees as standard practice. If landing distances are negligible enough to not even require updating the POH performance charts, then why chance it?

I will notify the FAA.

Chris Brown,

Thanks for the info. and glad you are looking into it.

I'd be interested if you can confirm that the '79 and later Calco transmissions have the same ball bearing interface between the jack shaft and the tube that connects to the flap. I found a picture of the Calco transmission online, but cannot tell for sure. It looks almost identical to the Dukes that the earlier models have, I wonder how else they differ? They both appear to be the same 40:1 gear ratio from what I can find.

Also Piper SB 739 is more pertinent to this discussion than 494B. The loss of control is specifically addressed there. It still only references pre '79 models, but that is simply because those were originally the 20:1 transmissions. The performance will be the same for all models. And other failure modes while apparently less likely, are still possible.

Maybe it has to do with speed and arm from center, but (in an earlier life) I worked a Cessna AgTruck for a few weeks with one flap, while waiting for a flap pushrod on order. In the AgTruck it was no problem at all.
( no work=no pay...)

We used full flaps to get off the runway as well as on landing.

It has to do with the size and position of the flap compared to the size and position of the aileron.

Put the flaps to 40 degrees and put the aileron all the way up and then go stand behind the wing of a navajo and you'll understand why that's not going to work out.

Chris...good question, I dont know. I know the transmissions were made by 2 different companies (Calco and Dukes) but unsure if the design is different. We tried to look up the exploded view of the transmission on ATP, but it does not show it. There are shops that overhaul these transmissions, but they don't give out the exploded info either.

I have not looked at SB739 (need to have the shop pull it up on ATP). The way I read SB 494B is about testing the transmission, not about the motor or flexible shafts. Will have to pull it to see.

But...The planes with the "new" transmission (whatever the change was) do not have a SB or AD and dont need any more inspection than what is in the maintenance manual. Which I have not looked up the procedure on. That leads me to believe that they are a different design. But we need to find out to be sure.

The one other part about loss of control accidents is when the flexible shafts coming from the motor break and drive one transmission and not the other. From my research, this seemed to be the common problem back in the "day". The newer system with the amplifier eliminated the ability of the motor to run if the position of the flaps did not agree. Which would still be valuable in case of many types of failures. Just not the one you experienced. So that still begs the question if the failure you had can happen on the "new" transmission.

Yeah...this. I was going to take a picture when I was at the hangar tonight, but forgot. Will try tomorrow.