Yesterday when making an approach into my home airport in MVFR conditions I experienced a catastrophic flap transmission failure on my Piper Navajo. I thought I would post about it here because I know there are people on here with Navajos among others with similar systems who might learn something from my experience. It was by far the scariest thing to happen to me since I've been flying.

I had just reached the IAF altitude (2000' AGL) a few miles from turning onto the final approach course. I leveled off and bled off some speed (low power setting) so I could get the approach flaps in. I was right at the base of the overcast layer and was in and out of the clouds, but could see the ground. Autopilot was engaged. Right about the time I should have started about a 30 degree turn to the left to be on the final approach course my airplane began a sharp bank to the right. Within about 2 seconds it was passing through roughly 45 degrees of bank. By that time I had instinctively disengaged autopilot and put in full left aileron simultaneously. I didn't have long at all to process what had caused the drastic roll because it was so sudden. I thought maybe I was in a microburst, but after 2 seconds of full left aileron failing to stop the roll to the right I dismissed that. As I passed through about 45 degrees of bank to closer to 60 degrees, I threw in full power on the right engine and added in some left rudder. This arrested the roll, but I could not get it back at all towards wings level. Then my attention turned to the flaps. I brought the switch to up and as I did I realized that the indicator was at the full flap mark. As the left flap began coming up I glanced out the right window to see the right flap completely retracted.

The whole incident probably only lasted 7 seconds or so, but it seemed like forever to not have any control over the airplane. During that time I turned about 90 degrees to the right. I didn't lose much altitude because I was still fast and I added in power on the right engine to try to avoid a stall/spin scenario. Luckily I could see the ground so I didn't need to cross check instruments to make sure that I was really in a sharp bank before taking action.

I told the controller what happened and told him we'd be back on the approach momentarily and would be landing no flaps, but did not need any assistance. The landing was relatively benign other than ballooning in the flare a little because of the gusty winds and no flaps. When I got parked I tried to move the flaps and the right one just shuddered and made a grinding noise. When I got out and grabbed the right flap with my hand it fell from almost fully retracted to fully deployed. It can be moved by hand.

In retrospect I realized a couple things. What I believe happened is that the flaps began coming down together. Then right before I neutralized the flap switch (on Navajos the flaps are either moving or not, there is no detents so you have to physically lift the switch back to neutral to stop the flaps at the 15 degree approach setting)(I leave my hand on the switch while the approach flaps are coming down) the right transmission let loose which allowed the right flap to retract and causing the plane to roll instantly and drastically. I then grabbed the yoke with both hands because full left aileron was tough with one arm and I wasn't sure if I was fighting the autopilot going haywire or what was going on. When I took my right hand off the switch, apparently I hadn't got it back to neutral yet. So the left flap continued to full 40 degree deployment.

For those familiar with Navajos, I can tell you that the test pilots were correct when they say you cannot control the airplane with all 40 degrees of asymmetric flaps. Even with other inputs like asymmetric thrust and rudder I could only arrest the roll at between 45 and 60 degrees of bank, and it was still creeping to the bad side. Supposedly at 25 degrees you have enough aileron to be able to keep wings level, which is why some of the Navajos have retrofitted the stops to 25 degrees of full flaps. My airplane used a different compliance method that reduced the likelihood of both failure of the transmissions and asymmetric flap deployment but kept the full 40 degree range operational.

So here's the part that might help others that I never considered. The POH says to deploy the flaps in increments so that if there is a problem you can recognize it and move the flaps back into alignment. The problem is that if the flap transmission fails in the manner that I experienced, this might not matter. If it catastrophically fails when you are putting in the last 5 degrees on short final, apparently one of the flaps could come slamming back up and putting you instantly into a drastic roll very close the ground.

So I will be modifying my airplane with the 25 degree mechanical stops now, as well as adopting new personal operating procedures that include both incremental flap deployment (which you are already supposed to do per the amended POH, but easier said than done when you are busy on an instrument approach) as well as getting my last notch of flaps in while I still have enough altitude (time to get the flaps retracted and powered up) if one of my transmissions catastrophically fails.

I can follow up once I get into the repair with more details on exactly what failed and any potential causes.

Chris - outstanding airmanship handling what clearly was a very challenging situation. Glad you were just under the overcast and in and out of the clouds with ground reference when this occurred rather than hard IFR - you had your hands full (literally) and didn't need any more challenge thrown at you. The 28 degree flap down mod is the way to go for sure. Hat's off to you!

Great job!

This is a gotcha scenario at Simcom. Some will assume engine failure and make the problem much worse.

The Navajo has so much wing, I almost never need full flaps and I sure don’t want them out if I lose one on approach.

Great reminder. Glad you could see the ground! Thanks for posting.

Glad you recognized it early.
Here is a case where it went wrong.
https://aviation-safety.net/wikibase/wiki.php?id=37960

Wasn't there an AD or something back around 1979-80 that restricted use of full flaps on Navajos...? May have been temporary. I remember a Navajo guy complaining about some such back then.

Yes there was/is an AD. The initial AD called for mechanical stops limiting the flaps to 25 degrees. There was alternate means of compliance that allowed the full use of flaps to remain at 40 degrees which is what my airplane had done to comply. This involved changing the transmission to a different design and gear ratio. However, knowing what I know now, I don't think it is a good idea to ever have the flaps at full (40 degrees).

You should submit a service difficulty report on this.

Thank you for sharing! This is one of those scenarios that I saw for the first time when I started going to Simcom for recurrent training. It's one example of why simulator training is worth the money. Certain failures you can't simulate in a real airplane. I have a friend who actually crashed in the sim from a split flap. I now think about it every time I lower my flaps. It's no joke and you did an amazing job recovering from it!!

Great save!

That's a really scary situation. I can totally see being baffled at first in a gusty environment.
Glad you made it!

Chris,

Really nice job handling the emergency!! Glad you're making the AD change too so it won't happen again, to that extent.

PS I needed to change my diaper after reading this.....

Well done Chris!

In the '79 -325 I fly, it has the Calco flap system that has 3 potentiometers and a flap amplifier (think really dumb computer) to prevent this. If the 3 pots do not agree with each other, the amp stops any further flap travel. Fixed the problem that can happen in your plane, but could leave you with flaps 15 unable to retract (takeoff), or no landing flaps at short strip.

We had a pot go bad last year and left flaps out, unable to retract. While there is an "emergency" on ground way to retract flaps (have to jump 2 terminals on the amp), I didn't want to get to my remote destination and not have takeoff flaps. Short strip....

In the model I fly with the Calco system, it is flaps 40 at 200' on every landing, except when really gusty. Then I leave them at 15 (first detent) or somewhere around 20.

Chris,

Just be aware that if your transmission fails the way that mine did on short final (the interface between the threaded shaft and the cylinder) you will almost certainly crash into the ground.

It does not matter that the the system will not let you deploy the flaps in an asymmetric manner. Both of your flaps will be at 40 degrees close to the ground because they can deploy together. Then if this threaded interface fails on one wing (lets say at 200' when reaching the end of its travel in your case) that flap will then retract instantly and you CANNOT arrest the roll until you get the other flap up. You will be at 45 degrees of bank before you can even realized what happened. You will very likely not have enough time at 200' at final approach speeds.

I'll also add that the inspections required by the AD do nothing to address the failure method that I experienced as far as I have found.

The only way to ensure you don't crash into the ground in this type of failure is to limit the flap travel. I am surprised that the FAA allowed the flap travel to remain at 40 degrees after originally deeming the fix to be to limit the travel. Like I said I won't ever use more than 25 degrees again after this experience. I've done significant research into the history of the AD over the last 24 hours. I'd be happy to elaborate more if anyone is interested.

The way I understand my system is there is a transmission on each flap. The drive motor with flexible shafts runs out to each transmission.

So, the way I understand it, one of the transmissions has to fail to have to have a flap "snap back up". You are saying the flexible cable failed or one transmission failed?

(edit)
I just re-read Chris' original post and am now seriously concerned.

With the Calco system, if a transmission fails like Chris' did, depending on if the pot is still functioning, it may be IMPOSSIBLE to retract the flaps. In other words, the failed transmission flap retracts on its own, and the system may not let you retract if it detected a split. Ugghhhh!

Wow, yes this would be even worse not being able to bring the other flap up. It would be fatal no matter what altitude it occurred if flaps were already passed 25 degrees when it failed.

BTW, my transmission passed the AD inspection only 50 hours ago.

My advice is to mark your indicator at 25 degrees and never deploy passed that at a minimum.

I don't know for sure about the Calco transmissions, but on the Dukes that I have the interface between the threaded rod and the cylinder that attaches to the flap is similar to the one in the picture I attached. In my case somehow all the ball bearings were able to work their way out and so the cylinder can just slide on the threaded rod. It seems like this interface would be difficult to inspect because it's completely enclosed. Even now, there is nothing obvious externally or signs that anything is worn or out of place.

OK, did a bunch of research with the Maintenance crew today and found the following information useful.

1. Airworthiness Directive 82-27-13 R2 only applies to the earlier serial numbers
https://goo.gl/ZLYiK1
2. SB 494B only applies to the earlier serial numbers

It appears the transmission was changed (along with other parts of the flap system) from the early serial numbers to the later serial numbers. The SB and AD instructions to check for free play with a vise grip, do not apply to the later SN transmissions.

Chris...are you going to notify the FAA on this one? I know the words "AD" can be considered dirty words, but perhaps someone (Piper or FAA) needs to examine these earlier transmissions to see why it failed.

At this time I am not concerned about the later transmissions. Unless someone can tell me a reason otherwise... Below is a limited shot of the left flap jackshaft (can barely see the transmission itself) on a later SN plane.