Bound mixture cable engine failure

Any DA40 related topics

Moderators: Rick, Lance Murray

Post Reply
User avatar
Roxi5m9
3 Diamonds Member
3 Diamonds Member
FIRST NAME: Nick
Aircraft: DA40
Registration: N597RA
Airport: 5M9

Posts: 71
Joined: Sun Jan 07, 2018 3:13 pm
Has thanked: 32 times
Been thanked: 3 times

Bound mixture cable engine failure

Post by Roxi5m9 »

Admittedly, that subject headline is a bit “click-baity” :D , but I had an experience that could have been extremely serious.

Last weekend after starting the motor on my DA40, when I pulled the mixture back for taxi it felt unusual, lots of resistance, almost like it was grinding. After pushing if forward and pulling it back again, it would not go forward again! If I wiggled it back more lean, it was bound up even more and would NOT go more rich again. After only a couple wiggles, the engine was too lean to run at 1000 rpms and started to shutter. I had to shut it down. Imagine if this happened in flight.

Upon investigation with an on field mechanic, we discovered the cable was bound at the joint in the cable where it is attached to a bracket on the front of the motor, where the cable transitions from being a curved flexible cable to a straight metal tube that bolts directly to the mixture arm on the fuel control unit. The mechanic adjusted the bracket my rotating it as far forward, or counter clockwise as he could, unbinding the cable. (Imagine looking as the left side of the engine)

Does anyone have any experience with this? The mixture cable is working great now, but it leaves me a bit nervous. I worked with the mechanic as he resolved this, but its perplexing to me that such a small adjustment in the cable curvature or position, (the bracket only rotated a quarter inch to the left) could be the final solution to this and I’m concerned if there is more wear going on inside that cable than is suspected.
Attachments
9CDA9A71-BC65-4121-99D1-5087704B3CA3.jpeg
N597RA 40.444
‘04 DA40-180
ATP, CFI
5M9, Marion, KY
User avatar
briankelly327
2 Diamonds Member
2 Diamonds Member
FIRST NAME: Brian
Aircraft: DA40
Registration: N633DS
Airport: KCDW

Posts: 22
Joined: Sun Aug 28, 2016 11:09 pm
Has thanked: 1 time
Been thanked: 2 times

Re: Bound mixture cable engine failure

Post by briankelly327 »

I have had a similar experience on many occasions, but only with hot starts.

When I land and shut down for 20 minutes to pick up a passenger or refuel, often times the mixture control freedom is very tight.

This behavior disappears by the time I get to the end of the runway for runup.

Mechanics were never able to recreate the problem, and Ive attributed it to uneven cooling / contracting of different engine parts.

Not sure if that is helpful, but also curious if others have similar experiences.
User avatar
Ian Sage
2 Diamonds Member
2 Diamonds Member
FIRST NAME: Ian
Aircraft: OTHER
Registration: NONE
Airport: KAPA

Posts: 31
Joined: Thu Dec 13, 2018 12:29 am
Has thanked: 30 times
Been thanked: 33 times

Re: Bound mixture cable engine failure

Post by Ian Sage »

I have encountered this exact symptom with a similar solution on one DA-40 in the past. Fortunately it was discovered promptly after startup. It took a fair amount of trial and error tracing down the root cause. In the end the bracket in your picture was partially rotated to improve alignment with the articulating solid "strait metal tube" portion of the cable as you describe above. This made an improvement but the cable was still binding.

Further inspection of the cable using a strait edge reveled that "the strait metal tube" was no longer strait. It was still strait enough to appear strait to the naked eye but when held up to a known strait edge the curve was obvious. The total angle was very slight and it was not at a single bend, rather a gradual curve. Several cautious applications of force along the back of the newly discovered curve measuring between each application straitened the piece and the mixture cable operated glassy smooth once again.

This was discovered after one of our supercharger installations, and after several issue free full power runs. The installation does not require any interaction with the mixture cable or the bracket (other than verifying that they move freely without interference with the new parts) but work is done in the vicinity of those parts. My best guess is that the bracket was not bolted 100% firmly in place before arriving at our shop and it was inadvertently bumped during the supercharger installation. During the fifth or sixth ground run the tube portion and bracket bound up and either at that moment or as a byproduct of the accumulated attempts to moving the cable while diagnosing the issue the tube portion bent ever so slightly. We had not encountered that issue before and have not since, but it is the most likely series of events that I can come up with as a working theory.

Until now I had never heard of another person encountering this specific issue and diagnosis. It has become one of my "while i'm in here i'll check xxx and yyy" items ever since we encountered it. It would be informative to hear if any other mechanics or owners here have encountered the same. It's always interesting to hear what people add to their unofficial checklists based on their personal experience.
User avatar
Steve
5 Diamonds Member
5 Diamonds Member
FIRST NAME: Steve
Aircraft: DA40
Registration: N432SC
Airport: 1T7

Posts: 1160
Joined: Thu Aug 12, 2010 1:23 am
Has thanked: 26 times
Been thanked: 175 times

Re: Bound mixture cable engine failure

Post by Steve »

Nick:

I am very familiar with this assembly (having had to remove and install it for my recent engine work). Looking at your photo, I noticed that the safety wire is missing from the bracket bolt. Did the bolt loosen and allow the bracket to rotate out of position? Also, the degree of "cocking" in the cable sleeve on yours does not seem any more than mine, which has always worked smoothly:
DSCN6021.jpg
Steve
User avatar
Roxi5m9
3 Diamonds Member
3 Diamonds Member
FIRST NAME: Nick
Aircraft: DA40
Registration: N597RA
Airport: 5M9

Posts: 71
Joined: Sun Jan 07, 2018 3:13 pm
Has thanked: 32 times
Been thanked: 3 times

Re: Bound mixture cable engine failure

Post by Roxi5m9 »

Steve wrote:
Tue Mar 31, 2020 8:49 pm
Nick:

I am very familiar with this assembly (having had to remove and install it for my recent engine work). Looking at your photo, I noticed that the safety wire is missing from the bracket bolt. Did the bolt loosen and allow the bracket to rotate out of position? Also, the degree of "cocking" in the cable sleeve on yours does not seem any more than mine, which has always worked smoothly:

DSCN6021.jpg

Steve
Hey Steve, this photo was taken after the mechanic rotated the bracket to as far of a counterclockwise position as it would go (relieving as much cocking of the cable as possible), but before he safety wired the bolt. In fact, he not only safety wired the bolt like yours, but he also safety wired the bracket via the hole in it to the large engine bolt to the left of it (the same bolt that the bracket is wired to).


It’s interesting this topic hasn’t come up before on this site. Seems to be happening to enough other people and could be a critical mechanical issue in the right (wrong) scenario.
N597RA 40.444
‘04 DA40-180
ATP, CFI
5M9, Marion, KY
Post Reply