10 February 2004
I've been having some ongoing bad experiences with the S-Tec 30 autopilot I installed in my Tiger, problems that I suggest may seriously threaten safety of flight.
By the Summer of 2001 I had been doing a lot of business-related flying, plus some long-distance traveling. The Tiger had come equipped with a Century 1 autopilot, but that's a basic wing-leveler with the ability to track nav signals. It did not have any altitude hold, plus it was getting long in the tooth and was having trouble holding headings; it was suffering from the dreaded "wing-wag".
I did a lot of research on available autopilots for the Tiger, but it appeared there was only one viable candidate for my budget: the S-Tec series. I spent a lot of time reviewing their product line, and reviewing my wallet; after saving up the money I bit the bullet and decided to go for the STec 30. The -20 was effectively a modern replacement for the Century 1, offering a wing leveler and the ability to track nav signals, but the -30 added an altitude hold function. S-Tec also offered a DG with heading bug feature, but I did not have the money for that. While it has some really neat features, it's not cheap; out the door I spent $8988.00 for the S-Tec 30 without the heading bug.
During my acceptance flights I noticed a serious altitude hold problem: every time I'd key the transmit on my radios, the autopilot would pitch down hard. This baffled my installer, and they surprised S-Tec with the issue. Note that this is an autopilot that was STC-approved for the Tiger how long ago? Anyway, after researching the problem (and finding out that the New Tiger factory was experiencing the same thing), S-Tec replaced the pressure transducer (under warranty) and it seemed to fix that pitch problem.
For about a year, the autopilot worked flawlessly; I was very happy with it. I flew that autopilot for many hours in many different kinds of weather. I found it especially helpful during IFR approaches to minimums, where I'd set the altitude hold right at the minimum and look for the airport. I enjoyed the autopilot so much that I had it upgraded with GPSS and the heading bug; that nice upgrade set me back another $3900 between equipment, installation, and upgrades to my UPS GPS/comm. So, I was into an autopilot for nearly $13K, but I was still happy with it.
However, about a year-and-a-half into our relationship, I noticed that the altitude would start to wander, oscillating up and down slightly. At first it was only noticeable in very smooth air; it would be a slight porpoising plus and minus 25 or so feet. As time went on, the deviations began to increase; by the Summer of 2003 the autopilot was going up and down 100 or so fpm and plus/minus 100 feet. It made my wife ill and me annoyed.
Bringing it to the shop's attention in August of 2003, I was told it was probably an issue with the computer. The avionics shop removed the computer and sent it back to S-Tec. S-Tec found that there was a problem with a "repair of a shorted connector internal to the computer." Simply put, there was a manufacturing defect internally to the pitch computer, and S-Tec fixed it. Good enough, I thought, until I was hit with a $850 bill!!! I was told by the shop that the autopilot was out of warranty, and despite their efforts S-Tec would not offer goodwill on the defect. The repair cost was $400 to S-Tec plus labor to the shop to fix a radio ground repair). So, now I was into this autopilot for nearly $13,500.
To make matters worse, the autopilot wasn't fixed! Not only were the altitude deviations not gone, they were increasing! It got so bad that by Christmas 2003 the autopilot was a bucking bronco: plus and minus 500 feet per minute, deviating plus and minus 250 feet, with airspeed going plus and minus 30 knots, and the trim warning lights screaming at me at each apex! I tried to use it once for an IFR approach and when I set the altitude hold at minimums I was so busy fighting the altitude deviations that I busted minimums. The problem had gotten so bad that it scared a fellow pilot buddy; I disabled the autopilot and contacted the avionics shop. The avionics guy asked me to have my IA remove the pitch servo and computer and he'd send them to S-Tec for review.
Much to my surprise, I got the following email a week or so later:
Not so good news on this autopilot - the Pitch Servo and Pitch Computer are at STEC and they confirmed the trouble - the servo is TRASHED... excessive water intrusion. This is clearly evident just by eye-balling the screws on the case that are rusted, as is the connector. STEC says the motor looks like it has been submerged in water for months and asked if this airplane crashed in the ocean...I spoke with [your IA] and he agrees that it indeed gets wet back there but not sure what to do, as there isn't much protection from the elements by virtue of design. A cover might help, but tough to say...STEC says it needs a new motor and accessories and this will cost every bit of $700 far as I can tell....I told STEC to wait for me before they did anything.
As you can well imagine, my patience was more than worn thin on S-Tec by now. I wrote a scathing letter to the avionic shop, expressing my displeasure:
I've dumped far too much money into this autopilot over the last short amount of years, including...repairs to it last August, to stand idly by with an open wallet. I was, and am, expecting an autopilot to last more than two years, given the thousands I spent on it...
S-Tec received FAA approval on this system as installed, and I expect them to stand by that design or correct it to my satisfaction. I cannot, and will not, be held financially responsible for an autopilot that was installed as designed and used in a proper and expected manner...I expect this issue to be resolved by S-Tec...at no further expense to me, including reimbursement for the labor used by [my IA] to remove the pitch components...
At this point I also began querying the Grumman owners' mailing list to determine how widespread this problem might be. I was amazed at the response. I have not yet tallied what I know, but there are numerous owners out there with problems in regards to S-Tec altitude hold autopilots. Some are minor, such as the deviations I experienced at first, and some are major such as those I had in the end. While not scientific, it sure that a lot of the installations out there are troublesome. I was even getting responses from owners with New Tiger and Cirrus with S-Tec problems!
Researching further, I asked the avionics shop if the entire installation was done exactly to the S-Tec STC, and they assured me it was. I also asked my IA to review the airframe and tell me if there was anything out of the ordinary that would cause excessive water to get into the pitch servo; he assured me there was not. Therefore, since it was installed as per S-Tec's design, I can only assume that the problem is a flaw in their installation. Given this, I expect them to do me right by it. I updated the avionics shop via email:
Here's some photos of your original installation. Look at the floor: it does not look like it gets wet back there. Ideas?
You are correct, not a whole lot of water intrusion evidence. What is very obvious, though is the rusted bolts in that structure just aft of the servo...
Not wanting to just take anyone's word for it, I went out to look at my Tiger myself. The airplane is currently in annual, and the panels are all removed so that they could get to the servo. I took some more photos (see above link and below photos), and looked around back there real good. I contacted the avionics shop again via email:
I went to look at the airplane myself today.
First, there's just no way that the area of the fuselage where the servo is mounted is getting flooded by water or getting submerged. Absolutely no way. The floor is dry and the dust down there has no evidence of water dripping or pooling, plus the servo is mounted 6 inches off the floor. Further, the mounting bracket for the servo has no stains or evidence of water drips. In fact, there is no evidence of water actually getting in that area at all except for the rusted screws on the electrical connector housing for the servo.
Second, the servo mounts to the bulkhead immediately forward of the ELT. That is right at the leading edge of the ELT access cover (you can see the cover rivnuts at the top), so there's no way that the cover can be sealed from moisture intrusion without affecting the ability to get to the ELT in case of an accident.
Third, the area aft of that bulkhead, where the ELT and the Whelen strobe power supply is, has evidence of water, but that's to be expected given the access cover. The rusted screws you saw in the background of the photos are the horizontal stabilizer attach bolts. All of these other components (including the electrical ones) are in perfect working order. As an aside, the material/trash that you noted in one of my photos is residue from the paint process; it appears to be remnants of paint and paint remover.
Fourth, I looked over the components before they were shipped to S-Tec, and there was no obvious evidence of water damage except for slightly rusted screws on the connector.
Finally, I am being contacted by NUMEROUS Tiger/Cheetah owners in regards to pitch control problems with S-Tec 30 and 50 autopilots. Mine is not an isolated case. I am working within the Grumman type club to determine how widespread this problem is. Once I have gotten my arms around it I will be moving forward for a long-term resolution.
I have not yet seen the servo internals myself, but for the moment let's accept S-Tec's diagnosis that the servo is "trashed" due to "excessive water intrusion". My theory as to how this is happening is that the connector for the servo is getting wet due to its proximity to the ELT access cover (see below URL). Because that water has nowhere to go it is traveling down the wire harness and directly into the servo body, where it collects and thus damages the servo. I am assuming there is no weatherproofing where the wire harness goes into the servo body, thus nothing to stop the water from getting inside the servo.
http://www.gatm.org/flying/images/ap/DCP_1404.JPG (Large file)
I am further assuming that the STC calls for that connector to be mounted at that location and not routed below the control cables.
Given this, I would like the following:
- From S-Tec, an official letter detailing the results of their inspection of the servo, their suspected cause(s) for the damage, and their official price quotation for repair. Further, I would like their recommendations to us for adjustments in the STC-approved installation to assure this will not happen again two more years down the line.
- Also from S-Tec, the name and contact information for a technical customer service representative that I can work with directly to resolve this issue.
- I would like the servo returned to me immediately for my own inspection.
- I would like, from you, an informal recommendation for installation adjustments to assure this will not happen again.
If my theory is correct, then we must adjust the location of that wiring harness to stop water from traveling down it into the servo. If you have any alternate theories as to how water is getting in there, I'm all ears. I would encourage you to personally stop by Szigeti Air Service ASAP while everything is pulled apart and have a look at it for yourself.
Ultimately, I would like S-Tec to cover the repairs to this servo and pay for its R&R, given that it was installed in the manner required by the manufacturer. If they are willing to do this and adjust their installation methodology to assure it will not happen again, then I will be satisfied and work with them as a liaison to other Grumman owners who may be affected by this problem.
I will expect an official response from S-Tec by the middle of this next week. Please contact me at your convenience so we can move forward.
I agree, pitch troubles have been seemingly widespread based on recent feedback from owners and not isolated to Grumman applications. S-TEC has always been known for very good customer service plus they have many years of experience building autopilots for little airplanes and I'm sure they are willing to solve your problem. [We] will do all that is possible to help you as well, as always.
One work day later, I received the following from him:
Here is an update on my dealings with STEC to date on your servo problem: I spoke in depth with STEC's Scott XXXX, who has inspected your servo. I have forwarded to him your previous emails. I have dealt with Scott Howard over the course of at least 15 years and it is my opinion that he is extremely knowledgeable with the STEC product and their STC'd applications. He indicated that the only solution is replacing the motor (there is no rebuild options on the motors). He was involved in the initial STEC process of the STEC product in Grumman aircraft as early as 1980's and according to him, is unaware of any water intrusion issues that will effect autopilot components that are installed per the blueprints. He has viewed the photos of your tail section and the mounting/configuration of the pitch servo. His theory is similar to those of [the avionics shop], that water is perhaps dripping down the cable and making its way into the servo connector and ultimately into the servo motor. This is evident by the rusted hardware on the mating connector as well as the hardware on the servo case. His thought is to perhaps position the connector in a way that takes it out of the down-stream of the incoming water ("water can't travel upward"...was his comment).
He is going to send the servo directly back to [us], as [we] shipped the unit in for evaluation. He is not going to write a detailed summary of the findings of the servo problem as according to him, the situation is straightforward: The motor was damaged due to apparent water intrusion and thus is the likely cause of the pitch oscillations reported during flight. He said that he will send the servo back with the usual inspection paperwork associated with normal STEC factory operations. He did mention that he would speak with you directly, if you wish. His phone number is xxxxxxxx.
During our conversation, I mentioned your dissatisfaction with the service and reliability of your System 30 and briefed him on the repairs accomplished since installation. He didn't think that the costs associated were unreasonable for this system. During warranty period, a Pressure Transducer was exchanged at no charge labor or for the part. 8/11/2003, a factory repair was accomplished to the Pitch Computer by repair of a shorted connector internal to the computer. The cost of this repair was $355, which includes the factory repair, [our] troubleshoot, removal, reinstallation and certification. It doesn't include freight to and from Texas, a $62.30 total charge listed as a separate line item on the invoice. [We] submitted a warranty claim for the labor portion, but it was declined by STEC for reasons that the warranty period had expired. During this repair, [we] replaced and reworked the aircraft grounding bus, the root cause of failure to the SL30 Com radio. The $400 labor associated with this repair is unrelated to the autopilot repair and is spelled out in detail on the Maintenance Release. All of this is documented on [our] Maintenance Release 5414, dated 8/11/2003.
[Our technician] is aware of this entire situation and has requested that you contact him directly for further comments on the situation. Please tell me how I can help you further.
16 February 2003 Update
From the avionics shop:
More information from S-TEC, and this is what we are told:
The pitch servo has high start voltages - Rust and contamination on the motor cap shows indication of water damage and there is same evidence found in motor housing portion of cover. They say motor requires replacement, which equals 1.5 hours of factory labor and the motor cost of $414. This leaves a dealer cost of $510.46.
Our theory of water entering the servo by flowing down the connector pigtail seems sound to S-TEC. The actuator cover has a small drain hole provided primarily for installations where servo is mounted vertically with the capstan up. This hole is not in a location to provide adequate drainage in horizontal mount, where water collection has "not been recorded to previously occur". They say current production models have additional drain holes. S-TEC says that their install instructions do not specify exact routing or securing of the cable - and they say "in the absence of any other reports of this similar occurance, we will not be revising the installation". Further they say "our recommendations in this case is to reroute and secure the cable , providing a drip loop or point lower than the pigtail area entrance to the servo". That was our recommendations earlier and they seem to agree. I think this is the only solve of the problem, if the water drip can't be cured.
S-TEC also feels that the design of the servo interface in this model aircraft (since 1985) is "so successful and has worked too well for too many years in too many airplanes to acknowledge a design defect."
They are asking what to do with the servo at this point (an open ended question, I'm sure from your view...).
In other words, "S-Tec says go pound sand."
At this point, I'm not sure what I'm going to do long-term. My immediate goal is to obtain that servo back and look it over, see what I can learn about the problem. Short-term I am definitely not going to spend any more money on this autopilot, especially one with a systemic problem that will no doubt recur. We are going to work with our local FSDO to find out what will be required to mark the altitude hold as "inoperable" and also find out how much of the STC we can deviate from should I decide to pay for a new servo. In addition, we are working with members of the Grumman type club, the American Yankee Association, to find out how pervasive this problem might be and see if we can't come up with a solution. I will be creating an input form for owners to let me know how they're doing with their autopilots. If I'm correct and this is a design flaw, it should not be isolated to my airplane. However, because I consistently fly my airplane in all kinds of weather that could explain why it happened so soon to me.
If you have any feedback for me, please feel free to contact me at grega*at*pobox.com. I'll update this web page as information is available.
Obviously, I cannot recommend S-Tec autopilots in any shape or form right now. We'll see what the future holds.
5/7/2020 Update: man,I can tell I was really frustrated back then...sorry I never came back and updated this. But the autopilot was working perfectly fine when I sold the airplane circa 2014. I'm pretty confident I likely paid to replace the servo and had the avionics shop re-route the wiring harness with a drip-loop in it to keep water from dripping into it. Such a pain.
Initial installation photos. Higher quality photos at: http://www.gatm.org/flying/images/ap/
Photos taken 02/07/04: