Autopilot 400

[Bob Cook]

GMAS

You are only telling 1/2 the story. They are two different animals..... one is a role rate (s-tec) and the 400 is a pitch/roll autpilot that has its input from the AI and DG. The S-TEC is a rate of change device that gets its input from the turn and bank.

Big difference.

I do not see 737 / DC8 autopilots being replaced with new ones, and these are far older than most 400 series autopilots. BTW mylar caps do not dry up or become leaky. The power supplies were the weak side of things and you need some good support equipment to set them up or diagnose problems with them. There are still good service people that support the 400 series autopilot and there are lots of parts around from those who have been convinced a new autopilot is necessary.

The 400 was and is a good autopilot if serviced properly. The aircraft does not need to change direction BEFORE the autopilot senses the change.

Each case is different and must be subjectively analized. If the 400 is working properly it is a damn good autopilot. I can do fully coupled approaches with mine and I have not seen a hiccup yet. Let's me more objective.

my two cents.

Bob

[Sten]

Hi,

I called S-Tec Friday afternoon and talked with Scott Howard. Can’t say he was over enthusiastic about the idea to modify my 50-system that came out of a C172, to fit the 337. Also said installation had to be done by a S-Tec dealer. I told him we have our own maintenance shop, and if it can be done, the 50 will be installed here with or without their support.

Anyway, after a lengthy discussion he explained their part # system. He also faxed a parts list for the 337 S-Tec 50-System.

The computer is fairly easy to modify, it’s just a matter of “replacing a few resistors”. The servos are different, as there is a specific servo speed for each aircraft. It has to be the right servo, and they are “very expensive” to modify!

The gyros are all the same except for the turn coordinator voltage, and the lamp voltages if so equipped.

For most Skymasters, the Computer part # is 0132-0-7, Roll Servo # 0106-R9, and Pitch Servo # 0108-P4. These are the numbers for a 28 volt system. The 14 volt part # ‘s change only the first four digits, the dash # of the computer and servos remain.

So, I’m still as confused as ever! Seems also to be some different opinions about the excellence of the original 400 System!

Next, I will have a look at the dash numbers of the 50 System I have. And try to locate a manual of the 400 System in the aircraft, so I can construct a test device to check the servos.

If the 400 servos are OK, I’ll probably try to find a good computer for the existing system. If not, and the dash numbers are right, it will be a winter project to install the 50-System!

Comments appreciated!

/Sten

[MikeZ]

Like any other item of complicated electromechanical equipment, what Bob said is true, like if it ain't broke, don't fix it.

I think the more appropriate way to look at it for Sten and others facing the same issue is one of putting good money after bad.

Even if one bought the S-tec on credit, I'd wager that the payments would be less than the ongoing repair and maintenance costs for the old (renovated) 400 autopilot. Not to mention downtime and aggravation.

So its a personal decision after all.

[skymaster]

howdy: recommend you talk with ron hitchcock exec. autopilot in sacramento ca. he can check your boards. a dealer can exchange servos . currently he is working on my s-tec 65. the servo test is being performed by a local s-tec dealer. Ron's electrical and bench testing abilities are superb. j

[Sten]

Skymaster,

Thanks for the tip, but the logistics of sending the servos away for testing gets a little complicated as I'm in Sweden...

I have a home brew tester for S-Tec servos. It plugs in to the servo connector and tests the servo in the wing/fuselage.

I'm now looking for the schematics for the Navomatic 400 so I can test primarily the servos, but also the control head. If they are OK, I'll try to find a good computer box...

/Sten