VDO ST443-3 Nozzle Area Position Indicator

 introduction
The Nozzle Area Position Indicator is fabricated by VDO and is used in the Panavia Tornado fighter jet. The partnumber is ST443-4 and is marked with nato number 6620-12-178-4916. The description on the type plate is 'Nozzle Area Ind.' The Nozzle Position Indicator shows the aperture of the engine outlet-nozzle. In idle the nozzles are open. When applying maximum power, the nozzles opening is reduced. When the afterburner is activated, the nozzles are opened.

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 description
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 disassembly
I spent some time disassembling the unit. It's a puzzle... After some fiddeling around I found the 'magic trick' to disassemble the module. [1] I loosened the four screws of the rear panel. This releves the pressure on the rubber gasket. The screws don't have to be removed, but it's possible. Removing the screws makes it possible to remove tha gasket, but beware not damaging the kapton cable. [2] I removed the four screws from the rear part of the housing. [3] The knob must be removed, this is probably the hardest part. The knob isn't just a knob, there's are reductor gears built into the knob. (Nice engineering!) The know don't have to be disassembled as I did. Near the axel behind the knob is a c-shaped ring. Remove the clamp with a screwdriver in the housing direction. The clamp is placed in a groove, so a little pressure is needed. Afte removing the clamp, the locking pin is visible. There's a pin pushed trough the knob and axel. Push the the pin through the knob until the pin falls out of the knob. Now the knob cna be pulled of the axel. [4] Now the frontpanel can be removed by removing the four screws from the front panel. [5] The assembly can be pulled out of the housing via de rear opening.

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The internal construction can be devided in the electronics (rear) part and the mechanical (front) part. Both modules are connected by four screws and a electrical connector. The electrical part has the powersupply at the rear end. There are also a couple of printed circuit boards with probably some servo driver circuit.

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The mechanical module contains a DC motor (left top) which drives the dial indicator via some gears. There's also a stacked/double variable resistor (right top) for position encoding. Since it seems that there are two three-wire parts stacked, two variable resistors are likely but a position resolver can also be possible. This has to be find out later on... There's also a manual indicator for setting a threshold level. By rotating the dial, the marker points out the setpoint. The position of the marker is also monitorered by a 2K2 variable resustor (left bottom).

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 repair
loose DC motor
Avionic parts sold on the internet is like gambling... It's always a guess what the condition is of the instrument. Instruments are discarded/sold when the are broken or worn out. If you're lucky, parts are sold if a whole aircraft is scrapped and the instruments are in working condition. But it's always a guess when buying stuff via eBay. In this case the device was rattling when is was moved or rotated. So it's clearly that there is a mechanical problem. After opening the instrument I saw that the DC motor was loose. There are two clamps which should hold the motor, but apparently the screws came loose. This is odd since these screws are usually locked with liquid thread locker. Due to vibrations or a production fault (...) the motor came loose after a while. The good thing is that the repair is very simple. The motor is installed an the two screws are tightened. I didn't use thread locker since the device is probably never to be used in the air again. I wonder what happened in the air when the pilot encountered the problem. The indicator would 'freeze' so the servo loop was never locked. 'Smart' instrument have a warning system built in if the lock time is too long, an error signal was sent via the wire harness to some warming handling device. I don't know (yet) if this warming system is also built in here...

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The DC motor was loose in the housing. Luckily ther was no other damage.

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The DC motor and the DC motor position are clearly visible.


wrong counter indication
There's a white arrow indicating the set threshold level. There's also a 000...999 counter which should correspond with the manual set threshold level. But the indication is way off. I guess I have to reset the counter by removing a gear, resetting the counter and replacing the gear. Than the counter should match the set point. This is a future project...

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 connector
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pin
function
details
A
??
B
??
C
??
D
??
E
??
F
??
G
??
H
??
J
??
K
power input115 VAC 400 Hz (L)
L
power input115 VAC 400 Hz return (K)
M
dial illumination+5 VDC 210 mA (N)
N
dial illumination+5 VDC return (M)
P
??
R
??
S
??
T
??
U
mechanical groundconnected to housing
V
??


 dial illumination
There are three parallel connected small lightbulbs positioned at the left top of the dial. After testing the voltage should be around 5 VDC. The current draw of the three bulbs is approximately 210 mA. This makes that the three bulbs use approximately 1 Watt of electrical power. There are three bulbs to get redundancy. If one bulb fails, there are two other bulbs to keep the dial illuminated. Since hot filaments are susceptible to failure due to shocks and vibration I suspect three bulbs are used as a backup for the backup. Pins M and N are used to power the lightbulbs.

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 teardown/reverse engineering
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internal connector


dial illumination bulb; 5 VDC
pin A
pin B

DC motor is dia lindicator
pin F = positive (red)
pin D = negative (black)

manual setpoint position variable resistor (2K2)
J = variable contact
K = CW end of resistor
H = CCW end of resistor

dial position variable resistor 1
L, M, N, T, U, V, W or X = variable contact (to find out)
L, M, N, T, U, V, W or X = CW end of resistor (to find out)
L, M, N, T, U, V, W or X = CCW end of resistor (to find out)

dial position variable resistor 2
L, M, N, T, U, V, W or X = variable contact (to find out)
L, M, N, T, U, V, W or X = CW end of resistor (to find out)
L, M, N, T, U, V, W or X = CCW end of resistor (to find out)

warning flag indicator
L, M, N, T, U, V, W or X = (to find out)
L, M, N, T, U, V, W or X = (to find out)

not connected
B
E
P
R
S


 to do
Reset the counter to the correct indicator position.
Solve the sticky manual set knob.
Clean the instrument.
Reverse engineer the electronics/wiring.
Find out the control signals.
Lubricate the mechanics.
Create a demo setup?




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