Litton INS - LTN-72RH
 

Hi there - there is a long history of various inertial navigation systems from Litton from very early
mechanical ones until recent ones based on laser gyros and GPS fusion. Recently I obtained a
LTN-72RH made by Litton Aeroproducts, Canada; part number 1520°0-05-02-36-71, serial 2836:

https://cimg3.ibsrv.net/gimg/pprune....fca059c561.jpg
https://cimg6.ibsrv.net/gimg/pprune....4aa863f0a7.jpg

This unit obviously is of the latest generation using mechanical gyros on a gimbaled platform...

https://cimg7.ibsrv.net/gimg/pprune....2d606b5e51.jpg

...together with a digital computer for navigation and user interaction. I have essentially three
questions on this unit and I am sure lot of experts and/or users should be in this forum:

(1) Does anyone know, where this LTN-72RH was used - I have been told, it was on DC10-300,
but not sure on this. Also would be interesting, what the additional letters "RH" mean - probably
some radio-nav addon included (Software revision is 72-71-06)?

(2) Of course: Any manual/schematics/pin-outs around helping me to better understand or even
fire up this unit? BTW: I have some experience in making equipment like this work again ;-) -
see my work on a or an :8

(3) Which CDU does match the unit? From the WEB I know a very old type of CDU as used with
LTN-51 (Concorde and probably many others)...

https://cimg6.ibsrv.net/gimg/pprune....3455cfceda.jpg

Than there is a midle aged CDU which obviously already has alphanumeric displays:

https://cimg0.ibsrv.net/gimg/pprune....fdaff9a9e7.jpg

And I came accross rather modern CDUs by Litton which more look like a FMS:

https://cimg8.ibsrv.net/gimg/pprune....4667ec5be1.jpg

Any hints/discussion/comments are welcome...

Best wishes, Erik - erik - at - baigar.de
https://cimg2.ibsrv.net/gimg/pprune....b24796d424.jpg

I dont know if these people can help

https://www.vmars.org.uk/

I associate the first CDU as being used with Litton INS, I have never seen the two later CDUs. I worked with multiple operators over the years until I retired in 2008. And would expect to find that equipment on DC10-10 and -30, Tristar and 747 2/300.
Concorde which you mention was as far as I am aware Carousel equipped.
Carousel was a quite common fit to Tristar too. Litton more usual on DC10 and 747 but as ever the fit was an operator option.
I do remember the INU being heavy !

Hi, thanks for the answers! Will try my best with the guys at the VMARS, although they may be centered on radio stuff? Thanks also to Wodrick, yes, the unit is very heavy (25+kg), so for the tree units probably having been on the DC10 this makes the weight of an additional
passenger ;-)
I have seen in document 34-42 on the DC10, that there should have been three INUs and one CDU. But the first CDU in the series (which I actually have got) does not have the propsed switch to select which INU's values to show on the display. Also checking some signals on this CDU they do not match what is given in the sheet 34-43-1 I have got on the DC10, so there must be a different kind of CDU still...

As the aircraft age then they get modified. "My" DC10s both 10-10 and 10-30 had triple installations with three CDUs one for each pilot and a third in the roof panel, usually over the Captain's head. I worked on DC10s from four different companies and all were different in some respect.
Have fun, sorry I can't help further, as I retired all my manuals and notes went to the recycling.

Thanks for your comments, Wodrick! Sad, that your notes etc. went to the recycling, but can understand this after your retirement. Probably you remember whether you often encountered failures in these systems? I guess the navigation computer calculated a mean/most probable position from the input of the three drifting INS systems? Perhaps it is to long ago to remember how much miles they have been off position after e.g. crossing the atlantic?

I might be confusing other systems, trans Atlantic up to eight miles or so. Could often be less. When they got to a consistent 20 miles or so time to change INU. In my company it was a Tech Log note INS Drift #1,#2,#3.

Wow - cool; thanks for the information. So good to know and for the long flight time over the
Atlantic, 8 miles are not so bad. The military Ferranti systems from the 1970ties I restored
and have one working one mile per hour is claimed, although I guess they rarely really
where that good. You also have been pretty right, that there are lot of very different configurations
of panels if one looks at cockpit pictures on the WEB! In a PanAm manual available online I found
the following panels related to the INU kit (Claimed to be a DC10-30):

A mode selector panel:

https://cimg9.ibsrv.net/gimg/pprune....5b7b44d17f.gif

Than a sensor display panel (with switch to select one out of three sensor units):

https://cimg5.ibsrv.net/gimg/pprune....4b2b643898.gif
And what they call CDU, an unit doing navigation already with waypoints etc.:

https://cimg3.ibsrv.net/gimg/pprune....3634acfc3d.gif
So the search for the right panels will go on and I will check, whether the sensor pinout matches
what I have got...

The pt. No. of your INU is: 452080-05.

The TSO for that unit is TSO-C5C which shows a LTN72 control panel as pt. No. 452090-02

That would suggest the first control panel depicted above, with the Dim selector in the middle, would be suitable. Litton were taken over by Northrop Grumman.

Inertial systems at the time had allowable drift of X number of miles.

Where X = 3+3T and T was the sector time.

For example, an 8-hour flight would mean the maximum allowable drift would be:

3 + (3 x 8) = 27 miles. The systems were quite reliable unless not used for some time, in which case they tended to fail at start up.

I have no idea what the RH represents, There are R and RL models as well.

Hi QA1 - many thanks for your informative post! Given the part number for the LTN72 panel, I
indeed found a pictures on AeroBay - the panel indeed looks quite similar to the first, oldest
ones on my first posting. The panel I have got looks identical although it has got a different
part number.
There is a catalog of commercial items available from Northrop Grumman giving more part numbers
of various navigation kits - so also thanks for this hint! Will be a good starting point for more
research...
Thanks also for the formula on calculation of accuracy. Of course this linear+offset one is
some estimation - from a physical point of view (and what I saw in my restored systems) is,
that there usually is a strong oscillatory component in position (Schuler period) too ;-)
Thanks again for all the input!!

I do know a bit about the construction of this Litton P-1000 4 gimbal platform. It has two G-1200 non fluid floated tuned rotor ball bearing gyroscopes each capable of measuring two axis of rotation. The rotor uses a heavy tungsten wheel with precision machined mechanical flexures similar to the Kearfott "Gyroflex" dry rotor gyro. The three A-1000 accelerometers are non fluid floated hinge restrained force rebalanced. They use speaker type coils and permanent magnets for servo restraint. Both inertial instruments use Invar as their main structural material for high thermal stability.

Similar Litton P-1000 platforms have been used on the early U.S. Air Force F-15 fighter aircraft, the A-10 Warthog attack aircraft, some of the early versions of the U.S. Air Force air launched AGM-86 ALCM and U.S. Navy AGM/BGM/RGM-109 Tomahawk cruise missiles.

Hi there - thanks for the input! Sounds quite interesting - if you have more stuff to share, you are welcome to get in touch via email at [email protected]! Yeah, the unit look very interesting and meanwhile I got some documentation from a Laker airways DC-10 showing the pinout of the unit and which is according to my first measurements accurate. I also obtained a matching CDU panel recovered from an Atlas 747 being scrapped in Rio/Brasil and a mode selector panel from eBay. So I have all stuff to give the unit a try but need to prepare some experimental cabling and then it is going to get interesting ;-)

I think the litton 72 was a follow on from the 52 which was a fixed platform, lots of the older 707s had 52 and were limited to latitudes because the ins couldn’t cope beyond certain limits, the 72 was much better but still not as good as the carousel

Hi Rimmer, thanks for your posting - yes, the 72 is derived from the 52 but this also had a gimbaled (not fixed) platform. Those days all these systems worked like this for some reasons (1) the instruments are much more accurate if they are not rotated throughout the flight as e.g. each accelerometer also measures a little of the rotation. (2) Computation was much more easy if the accelerometers measured N/S, E/W and Up/Down - so one stage of transformation was omitted in this way. (3) Just by routing the signals from the platform sensors via buffers to the plug of the unit, attitude signals are available which are very accurate and for free - these where used in the attitude indicators etc.
The later carousel units slowly (minutes) and constantly rotated the platform with the instruments thus averaging out any bias in the N/S and E/W accelerometers - that was a really clever idea... Have a good time, Erik.

Hi!
So, i managed to get my hands on a LTN-51 CDU (PT NO 663550-04-01) and I noticed it on one of your photos and im trying to power on mine, do you have by any chance the pinout for it? At the time i have only figured out that pins b and a (E and D on the J2 connector) are used to turn on the lights for the numbers, DIM and WPT.

I was also wondering if anyone had any idea on where did this bad boy flew. Its serial number is 0983 and its manufacturing date is 9-20-74.

Any info helps at this point, so thanks in advance :)
BTW: Awesome job with the FIN-RPMD!!

Hi there and thanks for the positive words on my FIN1010 and FIN1012 related work! It is still an exciting journey and almost daily interesting learning! Cool to see, that others out there also have some interest in these systems!
My focus in this thread is the LTN-72RH I obtained to see the difference to the UK made Ferrantis, so I do not have got any information spcific for your LTN-51 CDU and maybe this pinout (acquired with the help from the nice guys here - THX!) does not match your CDU (although it also has got two plugs, your known pins do not the pinout I have got). So probably not of much help for you:

https://cimg8.ibsrv.net/gimg/pprune....0e088fceb9.gif

Just out of curiosity: Do you have got a LTN-51 and what are your plans? Do you want to fire it up or are you just interested in lighting up the CDU for some purpose?

Hi!!
First of all: THANKS FOR THE HELP!, I'll try to make some sense of it when I get home and I promise to let you know if I manage to get something to work.
I don't really have any plans for it since I only own the CDU but at the end I would love to be able to fire it up!
I found the CDU inside a Learjet (Probably a Learjet 25) that someone cutted in half at a local airport (MMJC) for a project that never came through. The CDU is missing some of the minitron displays (I've only got 5 of them) and both of the knobs but its pretty much complete (BTW, if anyone's got some extra minitron displays I'll be happy to store them for you haha).
Also, there is a LTN-51 INU that supposedly belonged to a Concorde for sale on Ebay if anyone's interested (I would post the URL but i'm not allowed to do it until I have 10 posts, sorry)

If anyone else have any other info or advice for me I'll be very gratefull

PS: If you want, I can try to post a photo of the CDU later :)

Hey!
So, I've been trying to figure out the pinout and from what I understand it states that I should plug the 28VDC into the a and b pins on the J2 connector but those connectors are bridged over each other (they share the same cable). However, it looks that those pins on the J1 may be the one's I'm looking for (although the pinout says that those are not used). Am I reading the pinout correctly?. I also noticed that, in the pinout all the lower case letters have a "_" below them and in my connector, only a few of them have the "_". Is that important for some reason?.
The other pins I found (E and D of the J2) are correctly stated in the pinout so, just for the fun of it, I reassambled the CDU to see how the lights looked like and I have to admit that they do look pretty cool haha (I would post a photo but the site doesn't let me do that yet).

I'll keep you posted when I find any other clues and as always I would appreciate any other info anyone might have! :)

Hmm, as I stated - the schematic above matches my LTN-72 CDU and I also have CDU (3) from the first posting where the pinout does NOT match. Probably the CDU (3) above is what you have got, too. But I guess the basic functions are the same as are the internal electronics. As you discovered: Probably only some pins are swapped. Would be cool to see a picture of your illuminated CDU ;-)
Unfortunately I had no time so far, giving my CDU+LTN a try although the test cabling is prepared. Here for reference the two control panels I obtained/resuced over the last weeks to go with my LTN72-RH:

(A) A Mode-Selector panel, type 452100-02-03, serial 6825 from a scrapped Atlas airliner:
https://cimg2.ibsrv.net/gimg/pprune....ce3df51e1f.jpg

(B) A CDU, imported from Rio/Brazil (AeroBay made a good hobbyists price); type 452090-02, serial 0265 and it came even with paperwork and test protocol dated December 2010 and the origin was a scrapped TAP aircraft:
https://cimg0.ibsrv.net/gimg/pprune....d72b6ab51e.jpg
Will post an update after first tests - but probably not before February 2021 due to the pandemic...

Ex DC10 driver here...I can shed some light on this.

These units were indeed DC10 fit

The first unit is the Litton 72. From memory only 9 waypoints could be entered in sequence, all using Lat and Long co-ordinates.

The second unit is the Litton 92 which had a database containing user waypoints which saved entering Lat and Long co-ordinates.

I think the 92 had 100 waypoints that could be strung together simultaneously to create your route.

I have both these units on my shelf and used both during my years flying the DC10 itself.

Waypoints were entered on one unit only whilst the other two were set to ‘slave’ or RMT. The waypoints would then automatically be transferred into the two ‘slave’ units and after completion of data entry all three units were placed into AUTO and made their own calculations as to their position. This would in turn provide the triple mix position to the Navigation system and reduce position error. Radio updates from DME/DME would periodically bring about a course correction. When flying trans Atlantic and passing Lajes, the aircraft would get a radio update and would usually turn slightly to put itself back into the exact position.

Accuracy of the units was pretty good. Usually less than about 0.3nm/hour of operation from what I can remember.

Like all INS systems, POS INIT at the start of the flight was quite critical as getting it wrong on the ground meant the boxes couldn’t be re-initiated once in flight.

I still have the user manuals for both these units from the manufacturer and the Lam Schematics for the aircraft fit.

Hi G4PoweredDecent, thanks for your answer! Always great to hear first-hand experience! Surely you had plenty of time playing with the systems during your time riding the DC10 ;-) Some questions after reading your cool post:

Hm, I guess in real life you never used more than a hand full of waypoints, even on longer flights, right? Before advent of digital means for exchanging route information it was probably to cumbersome to enter all these waypoints correctly using the numeric keypad?

Ups - so the pilot taking the aircraft over from you for the next flight was wondering how to navigate without a CDU to operate the INS? (Just joking :))

That is an interesting point - so there was no intervention with the INS necessary and the correction was entered automatically via the digital connection between the Radio nav and the INS or did you at least have to confirm a correction on the CDU before it got active?

Wow, that indeed would be auite nice. But I guess this value is from the navigation accuracy mixed from the three INS installed and maybe regular (1/h) radio updates?

Amazing! Meanwhile I got hands on the relevant pages of DC10 documentation for the wiring (that is what you probably refer to as Lam Schematics?) if the INS to the CDU and to each other. Are the manuals you mention the "Pilot's manuals" or do they contain more technical information (or even schematics?)? You can always reach out for me via private mail using erik -at- baigar -dot- de. Thanks for your efforts in replying here!

That here is my LTN72RH-Experimental kit: Once I will have access to the INS unit after Corona, I will use the documentation from the Laker Airways DC10s (bottom left), to wire up the INS to my 3 phase supply using a prepared transformer (left, top). The mode select and CDU panels (top row, middle) will be connected using cables prepared with the appropriate pins&sockets (top, right) and being plugged directly into the INS's rear ARINC404 plugs (numbered layout of these see bottom, right). Will be fun to see whether there still is life in my unit:
https://cimg6.ibsrv.net/gimg/pprune....88ac56b524.jpg

Looks quite like the Litton 51 CDU we had in most of our DC-8-63s. A couple had Omega (pure crap), and I flew one or two that had Carousel INS. Loading the first 9 waypoints was the F/Os job, sometimes (actually quite often) the crossload of waypoints to the Left unit (remote) would not work so it had to be done all over again on that unit...


https://cimg0.ibsrv.net/gimg/pprune....015b0c23d.jpeg

Hi Erik,

I stumbled across this forum today looking for LTN-72 INS pics. I was actually about to create a video on disassembling the LTN-72 Platform.

I have 30+ years experience on the 72 gear. Worked in Defence for 15 years and became the guidance system specialist and then was poached by Litton / Northrop Grumman to work as their guidance specialist based in London for another 15 years.

If you're still looking for info and have any specific questions unanswered I'd be happy to try and help.

Your system has seen better times. Corrosion on the outside and inside. It may have come from a DC10 or equivalent that ditched in a lake somewhere in Africa. I seem to remember pics of the unit that was doing the rounds a couple of decades back. Some second hand dealer was trying to sell it on the open market.

I can see that the platform has never been worked on (has original glue tacks from Litton)

The CDU in the pics above is a 72 variant - all good there. The MSU is a LTN-92 variant - has the amber align light rather than the green ready nav light which can be found on the LTN-72 MSUs. No issues here as it is just an on/off switch for the INU. Also remember that behind the middle panel on the MSU is the hidden test/bias switch. With power on this switch can be turned to bias and from there the current bias and scale factor figure can be read direct from the INUs core memory. These will be updated figures that are more accurate than the figures that can be found on the INUs front bias label. Erik as we have discussed in PMs the INU requires two power sources to turn on. A primary 115vac 400hz and a +28VDC (battery backup) source. It requires this 28v just to initiate turn on. This power can be removed after turn on and the INU will continue to operate. When a source of 115vac 400hz is fed into the INU a high pitched frequency whine will be heard (from the power supply) prior to turn on. This just tells me that there is power going to the INU.

The INU has some BITE (built in test) indicators. 3 can be found on the power supply externally and another 5 bite indicators can be found on the monitor board behind the panel beneath the power supply. Take this smaller cover off and the bottom cct bd will have 5 round disks - hopefully they will be black. If any are yellow this indicates that the unit has seen some fault during its life. These bite indicators can be easily reset with a magnet as they are magnetically latched.

From memory bite indicators are: Power supply PSM1 - Power converter issues. PSM2 - +5VDC fail. PSM3 - +13VDC fail
Monitor board DS1 - Gyro spin issues - from memory
DS2 - servo no-go (round servo board problem on the gimbal assy)
DS3 - Overtemp
DS4 -
DS5 - Quantizer board problems (large board in the front gimbal area
DS6 - Bias wipe Computer board issue

Thanks Simon for the tips and efforts in digging that out. Just verified: While the sticker with the calibration data of the LTN72RH can be seen on the pictures I got from the seller, it got lost during transportation - the front just shows the sticky stuff of the sticker, but not the paper any more; zooming in on the seller's image gives a rough impression of the numbers:

https://cimg2.ibsrv.net/gimg/pprune....fb54f51fa9.jpg

But fortunately the memory is an EEPROM, so that should not "forget" the stuff easily and may still be readible:

https://cimg4.ibsrv.net/gimg/pprune....c89821b106.jpg


Also easily verified that PSM1-3 have not been fired (or resetted):

https://cimg0.ibsrv.net/gimg/pprune....86f6a3d272.jpg

The platform trimmer board (the round one) is -08 - one which is not the worst as far as I understand your video:

https://cimg3.ibsrv.net/gimg/pprune....6ebd34288c.jpg

There is one of the little devices on the platform inidcating that there was no excessive shock (otherwise the small glass tube inside would have been broken and the color would have turned red) - so at least one good news:

https://cimg3.ibsrv.net/gimg/pprune....ec08caa6a2.jpg

The platform moves smoothly in all axes, but as mentioned via email - to my taste, there is quite some friction and I doubt that can be overcome by the torquer motors easily. At least the other INS systems I know (LTN51 and Ferranti INS) have less friction here. Looking at the nice video by Simon on youtube, , the platform stops moving quite quickly there, too. But I guess there is no specification of friction in the platform maintenance manual? Although there is some corrosion on the chromatted aluminium parts, all plugs and other aluminiumparts look pretty OK. Quite strange looks a small PCB which definitively looks like some homebrew add-on done at the RAE in Boscombe Down:

https://cimg3.ibsrv.net/gimg/pprune....9c93165f2e.jpg

Last but not least I have not been able to find DS1-DS5 (the only external flap on the rear covers some plug and beneath the power supply there are no indicatiors):

https://cimg7.ibsrv.net/gimg/pprune....5a0eb5334d.jpg

https://cimg5.ibsrv.net/gimg/pprune....add2ae0cb8.jpg

The bias figures are not readable. My best guess for the GBX,GBY, GBZ values are: GBX: 7770 3600 GBY: 0015 2200 GBZ: 0010 0000. For info bias figures are between 7765 0000 and 0024 0000. Remembering that these are octal values with the middle figure being 0000 0000.

If the unit turns on, the first thing I would do as an absolute priority is to get the bias figures from the core memory. Procedure is:

MSU to Stby (remove small grey cover from msu - 2 scews). Turn rotary knob to bias. The write down the bias figures as shown on the CDU. CDU rotary knob should be in 'Pos' and wpt thumbwheel at 0

GBX (wpt 0)
GBY (wpt 1)
GBZ (wpt 2)
SFX (wpt 3)
SFY (wpt 4)
SFZ (wpt 5)

Once you have these figures written down you then have some sort of insurance policy against memory wipes.

Turn bias switch back to 'off' and you can then switch msu off or continue to align.

With regard to the Boscombe Down cct board mod. I have not seen this on any unit before, so this unit must have been their test bed. I did help the guys at Boscombe Downs a few times in their workshop but was not aware they were modifying the units. It looks like its attached to the Qantizer cct bd, which is the large long board next to the platform. It may have been a data collection board. The qantizer board receives the raw acceleration data from the accelerometers and converts it into velocities and distance and then sends that data to the computer boards. The other boards in the gimbal area are: Gyro spin board, Temp control board, frequency control board (2 cct boards - one on top of the other, and the other board is the 26v aux pwr supply.

I wouldn't be too worried about the excess friction in the gimbal. The excess friction would cause high drift on long flights. Something that you will not be doing.

The main priority is to get the unit to turn on without smoke coming out :-)

Looking at the pins on the rear end, they look okay. The main ones to worry about are the power and ground pins, they should be clean.

FYI - the top rear port (J3) is a test plug for the workshop.

FYI - I'm also trying to get my posting count up as I am unable to post pics until I have at least 8 posts on this forum. :-)

With regard to the hidden BITE indicators, they can be found behind the smaller black circuit board side cover, found just under the power supply which you removed in the pic above. 9 or so screws to undo. The monitor board is at the bottom, and you may have to slightly move the handle so as to view the bite indicators.

DS1 on the left and DS6 on the right.

Another note of caution. The unit like to run hot. In that the gimbal assembly requires a lot of heat to keep the gyros happy. The front of the unit will be quite hot to touch, whereas the rear of the unit where all the computer cards are likes to be at a cooler temp. When installed in an aircraft there is a constant movement of air thru the rear of the unit. If you get it up and running I would place a fan at the back of the unit to force air thru the rear which will then escape thru the underside of the unit.

The front side covers of the INU can be removed if there if a lot of fluff stuck in the heat exchanges. This fluff can be removed by a strong brush, just make sure the top lid if firmly in place so that nothing gets into the gimbal area. These older mechanical systems are a little finicky. :-)

Hi Simon, thanks for the feedback and the additional hint. Found the indicators (they have been hidden behind the label of the lowest PCB) and no one is triggered:

https://cimg3.ibsrv.net/gimg/pprune....d4598b5c1c.jpg

But probably discovered the no-go under that panel as there seem to be two boards missing here: Slot BY (2nd from top is empty) and there
is a open plug/empty slot between CU and CV (3rd from bottom):

https://cimg4.ibsrv.net/gimg/pprune....e83f8eb4ea.jpg

A pinout of the "workshop-plug" J3 would be quite helpful as this surely has access to all supply voltages - so it would be easy checking for shorted caps there. Thanks also for the hint on heat philisophy in the LTN72. Of course I am aware from my experienct that slip rings and bearings to not like dust at all ;-)

Well that's weird. Missing one of the D/A boards. The other missing board is not an issue. The board above the Mode board is an optional board (Radio Navigation Interface board - RNI). Not required for this INU.

But with regard to the missing DA board, it should operate without. It's just a board that converts digital pitch/roll signals to analog format for some older avionics.

Initialization of the INU.



The INU goes thru a number of statuses in order to complete a full alignment.

Status 90: MSU to STBY, Power applied to gimbal, heaters. Gimbal is forced into a cage mode and held steady whilst the gyros come up to speed. Approx. 50 seconds

Status 80: Once the gyros are up to temp and speed the caging system is released and the gyros now run the gimbal. The gimbal is now at a local level state. The INU is held in this state for a further 2 minutes.

Status 70: Course alignment where it starts to calculate true heading via slewing of the azimuth gimbal. Lasts for 3 minutes

Status 60: True heading is calculated and finalised. 1 minute

Status 50: True heading is now available via the CDU

Status 40: The beginning of fine alignment. Lasts around 3 minutes

Status 10: Finalising fine alignment. Lasts around 4 minutes.

Status 02: Alignment complete. INU can switch to NAV mode on the MSU

Total alignment time is 15 minutes



The INU can be used in two modes. One where the INU requires the computer to be fully functional and a full alignment is successfully completed. The other mode is ATT mode where the computer is shutdown and only the raw attitude info from the gimbal is fed to instruments.



To prepare INU for first turn on.

Side cct bd black panels and gimbal top black cover to be removed for initial test.

INU requires 115vac 400hz and +28VDC for initial turn on. Due to the high heating requirements at turn on, a lot of current will be required initially.

Switch the CDU rotary switch to DSR TK/STS

When the MSU is turned to STBY – power is sent to the INU and cage mode will commence instantly.

To ensure the gimbal has the ability to drive the gimbal it’s best to offset the gimbal prior to power up. Do this by manually moving the gimbal. Move the outer roll by 45 degrees, move the azimuth by 45 degrees and slightly offset the pitch as per my pic. The 8 OPAmps on the round servo board are in charge of driving the gimbal during cage mode (see pic). 2 for each axis (O/R, I/R, Az, Pitch). If the gimbal just spins wildly at turn on, then turn off immediately. Replacing the faulty amps will fix the problem.

If the gimbal cages then this is good news. Gyros should then spin up and upon successful spin up the INU will go to Status 80.

The INU will remain in status 80 until a lat and long has been entered into the cdu and the MSU is turned to ALIGN.

In instances where the INU has been transported between hemispheres and not been turned on, the computer may show an error code as it is unable to recognize its recent (last known) turn on location. This error can be over ridden by recycling the INU. Action Malfunction code of 5-03 will be displayed.

If the INU does turn on the first priority is to collect the bias figures from the core memory. Turn the switch under the hidden panel on the MSU to BIAS and flick waypoint wheel to 0. Write down the 6 bias figures – Move waypoint thumbwheel from 0 to 5. Once complete turn the bias switch back.

If you get this far, then things are going extremely well. Once lat and long has been entered into the CDU and MSU has been set to ALIGN, then just watch the status. If it gets to Status 40 or 10 and a flashing warn comes up (AMF 2-19 / 2-20 / 2-21) this is expected as the unit is so old and out of calibration. If you get this far then we can perform some calibration to try and trick it into performing a successful alignment. Usually takes a week to fully calibrate an INU in a workshop environment.



Let’s see how far you get 😊

https://cimg4.ibsrv.net/gimg/pprune....cd9b043e90.jpg
OpAmps on Servo Bd

https://cimg1.ibsrv.net/gimg/pprune....39bd755786.jpg
O/R, Azimuth and pitch offset manually prior to first turn on

Thanks Simon again for the interesting and helpful information (your efforts are highly appreciated); I will definitively follow your advice and in case of problems swapping the driver OpAmps is not a problem at all ;-)

Do you have got an idea how much power is required on the rails? I have no problem with 115VAC/400Hz as I built a powerful inverter, but currently its 28VDC output is limited to 2A.

Minimal current needed for the 28v. This is really only required as a safety interlock for initialisation. It needs to see that 28v is present so that if 115 is lost in flight it can survive on the backup. In reality as soon as the ini turns on the 28 can be removed. You’ll get a warning but the inu will still operate as normal.

when the big day arrives, remember to take a video of the gimbal starting up, so we can see any issues and advise. :-)

Today connected wires to the mode panel and the CDU and did some measurements to verify what my pinouts tell me. For both panels I seem to have the correct pinouts and the CDU even sends out digital data when powered up (28V DC, drawing around 0.5Amps):

https://cimg3.ibsrv.net/gimg/pprune....1a9db9223b.jpg


But I am somewhat struggling with the pinout of the plugs on the rear side of the LTN72...

https://cimg4.ibsrv.net/gimg/pprune....e92d20cd46.jpg


...here my pinout says gound on J2A 7, J1B 2,4. But I do not see the ground pins being connected. So am I right, that the A parts are the upper halves and the B the lower ones and that the numbers given on the plugs are OK? From my DC10 (Laker airways) schematics I see that 115VAC should be connected to J1B, pins 1 and 3 and 28VAC (excitation for attitude outputs) is to be sent to J2A pin 6.

The power pin outs are the same for all LTN-72s and LTN-92 INUS

115VAC 400hz (Primary Power) 5 amps
High - J1B Pin 1
Common - J1B Pin 2

115Vac 400Hz (Heater Power) 10 amps
High - J1B Pin 3
Common - J1B Pin 4

J1B Pin 5 is chassis ground. This is connected to the aircraft frame / INU tray

Battery (+28VDC)
+28VDC - J2B Pins 3 and 4
-28VDC - J2A Pins 3 and 4 (this is battery return)

No other power is required to run the INU. All other voltages are created by the INU and fed throughout the unit.

Your assumptions on the plug layout are correct. A is top and B is bottom.

https://cimg8.ibsrv.net/gimg/pprune....23e8bddb50.jpg
https://cimg9.ibsrv.net/gimg/pprune....a222b8209e.jpg
https://cimg0.ibsrv.net/gimg/pprune....3e864d497e.jpg

Wiring of the LTN72 to Mode-Panel and CDU is complete with all required connections. Impedances look good and Inverter is ready with a 3A fuse permitting around 320W maximum into the INS. First try will be without heaters, but will re-check wiring next weekend before giving the smoke an opportunity to show up in my lab ;-) Stay tuned (event of first power-up will be recorded on video)...

https://cimg9.ibsrv.net/gimg/pprune....c613bc41c6.jpg

This is a very big moment.

Remember when power (specifically 115vac 400hz) is first supplied to the unit (before switching the MSU on) there'll be a high pitched whining noise from the back of the INU. This is good.

Regarding the video, it's only important to capture what the gimbal does at turn on (video to show the CDU display + Gimbal movement + your hand switching MSU to stby - if possible). We can gain a lot of knowledge from what the gimbal does at switch on.

If it does turn on without smoke - remember to write down the bias figures (as a priority) - as previously mentioned in the thread.


This is what I expect at first turn on.

60% chance that nothing happens - other than smoke from the power supply (not good)

30% chance the gimbal cages (or tries to) and then maybe spins wildly after cage mode is complete - possibly due to failed gyros/accels/servo bd. (possibly be able to fix)

5% chance that system tries to align but then fails due to the calibration being way off (High possibility to fix with some calibration) Action/Malfunction code will be displayed and a flashing 'Warn" on the CDU.

5% chance of successful alignment - nothing to worry about.

Good luck. :-)