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A project log for Boeing 747-200 PMS/CDU Restoration

Restoring a classic bit of aviation history with a modern twist

jeremy-gilbertJeremy Gilbert 06/14/2017 at 16:410 Comments

For many years now, I have been periodically scanning eBay of used airplane avionics, or aviation instrumentation. Once or twice a year I'll see something cheap enough (under $50) and fascinating enough that I'll buy. In the past I've restored or made operable all sorts of unusual little devices and learned a lot about the way complex instruments like this used to be designed.

The 747-200 CDU is my most ambitious project to date. This device is basically an early computer, probably one of the only computers actually onboard an airplane when it was designed. It was a predecessor of the now-common flight management system, which is basically a overall control system tying together the autopilot, auto-throttle, position, altitude and course management systems together.

This CDU is basically a "head end" terminal to a complex computer that lived in the avionics bay of a 747. This computer itself is long-gone, but for my purposes it doesn't matter.

I originally found this device on eBay. This is the listing:

The auction description reads:

This is a control display unit (CDU) used on Boeing 747 aircraft and possibly other types as well. This one has a parts tag showing removal from N628US on 5/23/08 during upgrade or tear down has been in storage since. It is in good condition without damage to the housing or connectors. The display is not cracked BUT has some delamination on bottom edge and all controls are in place. The data plate reads CONTROL DISPLAY UNIT, PART NO. 160080000-021, POWER 115V 400 HZ 0.8A, 5V 400 HZ 1.5A, CUST. NO. 60B40150, DELCO ELECTRONICS, FAA PMA 747 SERIES. It is not tagged and I have no way to test it so it is being listed AS IS. Thank you for looking and please see our many other aviation auctions.

This item was appealing to me because normally the PMS/CDU units are much more expensive. This one had a delaminated front which apparently knocked the price down quite a bit.

I looked up the aircraft N628US on PlaneSpotter:

Planespotters even has a photo of the original plane.

This plane was made on April 1980, and must have flown millions of passenger miles with its little PMS/CDU before it was decommissioned in the 2000s.

There is even a slight outside chance that as a young boy I even rode on this very aircraft! Back in the 70s I traveled from Boston to Portland, OR routinely with my family to visit my Dad's folks out west. I distinctly recall that often those trips would be on a wide-body plane like the 747. So who knows? It would be impossible to know now if I had ever ridden on N628US but I was certainly on a plane just like it.

The PMS/CDU was only used on certain NorthWest Air 747s. You can see a photo of it here:

If you look carefully, you can see that this in fact is the ONLY interactive computer interface onboard the entire cockpit. The only other major "computers" on this aircraft that I am aware of were the INS and the Autopilot which have much simpler knob based interfaces and i'm not even certain if they were proper microprocessor based computers.

I posted a question out to and someone there was nice enough to read the description of what the PMS was supposed to do.

Performance Management System (PMS)
Introduction The PMS is an interface between the autopilot and the autothrottle. It calculates aircraft performance by using pilot inputs, stored data, and real time information from several aircraft systems. The purpose of the system is to provide fuel conservation and optimize aircraft performance.
The CADC and INS provide vertical climb rate and acceleration information to the PMS to determine if the aircraft is in climb, cruise, or descent. The PMS selects the applicable
control mode to become active when engaged. When engaged, the PMS provides outputs to the A/P, F/D and A/T to control the aircraft during climb, cruise, and descent.
The components of the PMS include:
control display unit (CDU)
performance management computer unit (PMCU)
switching unit (SU)
engine interface unit (EIU)