One of the slower projects is the conversion of two B737 CDU units. The two units were purchased from an aircraft scrap-yard in the US and were formally used in a Boeing 737 operated by United Airlines.
LEFT: Straight from United Airlines to me. Two OEM CDU units. The rear unit has already had its CRT display removed and is partially 'gutted' (click to enlarge).
The two CDUs came from an airframe of a B737-500, which in 2008 was retired along with other Boeing classics, due to United Airlines decision to adopt the Airbus A-320.
The CDU presently used in the simulator is manufactured by Flight Deck Solutions (FDS) and although I have been pleased with its operation and reliability, there is little resemblance, other than appearance, to the OEM unit.
LEFT: Detail of the keyboard and DIM knob. Interestingly the DIM knob dims the actual screen and not the backlighting (click to enlarge).
The prominent difference is external build quality and the tactile feeling when depressing the keys on the keyboard; the keys don't wobble in their sockets, but are firm to press.
The OEM CDU is large and VERY heavy. I was surprised at the weight - a good 6 kilograms. Most of the weight is made up by the thick glass CRT display screen and other components that reside within the sturdy aluminium case.
LEFT: The casing removed to show the electronic boards that are secured by lever clips. Like anything OEM, the unit is made very well from solid components (click to enlarge).
Like the casing, the internal structure is also made from aluminium and has four rails to enable the electronic boards to be installed and secured into place.
Whenever I look at anything OEM, I am amazed at the workmanship that has gone into producing the item; the CDU does not fall short in this area.
A myriad number of small screws hold together the aluminum casing that protects the internal components. Not only screws are used, but also special miniature DZUS fasteners than enable the side of the casing to removed easily for maintenance.
When discussing the CDU there are three similar terms that are often used interchangeably: CDU, FMC and FMS. In this website, I use the terms CDU and FMC interchangeable which is not quite correct - let me explain.
LEFT: Protective cover removed to show the main pin-out board, rear of the CRT display, power supply, and electronics. These parts cause the CDU to be quite heavy. The two Canon plugs are just visible at the right of the picture enable connection to the aircraft. (click to enlarge to see detail).
The Control Display Unit (CDU) is the interface that the flight crew use to interrogate the data from the Flight Management Computer (FMC); it's basically a screen and keyboard. The FMC in turn is but one part of a complex system called the Flight Management System (FMS). The FMS is capable of four dimensional area navigation. It is the FMS that contains the navigational database.
CDU vs. MCDU
The older units used in the classic airframes are always referred to as a CDU, while the NG units are called a MCDU. M stands for multipurpose or multi-function. Basically, the MCDU has a different key called a menu key. This key, when pressed, accesses another layer of information that is not available in the earlier CDUs.
For those more military-minded, the CDU in military parlance is called a mission computer.
The CDU dates from 2008, therefore; it is not exactly identical to the CDU used in the Next Generation airframe, however, it is very close.
Main Differences - 500 series to NG
(i) The dim knob is a slightly different shape;
(ii) The display screen is rounded at the edges (the NG is more straight-edged);
(iii) The absence of the horizontal white lines located on the inside edge of the display frame bezel; and,
(iv) The display screen is different - cathode ray tube (CRT) verses liquid crystal display (LCD).
(v) Two of the keys are different. The NG has a menu and space key whilst the older CDUs have a DIR INTC and a blank key (no lettering on key).
Other differences, not important in the simulator environment, are the colour of the fonts used; older units have black and white or green font while later model NG units use multi-coloured font.
To a purist, these differences are probably important, and if so, you will have to contend with a reproduction MCDU or pay an exorbitant amount for an NG unit.
The software used in the OEM CDU is not used in the simulator. The CDU functionality is dictated by the avionics software (ProSim-AR) in use. This is also true for the font type and colour.
LEFT: Completely gutted. All unnecessary and unusable electronic components have been removed. These two CDU units will soon operate flawlessly with ProSim-AR and flight simulator (click to enlarge).
Converting the CDU
I am liaising with an Australian company that specialises in converting avionics components used in commercial flight simulators. This company has had considerable experience converting B747 avionics and is keen to see if their expertise will similarly work with the B737.
In a second article, I will explain in more detail how the conversion was done, and examine some of the problems that needed to be resolved. I also will discuss the mounting of the unit into the CDU bay.
More photographs of the CDU are located in the image gallery. Additional images will be added to the gallery in due course.
OEM - Original Equipment Manufacture (aka reral aircraft part).