OEM Boeing 737 Control Columns - A Closer Look — Flaps 2 Approach

OEM Captain-side 737-500 series control column. Previously used by Croatian Airlines

The two control columns have been refurbished and installed into the simulator. The control columns previously were used in a 737-500 airframe operated by Croatian Airlines.

I was fortunate to have been able to secure these columns, and although there is some wear on the yokes, the buttons, electric trim switches, chart holders, and trip indicators are operational and in good condition. Furthermore, a working stick shaker is attached to the captain-side control column.

In this article, I use the words control columns and yoke interchangeably.

Mechanical Set Up

To allow the two columns to be fitted to the 5 inch high platform, the lower cogs have been removed and replaced with bearings. The bearings support a high strength stainless shaft that connects to a rotating disc beneath each of the columns; movement is synchronised between control columns.

Physical movement of the control column is registered by high-end string potentiometers and any movement converted to an electrical signal that can be read by the interface card. The interface card used is a Leo Bodnar 836X joystick controller.

The interface card, electrical wiring and potentiometers are installed on a piece of plastic board mounted to a dust proof box and attached to the underside of the platform. Access to the box is via the front of the platform.

Push and Pull Pressures

In the real Boeing 737 aircraft the control columns are hydraulically driven, and a fail-safe cable mechanism provides redundancy should the hydraulics fail. The 737 is rather unique in that, although hydraulics control movement of the control column, the pressures needed to move the columns (by hand) are quite stiff. Therefore, hand flying a 737 can be quite tiring; you must use a little muscle to move and maintain the position of the controls.

The specifications for the real aircraft state that the control column has a 37 pounds push/pull value +- 4 pounds, while the roll pressures are 12 pounds +- 3 pounds. These pressures can differ from aircraft to aircraft, but fall within the published specifications. To replicate the push, pull and roll forces as accurately as possible, four heavy duty springs have been fitted to the column mechanism.

Heavy duty pre-tensioned springs provide accurate static control loading

The control column pressure can be adjusted by either replacing the springs with higher or lesser tension springs, or by disengaging the outer springs.

A pressure test determined that push/pull pressure is 20 pounds and roll pressure 15 pounds. The push/pull pressure is on the low side, however, will be left as is for the time being. Springs have been used rather than hydraulic rams due to the simplicity of a spring and ease of replacement.

Although the use of springs is rudimentary, it's acts as an interim measure until control force feedback is installed. When this is done, the force required to move the control column will alter based on aircraft's speed, flap setting, landing gear position and other environmental variables.

The video at the bottom of this article demonstrates the linkage mechanism and springs in motion.

Configuration - Movement and Buttons

Configuration of the control columns is straightforward. Although there are two control columns, each column is linked to the other. Therefore, only one interface card is required. The buttons on the yoke, and the electric trim switch are connected to the outputs on the interface card.

Initial registration of the movement of the yoke and buttons is established in the Windows joystick calibration software. Further calibration is either done directly in the flight simulation program, FSUIPC, or in ProSim737. Although it is possible to assign buttons directly via the flight simulator set-up menu, the preferred method is to use FSUIPC or ProSim737.

Backlighting (Trip Indicators)

The actual yoke doesn't have backlighting; any illumination of the yoke is achieved by focusing the map light which is attached to the overhead panel. However, the numbers on the trip indicators do have backlighting (to illuminate the numbers).

Trip indicators are an airline specific option and do not come as standard issue. Pilots use the trip indicator to 'scribe' the flight number of the flight, or to document the Vref speed. Some crews never use the indicators. I use the trip indicator as a ready memory pad to document the landing Vref speed (Vref+5). The backlighting for trip the indicators is powered by 5 Volts.

Chart Holders

The chart holder is used to secure the approach plate (paper chart) in an area that it can easily be read during flight operations. The chart holders have a folding mechanism beneath the plate that allows the holder to be either pushed flush to the yoke, or positioned at a user-selected angle.

Another function of the chart holder is to provide a ready memory jogger for specific flight modes (checklist). The adhesive transfer on which this information is printed is specific to each aircraft type and /or airline. illumination of the chart plate, like the yoke, is achieved using the map light.

OEM Verses Reproduction

Several companies manufacture reproduction control columns: Precision Flight Controls (PFC), CH Products, Revolution-Sim and Ace Engineering to name a few. Over the years I have used products from ACE, CH Products and PFC. Without transgressing into a 'tit for tat' argument, you get what you pay for.

A CH yoke retailing at $100.00 cannot be compared with an ACE yoke retailing around $1300.00, however, both products have been manufactured to cater towards differing segments of the market. This said, the difference between ACE and PFC is marginal. I cannot comment on Revolution-Sim having not used their products.

So what is the different between a high-end reproduction yoke and a OEM yoke and column?

The main difference is the feel and finesse of the genuine item. Boeing has spent a lot of money (more than PFC, ACE or Revolution-Sim combined) in the development and engineering of the control column, and this is very difficult to replicate in a reproduction.

The OEM yoke and column is engineered to provide faithful service for many years. It's also built to suffer use and abuse from real-world pilots, and I am certain anything a virtual pilot can throw at it, will not cause any damage. The buttons and electric trim switches are solid, feel good to manipulate and are very reliable.

Yoke Performance

The yoke moves left and right across its range of motion with a smooth and silky feel without staggering, binding or rough patches. Likewise, the columns move forward and aft very smoothly.

The electric trim switches are far more responsive than the reproduction switches I have used. A slight application of pressure to the switch engages the electric trim. The electric trim switches response is a akin to a hair trigger on a firearm - it only needs a light touch to engage.

The control column is very responsive, and if calibration has been done correctly, very accurate. If the yoke is turned 15 degrees to the left, the measurement on the aileron tape is exactly 15 degrees.

Synchronization

I was concerned that synchronisation between the two control columns would not be perfect, however, my concern was short-lived. The use of high-end bearings at the end of the control linkages removes the opportunity for any slop (loose movement) to develop.

Appearance of Yoke - Used Look

If you carefully study the pictures of the yokes, you will observe that the yokes are not pristine condition, but show solid use (and probably abuse when it was striped from the aircraft).

The baked-plastic covering of the yoke shows scratches and some of the metal has been rubbed clean of paint. Some enthusiasts dislike this look and prefer a brand new 'out of the showroom' appearance. If this is you, then I suggest an OEM yoke may not be for you, unless you wish to completely overhaul the yoke and pay the large amount of money required to re-bake the plastic coating.

I like the 'used' look and feel it adds to the simulator. I have been in many cockpits, and very rarely do you find a flightdeck in brand new condition, other than in the first few months of service. More often than not, gauges, yokes and panels are scratched, dented and stained from many hours of sustained use from individuals that are more interested in flying, and going home after the flight, than maintaining the desk!

Additional photographs and information can viewed in: OEM 737 Yokes and Columns and Platform Construction.

Below is a short video showing the under floor mechanism, springs and linkage rods. If you listen carefully you will hear the springs creaking. This is not an issue when the simulator is running as any noise is cancelled out by the noise of the engines and flight deck ambient noise (electrics, 400 hertz noise and wind).

Glossary

Control Wheel - Yoke.

FSUIPC - Flight Simulator Universal Inter-Process Communication (interface software that provides a bridge between flight simulator and outside programs).

OEM - Original Equipment Manufacturer (aka real aircraft part).