Knobs — Articles / Posts — Flaps 2 Approach

OEM ISFD (Image copyright Driven Technologies INC)

The Integrated Standby Flight Display (ISFD) is mounted in the stand-by instrument cluster in the Main Instrument Panel (MIP). The ISFD provides redundancy should the Primary Flight Display (PFD) on the Captain or First Officer fail.

The ISFD is not a common panel to find second hand, and working units are expensive to purchase. I don't have an OEM ISFD, but rather (at least for the moment) use a working virtual image displayed by ProSim737.

Conversion of an OEM unit is possible, however, the unit would need to be fully operational, and finding a working unit at a reasonable price is unlikely. ISFDs are expensive and reuse is common. If a unit does not meet certification standard, it's disposed of because it's broken and cannot be economically repaired.

ISFD Knob

The ISFD knob that came bundled with the MIP I purchased is very mediocre in appearance – in fact it's a piece of plastic that barely looks like a realistic knob. I purposely have not included an image, as the design would be an embarrassment to the company that produced the MIP.

A friend of mine is a bit of a wizard in making weird things, so I asked him if he could make a knob for me. He made two knobs – one based on the standard design seen in the Next Generation airframe and the other knob a shorter version of the same type.

Attention to Detail

Attention to detail is important and each knob has the small grub screw and cross hatch design as seen on the OEM knob. The knobs have been made from aluminum and will be primed and painted the correct colour in the near future.

A 2 axis CNC lathe was used to fabricate the knobs. The use of a computer lathe enables the measurements of a real knob to be accurately duplicated, in addition to any design characteristic, such as cross hatching or holes to install grub screws.

The real item – a Boeing Type 1 General Purpose Knob (GPK) and issue packet. There can be nothing more superior to an OEM part, but be prepared to shell out a lot of clams

In Australia during the early 1980’s there was a slogan ‘Oils Ain't Oils’ which was used by the Castrol Oil Company. The meaning was simple – their oil was better than oil sold by their competitors. Similarly, the term ‘Knobs aren’t Knobs’, can be coined when we discuss the manufacture of reproduction knobs; there are the very good, the bad, and the downright ugly.

Boeing Knobs

As a primer, there are several knob styles used on the Main Instrument Panel, forward and aft overhead, various avionics panels, and the side walls in the 737-800 Next Generation.

If you search the Internet you will discover that there are several manufacturers of reproduction parts that claim their knobs and switches are exactly identical to the OEM knobs used on Boeing aircraft – don’t believe them, as more often than not they are only close facsimiles.

In this article, I will primarily refer to the General Purpose Knobs (GPK) which reside for the most part on the Main Instrument Panel (MIP). Boeing call these knobs Boeing Type 1 knobs.

oem 737 -800 next generation knob. note different location of set screw. knob used on overhead

Why Original Equipment Manufacturer (OEM) Knobs Are Expensive

Knobs are expensive, but there are reasons, be they not be very good ones.

The average Boeing style knob is made from painted clear acrylic resin with a metal insert. On a production basis, the materials involved in their manufacture are minimal, so why do OEM knobs cost so much… Read on.

There are two manufacturers that have long-term contracts to manufacture and supply Boeing and Airbus with various knobs, and both these companies have a policy to keep the prices set at an artificially high level.

Not all flight decks are identical, and the requirements of some airlines and cockpits are such that they require knobs that are unique to that aircraft model; therefore, the product run for knobs for this airframe will be relatively low, meaning that to make a profit the company must charge an inordinate amount of money to cover the initial design and production costs.

A high-end plastic moulding machine is used to produce a knob, and while there is nothing fancy about this type of technology and the process is automated, each knob still requires additional work after production. This work is usually done by hand.

Cross section of a Boeing Type 1 General Purpose Knob

Once a knob has been produced, it must be hand striped and finished individually to produce a knob that is translucent and meets very strict quality assurance standards. Hand striping is a complex, time consuming task.

Additionally, each knob must undergo a relatively complex paint spraying procedure which includes several coats of primer and paint, and a final clear protective coating. Spray too much paint and the translucent area (called the pointer) inside the twin parallel lines will not transmit light correctly. Spray too little paint and the knob can suffer from light bleed. There is a fine line during production when it is easy to ruin an otherwise good knob with a coat of thickly applied paint.

Finally, any part made for and used by the aviation industry must undergo rigorous quality assurance, and be tested to be certified by the countries Aviation Authority. Certifying a commercial part is not straightforward and the process of certification takes considerable time and expense. This expense is passed onto the customer.

Often disregarded during the manufacture of reproduction knobs is the inner metal sleeve. The sleeve protects the material from being worn out from continual use

Replicating Knobs - OEM Verses Reproduction

It’s not an easy process to replicate a knob to a level that is indiscernible from the real item. Aside from the design and manufacture of the knob, there are several other aspects that need to be considered: functionality, painting, backlighting, robustness and appearance to name but a few.

Backlighting and Translucency

To enable the knob to be back lit calls for the knob to be made from a translucent material. Unfortunately many reproduction knobs fall short in this area as they are made from an opaque material.

The knob must also be painted in the correct colour, and have several coats of paint applied in addition to a final protective layer. The protective layer safeguards against the paint flaking or peeling from the knob during normal use. In the photograph below, you can see where extended use has begun to wear away part of the knob's paint work revealing the base material.

Detail of the grip and metal set screw. The set screw is important as it enables the knob to be secured against the shaft of the rotary. This knob previously was used in a Boeing 737-500

Set Screws and Metal Inserts

Often lacking in reproduction knobs is a solid metal set screw (grub screw).

The task of the set screw to secure the knob against the shaft of the rotary so that when you turn/twist the knob it does not rotate freely around the shaft. Plastic set screws can be easily worn away causing the knob to freely rotate on the shaft of the rotary encoder.

The position of the set screws on the knob also deserves attention. Correctly positioned set screws will minimize the chance of rotational stress on the shaft when the knob is turned.

Of equal concern is the hole on the underside of the knob where the rotary shaft is inserted. The hole should be sheathed in metal. This will increase the knob’s service life. If the hole does not have a metal sheath, it will eventually suffer from wear (disambiguation) caused by the knob being continually being turned on its axis. Finally, the knob must function (turn/twist) exactly as it does in the real aircraft.

Reproduction knobs may fail in several areas:

(i) The knob has various flaws ranging from injection holes in the molded plastic to being the incorrect size or made from an inferior plastic material;

(ii) The knob does not use metal set screws, and the set screws are not located in the correct position on the knob;

(iii) The knob has a poorly applied decal that does not replicate the double black line on Next Generation General Purpose Knobs. The adhesive may not be aligned correctly and may peel away from the knob;

(iv) The knob is made from a material that does not have the ability to transfer light (translucent pointer);

(v) The knob does not appear identical in shape to the OEM part (straight edge rather than curved);

(vi) The paint is poorly applied to the knob and peels off. OEM knobs have several thin coats of paint followed by hard clear coating of lacquer to ensure a long service life;

(vii) The colour (hue) of the knob does not match the same hue of the OEM product; and,

(viii) The circular hole in the rear of the knob, that connects with the shaft of the rotary encoder does not have inner metal sleeve.

The time it takes to manufacture a knob is time consuming, and to produce a quality product, there must be a high level of quality assurance throughout the manufacturing process.

Older Classic-style Knobs

It's common knowledge that many parts from the classic series airframe (300 through 500) are very similar, if not identical to the parts used in the Next Generation airframe. Unfortunately, while some knobs are identical most are not.

The knobs may function identically and be similarly designed and shaped, but their appearance differs. Knobs used in the Next Generation sport a twin black-coloured line that abuts a translucent central line called the pointer, classic series knobs have only a central white line.

Rotary Encoders

Although not part of the knob, the rotary encoder that the knob is attached deserves mention.

A fallacy often quoted is that an OEM knob will feel much firmer than a reproduction - this is not quite true. Whilst it is true that an OEM knob does has a certain tactile feel, more often than knot the firmness is caused by the rotary that the knob is attached to.

Low-end rotary encoders that are designed for the toy market are flimsy, have a plastic shaft, and are easy to turn. In contrast, rotaries made for the commercial market are made from stainless steel and are firmer to turn.

Also, low end rotaries and knobs are made from plastic and with continual use the plastic will wear out prematurely resulting in the knob becoming loose.

Many reproduction knobs fit the bill, and for the most part look and feel as they should. It's easy to criticize the injected plastic being a little uneven along the edge, but this is unseen unless you are using a magnifying glass

Final Call

Whether you use reproduction or OEM knobs in your simulator is a personal choice; It doesn't play a huge part in the operation of a simulator. After all, the knobs on a flight deck are exactly that – knobs. No one will know you have used a reproduction knob (unless low end reproductions have been chosen).

However, the benefit of using a real aircraft part is that there is no second guessing or searching for a superior-produced knob. Nor is there concern to whether the paint is the correct colour and shade, or the knob is the correct shape and design – it is a real aircraft part and it is what it is. But, using OEM knobs does have a major set-back - the amount of money that must be outlaid.

But, second-hand OEM NG style knobs are not easy to find and often there is little choice but to choose ‘the best of the second best’.