Auto Brakes — Articles / Posts — Flaps 2 Approach

Enthusiasts don’t normally give a second thought to the rotary type auto brake mechanism on the Main Instrument Panel (MIP); it works and that’s all that matters. However, several reproduction rotarys do not entirely replicate the correct operation of the auto brake in the Boeing 737 aircraft.

In the real aircraft, a pilot has the ability to select between auto brake OFF, 1, 2, 3, and MAX. The first three brake indications and off are achieved by turning the knob in a clockwise or anticlockwise direction, however, the knob stops at MAX. To engage MAX, the pilot must pull the knob out from a retainer and then move it a further click to the right.

The reason for this is quite simple; engaging auto brake MAX results in severe deceleration which can be stressful and uncomfortable for passengers, as well as creating undue wear and tear on the braking mechanism of the aircraft. The function of pulling the knob is not replicated in reproduction auto brake switches.

Aviation Scrap Yard to Me

To my knowledge, the auto brakes operate and use the identical rotary mechanism throughout the Boeing aircraft series, the exception being the size and style of the actual knob mounted on the rotary. The early model auto brake assemblies have a slightly larger knob, while those used in the 737-500 have a smaller knob that is identical to that used in the Next Generation aircraft (with the exception of the parallel black strips which is distinctly Next Generation)

Larger and Robust

If you have inspected any genuine aviation part, you will have noted that the size of the item is usually quite large in contrast to reproduction simulator part. This is because a real part must be manufactured to take into account the nuances of pressure, fatigue, vibration, water and dust proofing, and be made as sturdy as possible to ensure operational longevity. Genuine parts regularly are designed to military specifications and can withstand considerable abuse.

Canon Plugs

The wiring used with an OEM part can appear complex with several wiring harnesses and a multitude of connections. In a real aircraft, these wires connect to circuit breakers and a magnitude of interconnecting electrical components and power sources. All Boeing 737 aircraft use Canon plugs. Canon plugs make removing a panel straightforward for a technician and also provide a fail safe mechanism to ensure that specific wires are connected to their correct mate. Each Canon plug is different and can contain up to 32 differently arranged pin-outs. It is almost impossible to miss-mate two Canon plugs.

With so many variables, it can be frustrating slow process determining which is the correct pin-out to use.

Autobrake Mechanism - Simpler than Most Conversions

The auto brake assembly is a lot simpler to convert than a more sophisticated avionics panel.

If you already have an auto brake rotary and installed to your MIP, all you are doing is replacing the reproduction plastic rotary with a real OEM rotary. The wires then connect to your interface card. The only tricky part is determining which connection outlet on the real rotary to connect the wires to. Determining this is either by trial and error or finding a schematic diagram for the rotary. I was fortunate that I had access to the later…

using a dremel bit to carefully enlarge the hole in the panel so that the rotary shaft will fit correctly

Conversion and Retrofit

Remove from the MIP the reproduction auto brake rotary and front light plate. Check to determine if the shaft of the genuine rotary will fit through the panel and MIP wall; the circumference of the hole may need to be widened. If this is necessary, it’s important that you do not damage any IBL back lighting that maybe used in the light plate. IBL is usually sandwiched between the back and front of the panel.

I used a dremel tool to gently and very carefully remove part of the light plate to allow the rotary to fit through the hole in the plate. Do NOT use a drill as this may fracture the light plate.

Fitting the Rotary Nut - Enlarging the Plate Recess

The rotary is securely connected to the MIP via a hexagonal-shaped nut. Depending on your MIP manufacture, you may need to enlarge the hole on the rear of the plate to enable this hex nut to be recessed in the plate. If you are using a MIP made by Flight Deck Solutions you will not need to do this as FDS have designed their MIP to fit genuine parts. Use the dremel to gently enlarge the recess on the rear of the plate. Be VERY careful not to damage the light plate; use the dremel tool very lightly.

owm 737-500 auto brake squib showing multiple connectors and pin out codes

Wiring

The genuine rotary appears to look like a squid (the sea animal) with an assortment of wires emanating from screw connectors.

Remove all the wires and screws and thoroughly clean the unit with a suitable cleaner. Do not discard the wires and connectors as you can probably reuse the high grade aviation wire; remember recycling is good and helps the environment. You will note that each connector is marked by a printed number.

The numbered keys for the autobrake squib keys and function are outlined below:

31 - common or earth
32 - RTO
33 - OFF
34 - autobrake 1 1
35 - autobrake 2
36 - autobrake 3
37 - autobrake MAX

Replace the screws in the appropriate connectors you will need to use. Then add a section of wire (use the old wire first) to the connectors. If you cannot reuse the sturdy clips, then use automobile electrical tabs.

If you have not done so already, before you cut the wires from the (to be replaced) “plastic” rotary, mark with tape and pen which wires connect to what function – RTO, OFF, 1, 2, 3 & MAX. This will make it an easier task when reconnecting or soldering the wires.

Aligning Rotary on MIP

Aligning the autobrake rotary is important if you want the selector knob to align correctly with the engravings on the MIP. If you look carefully at a reproduction rotary you will notice a circular lug that often is mated with a hole in the receiving metal - this stops the rotary from spinning on its own axis. OEM parts do not have such a lug. Rather, they have a circular washer that has a lug attached to it. This washer slides over the shaft of the rotary along a defined groove. The lug on the washer then meets with an appropriately positioned hole in the MIP to stop the rotary from swivelling.

Interface Card

If you are replacing a reproduction unit with an OEM unit, then an interface card is not necessary - connect the wires from the new rotary to those cut from the removed from the plastic reproduction rotary either by solder or using a terminal block.

However, if this is a new installation a Phidget 0/16/16 interface card will be required to assign the appropriate locations of the knob to their respective functions.

Is There a Difference?

Can I notice and feel the difference between the older reproduction rotary and the genuine rotary? In a nutshell - a resounding yes.

The genuine rotary is firmer to turn, engages with a distinctive audible click, and feels more robust. The knob also feels different to the reproduction knob; probably because the reproduction knob is made totally from acrylic and a genuine knob, although made from similar material, has a stainless shroud around the inside of the knob. This causes the knob to feel more secure on the rotary.

Annunciators

The autobrake has two annunciators - the Anti Skid INOP and the Autobrake Disarm korrys. The simulator uses OEM annunciators and although these korrys have been wired separately, their connection with the autobrake is done through the avionics software in use.

Below is a short video showing the use of the auto brake assembly. Ignore the speed reference knobs and fuel reset switch that need to be replaced with OEM knobs.

Auto Brake Usage

The auto brake is designed as a deceleration aid to slow an aircraft on landing or in rejected take off. The rotary switch has four settings: RTO (rejected take off), 1, 2, 3 and MAX (maximum). The brake can be disengaged by turning it to OFF, by activating the toe brakes, or by advancing the throttles; which deactivation method used depends upon the circumstances and pilot discretion.

RTO and MAX provide similar braking power (3000 PSI). 1, 2, 3 and MAX provide an indication to the severity of braking that will be applied when the aircraft lands. Often, but not always the airline will have a policy to what level of braking can or cannot be used; this is to either minimize aircraft wear and tear or to facilitate passenger comfort.

In general, setting 1 and 2 are the norm with 3 being used for wet runways or very short runways. MAX is very rarely used and when activated the braking potential is similar to that of a rejected take off; passenger comfort is jeopardized and it’s common for passenger items sitting on the cabin floor to move forward during a MAX braking operation. This 'safety feature' is the reason why Boeing airframes have been designed so that the pilot must pull the auto brake knob before selecting MAX.

If a runway is very long and environmental conditions good, then a pilot may decide to not use auto brakes favoring manual braking.

The pressure in PSI applied to the auto brake and the applicable deceleration is a follows:

Auto brake setting 1 - 1250 PSI / 4 ft per second.
Auto brake setting 2 - 1500 PSI / 5 ft per second.
Auto brake setting 3 - 2000 PSI / 7.2 ft per second.
Auto brake setting MAX and RTO - 3000 PSI / 14 ft per second (above 80 knots) and 12 ft per second (below 80 knots).

To activate the auto brake it must be armed by selecting the appropriate setting using the auto brake selector knob (1, 2, 3 or MAX). Furthermore, for the auto brake to engage the throttle thrust levers MUST BE in the idle position at touchdown. If the auto brake has not been selected before landing, it can still be engaged providing the aircraft is travelling no slower than 60 knots.

The auto brakes can be disengaged by either pilot by applying manual braking or selection the auto brake selector knob to OFF. Either action will cause the auto brake disarm annunciator to illuminate for 2 seconds before extinguishing.

Important Facet

It’s important to grasp that the 737 NG does not use the maximum braking power for a particular setting (maximum pressure), but rather the maximum programmed deceleration rate (predetermined deceleration rate). You can only obtain maximum braking pressure using either RTO or when depressing the brake pedals. Therefore, each setting (other than RTO) will produce a predetermined deceleration rate, independent of aircraft weight, runway length, type, slope and environmental conditions.

Auto Brake Disarm Annunciator

The auto brake disarm annunciator is coloured amber and illuminates when the following conditions are met:

Self test when RTO is selected on the ground.
A malfunction of the system (annunciator stays illuminated - takeoff prohibited)
Disarming the system by manual braking during an RTO or landing
Disarming the system by moving the speed brake lever from the UP position to the DOWN detente position.
If a landing is made with the selector knob set to RTO (not cycled through off). If this occurs the auto brakes are not armed and will not engage. The annunciator will remain illuminated

The annunciator will extinguish in the following conditions;

Auto brake logic is satisfied and auto brakes are in armed mode.
If the thrust levers are advanced during an RTO or landing ( 3 seciond delay is allowed after the aircraft has landed).

Personal Preference and Anti-skid

My preference for using auto brakes is, that when conditions are not ideal (shorter and wet runways, crosswinds) - I devote my attention to the use of rudder (for directional control) without concern for braking... the machine does the braking, and I take care of keeping the aircraft on the center-line...

Anti-skid automatically activates during all auto braking operations and is designed to give maximum efficiency to the brakes, preventing brakes from stopping the rotation of the wheel, thereby insuring maximum braking efficiency.

To read more on this subject navigate to: Rejected Takeoff (RTO) - Review and Procedures.

BELOW: Photo montage of auto brake assembly. Final conversion lower right picture - ready to install to MIP.