by Norman King


(Photo by Robert Phillips)

A familiar feature of the Boeing 707 is the unique streamlined High Frequency probe antenna mounted at the tip of the fin. However, lower on the leading edge of the fin (in line with the registration on VH-XBA) is a less familiar device which may appear to owe more to a steam locomotive than to a jet airliner. To explain the purpose of this device, we need to call upon one of the engineers on the XBA project. Norman King takes up the story:


A requirement for flight control systems is that the faster you fly, the heavier it should appear to operate that control, be it elevator, aileron or rudder, the three primary flight controls. This is called "feel".

The device in question is called the Q bellows intake. It is part of a system that creates feel for the pilot in the rudder pedals.

The rudder is the only powered control on the 707 family, and a way to give the rudder system feel that is proportional to airspeed had to be added, because with 3000psi of hydraulic pressure aiding the pilot, it would be easy to achieve the full 26.5 of rudder travel at any speed. You certainly need full travel at take-off and initial climb speeds, around the 150kt mark, when an engine failure at or just after rotate demands full rudder authority to contain the yaw, but at 300kts, full travel, suddenly, would cause major structural damage.

So, the Q bellows intake is just another type of pitot tube, a device that measures or uses pressure created by increasing airspeed. In this case, it acts on a component half way up in the vertical stabiliser, to alter the mechanical connections between the rudder actuator and the rudder pedals, making it gradually (artificially) harder and harder to push those pedals to achieve the same rudder movement as the airspeed increases above 130kts. It achieves this through changing the mechanical advantage within the linkages.

After the fin and rudder was reattached to VH-XBA in 2006, this system had to be tested.

Naturally, there is a bit of Boeing test kit designed for that purpose, but sadly ATCL no longer had one in the tool crib, so our engineers made up their own after ratting through the ATCL dump bins, where much good stuff found its way, and from where we sourced many a useful item for repatriation to Australia. From discarded bits and pieces a patented QFM 707 Rescue Team apparatus was manufactured. Basically, it was a way of allowing an air source to pressurise the Q bellows unit, and incorporated a gauge to measure the pressure and check to see that the 130kt switch did its thing at the correct "speed" as calculated from the pitot pressure. There is also switching that commands a rudder hydraulic pressure reduction above 130kts to further add to the safeguards, reducing it to 2250psi depending on flap position.

As in all pitot heads, whether they feed airspeed indicators or devices such as this, there is a heating element to prevent icing and this creates the discolouration on the shiny stainless steel tube that looks as if a blowtorch has been played over the surface. Those heaters are mighty powerful.

Norman King

April 2012




Added a better photo thanks to Robert Phillips.
Original issue