Avionics Bending: L-1011 Autopilot - Pitch Mode and Heading Panel - Part 3

L-1011 Autopilot - Vertical Speed Select

The VERT SPEED window and Vertical Speed Select thumb wheel are two pretty interesting components of the Heading/Pitch Mode Panel oft he Lockheed L-1011 autopilot. The Vertical Speed is indicated using a tape that is motorized and the thumbwheel allows adjustment of the value once the VERTICAL SPEED (VS) mode is selected on the panel.

The module takes +12 and -12V bipolar power. For todays hack I have worked on reading the vertical speed select values as output by the panel. The values are between -5V for 4000 feet per minute climb and -10V for 8000 feet per minute descent. I love the face that 8000 fpm can actually be selected on the L-1011 autopilot ... makes sense for an emergency descent ... of course, would probably also make about 80% of the 280+ people behind you loose their lunch. :)

Here is short video of the values produce by the panel ... for detail information please also see the chart below:

L-1011 Flight Instruments: Vertical Speed Indicator (VSI) (31-9)

The vertical speed OFF flag indicates CADC data or electrical power has failed. Pressing the ALT ALTN switch will provide opposite data to the affected altimeter and vertical speed indicator unless the actual indicator failed. 

Avionics Bending: Vertical Speed Indicator - VSI (Analysis)

Compared to the Airspeed Indicator from the previous posing, the Vertical Speed Indicator used with the Sperry Avionics packaged is a rather simple device. The indicator shows the aircraft's vertical speed in thousands of feet per minute ranging from 6000 fpm down to 6000 fpm up.

Here is a short video showing the basic performance of the VSI by simply using an ADC, an H-bridge and a simple micro-controller for data input.

The instrument an instrument-case is held together by two screws on the backside of the VSI.



Just like the Airspeed Indicator, the VSI has a gear portion in the front, one motor and one potentiometer that is used for position resolution, and a number of on-board electronics.





The "OFF" flag is actuated with about 12V DC and the motor in the instrument can be driven all the way to 47V DC.





The gear mechanism in the instrument is very basic and consists of a large gear attached to the motor and the potentiometer. The indicator is directly attached to the main shaft.

The large potentiometer reads zero resistance at 170 degrees of shaft angle.