Avionics Bending: AFCS Mode Indicator (Analysis)

Many people described the Lockheed L-1011 as the electronically most advanced aircraft of its time. I would agree with that. One of the center pieces of the L-1011 is its flight control system. the posting today shows the AFCS Mode instrument that is used to display the control mode that the avionic flight control system is in. I will give a little bit of an overview of each mode towards the end of this posting.



This particular type of mode indicator was also used in other aircraft, but this one was build specifically for the Lockheed L-1011 by Collins Avionics. Compared to other instruments, the casing is held in place by a massive amount of screws. There are a total of 10 of them.


On the inside, the instrument is rather basic. There are a number of solenoids that work in tandem to show 3 sides of each mode annunciator (typically one side is blank or just black). The solenoids stick out because they look like silver tubes or rods.

Top View

Right hand side view

Bottom view

Left hand side view


The annunciator has two test buttons on the top left and top right of the instrument. The TEST-1 button flips all indicator fields down and the TEST-2 button does the opposite. This would be used to test the functionality of the indicator prior to flight or during flight.



The next two pictures show the action of each of the two TEST buttons when they are depressed.




There are two general differences in the modes. The modes are either black letters on a white background or white letters on a black background. The two have different meaning:

1. White Letters on Black Background - is a mode that is ARMED but has not engaged yet. For example, ALTITUDE (ALT) capture can be ARMED (ALT ARM) until the aircraft reaches the selected altitude. Hence, first we would see a ALT ARM flag first and then an ALT flag.
2. Black Letters on a White Background - indicates an active mode or warning (e.g. Windshear).

Avionics Bending: Horizontal Situation Indicator - HSI (Analysis)

Today's posting i a little long, but it also covers an awesome instrument for the Lockheed L-1011 project, namely, the Horizontal Situation Indicator (HSI). The pair of HSIs that I picked for the project are made by a French company called SFENA.


This particular HSI was not part of the standard avionics package of the L-1011, however, it was used on L-1011s owned by the British RAF. Here are two pictures of the basic pointer elements of this type of HSI:


Unlike the standard HSI on the Lockheed L-1011, the SFENA HSI provides pointers for two additional NAV sources (VOR1/VOR2 or ADF1/ADF2). To select which NAVAID would be displayed on the yellow or the green pointer, an external selector panel would be provided with each HSI.

The instrument and the casing is held by eight rather small screws.


The top of the HSI is dominated by the circuit board that drives the Ground Speed and the Distance displays. The right hand side portion (which is the rear portion of the instrument) houses eight printed circuit boards on a bus.


Right hand side view:


Bottom View: The circuit board on the right hand side is the instrument bus for 7 of the 8 cards.



GLIDESLOPE

The glideslope deviation indication is on the right hands side of the rotating compass card. There is a flag that covers the pointer when the ILS signal is not present or not good. The pointer and is driven by small motor.



NAVAID Pointers

There is a yellow (not the course indicator) and a green pointer that can be switched as follows:

The yellow single line pointer can be: VOR1 or ADF1
The green double line pointer can be: VOR2 or ADF2

Which NAV source is used by the pointer is selected by an external switch panel.



The course indicator (large yellow T shaped indicator) can also be sourced from either NAV1, NAV2 or the INS (which would be the FMS in this case). The course indicator is broken into 3 segments, the top portion the middle portion which acts as the course deviation indicator and the bottom portion. The course indicator further has two flags that act as directional "TO" or "FROM" indications. The course indicator is also used to indicate the localizer beam of an ILS.




The HSI can display Ground Speed and Distance information. The distance would be to the next NAVAID (if it has DME) or to the next way-point a selected in the FMS.


The picture bellow shows the component side of the board that drives the digit displays on the top right and left of the HSI.




The heading information on the HSI can either be slaved to the magnetic compass of the aircraft or to a directional gyro system. The HDG flag indicates failure of the heading data and the smaller flag shows either MAG or TRUE to indicate the instrument source.

Avionics Bending: Altimeter (Analysis)

For the Lockheed L-1011 cockpit project, I picked a set of Altimeters made by ADS Inc. There are a number of altimeters throughout the cockpit; 3 of them on the main instrument panel and one on the pressurization panel. The instrument featured in this posting would be the primary altimeter used by the captain and the first officer.


Altimeters are corrected for barometric pressure at a flight level below 18000ft (180). Hence, all altimeters have a BARO correction method (a knob or something) to adjust for either In.HG or MB (Millibar are mostly used in Europe).


This altimeter has an interesting mechanism for barometric correction. It uses a number of servos to do that with. Here are the three standard views of the instrument:

TOP


RIGHT HAND SIDE


BOTTOM


LEFT HAND SIDE


There are total of five syncro devices in the instrument. Two of them are differential resolvers (CD) to encode the altitude reading and the barometric correction, one syncro limiter, a syncro transmitter and a syncro generator (basically the motor driving the instrument).



The flag arm is moved by a simple solenoid.



The altitude is displayed by four number drums. Two large ones for the 1000s and 10,000s of feet and two small ones for the hundreds and tens of feet.

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.