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.