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Aircraft Systems Topic 8.

Conventional Horizontal Situation Indicator


As the name suggests, this instrument advises your horizontal position, referenced to a VOR, or VOR/DME beacon. They can NOT be tuned into an NDB. These are the instruments fitted to pre-EFIS aircraft such as Boeing B727, early B737's, and McDonnell Douglas DC10's. You will also find them in smaller aircraft such as the Fokker F27, F28, and early model Beechcraft Kingair's.

They are effectively a VOR mounted within a Directional Indicator (DI). The compass dial is aligned to magnetic north by means of a "Flux Valve", which is mounted in the wing, or sometimes the tail of an aircraft, well away from the major sources of magnetic interference. Having the HSI "slaved" to the flux valve will mean you do NOT have to continually check and re-align the HSI with the floating magnetic (standby) compass every 15 minutes or so.

When tuned to a co-located VOR/DME beacon, the instrument can display range and bearing information from that beacon, provided valid signals are being recieved. Refer to the diagram as we review the display.

Item a.
This shows the distance to the tuned DME beacon.

Item b.
Indicates whether a VOR, or ILS frequency is tuned. The DME will automatically be tuned to the DME station that corresponds to the tuned ILS. When tuned in to a co-located VOR/DME station, setting the VOR frequency (VHF) will automatically tune in the DME (UHF).

Item c.
This is the compass rose, that is automatically aligned to magnetic north at the aircraft location by the flux valve.

Item d.
This is the course deviation indicator (CDI). It tells the pilot whether he is on the selected radial as set using the "Course Arrow" (item g). If the aircraft is off track the CDI will indicate how many degrees of error there is. If the CDI bar is say off to the right, then the pilot should turn the aircraft to the right in order to intercept the selected course. If the CDI bar is off to the left, a left turn is required to intercept the set course. The aircraft is on the 345 VOR omni radial FROM the station. The bearing TO the station is 165M.

Item e.
This is the scale that tells the pilot how many degrees the aircraft is left or right of the set course. In this case the aircraft is exactly on the 345 radial FROM the tuned VOR beacon. The calibration of the scale is different for ILS and VOR. When an VOR is tuned, each dot represents 2 degrees. When an ILS is tuned, the dots each represent half a degree (ie: 4 times as sensitive).

Item f.
This advises the pilot whether the aircraft current heading is toward or away from the beacon. Hence it is called the "TO/FROM" flag. In this case the aircraft is flying TOward the tuned VOR beacon as witnessed by the arrow (flag) pointing TOward the front of the miniature aircraft symbol. If the aircraft was flying away from the VOR beacon a flag would be seen pointing rearward. It should be noted that the TO/FROM flag is hidden from view when an ILS frequency is tuned.

Item g.
This is the "Course Selector Arrow", and it's position on the compass rose reflects the value set on the VOR tuning/selector panel.

Item h.
This red flag (sometimes red and white striped) indicates that valid glideslope information is NOT being recieved, and so an ILS can NOT be flown for fear of flying into the ground.

Item i.
This is the glideslope deviation scale. The centre of the scale represents the aircraft, and the white marker represents the position of the glideslope. If the glideslope marker is below the centre of the scale, the aircraft is above the glideslope, and vice versa if the glideslope marker is above the centre datum.

Item j.
This flag indicates that valid VOR or Localiser signals are NOT being received, and bearing information will be incorrect. There should be no flags showing, except when the radio is selected off, or the test button on the VOR tuning/selector panel is pushed.

Item k.
This is the selected heading as set by the pilot in the heading selector window, which in large aircraft is usually located within the autopilot selector panel.

Item L.
Represents the aircraft current groundspeed as calculated from the rate of closure to the tuned DME beacon.

Item m.

The value at the top of the compass arc is the aircraft current magnetic heading.

Item n.
This flag indicates that valid signals are NOT being received from the flux valve, and so heading information is corrupt,  and should not be used.

I hope that this mini training editorial from the ATPL course I conduct helps you in your studies.
Further ATPL training courses information is available within this website. You can enrol by E-mail. Some selected ATPL training texts, including assignments are available from most pilot supply shops around Australia, or through the secure ONLINE SHOP at this website for those of you with plastic money. A list of approved distributors is available within the Avfacts site.

Finally, good luck with your studies

Remember that it is better being down here, wishing you were up there, than to be up there, wishing you were down here.
Best wishes

Rob Avery

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Marty says ... "Goodbye to GA".