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Aircraft Systems Topic 6.

The Electronic Horizontal Situation Indicator


This instrument is that found in EFIS aircraft. We will discuss the various readouts and relevance of pointers, etc. It replaces the conventional (electro-mechanical) HSI, but is said to be more reliable, can display more information, and presents this information in colour which is more easily interpreted. The EHSI screen like that of the EADI is a cathode ray tube (CRT) just like your television. More modern screens such as fitted to the B777 are Liquid Crystal Displays (LCD) such as used in portable computers. The advantages of LCD screens are that they are even more reliable than the CRT, use less power, create less heat, and are easier to read in difficult light conditions.

Let’s have a walk around this particular screen display, which incidentally is called "MAP" mode, and is from the B767 aircraft as used in the ATPL examination.

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Item a

This is the distance to the next waypoint, in this case it is waypoint ‘TIPSI’. Waypoints not overhead a VOR/DME/LOC/NDB beacon, or not at an airport are shown as a five letter ICAO abbreviation, as found on en-route navigation charts. Waypoints that are situated at a VOR/DME station are identified by a 1, 2. 3, or 4 letter identifier. Airport reference points are shown as a 4 letter abbreviation (eg: YSSY for Sydney airport).

Item b

This is the command heading bug, and it represents that position on the compass arc relating to the value set in the heading window on the Autopilot/Flight Director panel. The dashed line extending from the aircraft symbol to the compass arc is to allow the crew to relate this heading for proximity to hazardous weather such as shown. The dashed line also assists the crew find where the heading pointer is, when the bug is NOT on that part of the compass arc that is displayed.

Item c

This indicates that the TCAS system is set in the "Traffic" (TA) only mode. That is to say, no "Resolution Advisory" aural or visual warning will be issued ( ie: no "Climb, Climb", or "Descend, Descend" aurals, and no red RA red squares. The highest level of alert is a Traffic Advisory (TA) when in this mode. Refer to previous training editorial on TCAS.

Item d

This is a weather radar return. It shows hazardous weather in the form of a probable thunderstorm to the left (east) of the aircrafts current position. Infact this storm shape shows all the hallmarks of a severe thunderstorm, with fingers, "U" shaped returns, scalloped edges, most of the precipitation at one end, all contour colour levels displayed (yellow, green, and red), steep rainfall gradients, and the hook at the back of the storm may indicate the presence of a tornado there. Keep well away from any storms with these characteristics - they will probably kill you if penetrated.

Item e

This shows the wind direction and speed. In this case the wind is from 160M at 40 kt.

Item f

This is the TCAS symbol for "Traffic" (yellow circle). The aircraft is in your 11 o’clock position. The +01 and the downward pointing arrow next to the yellow circle indicates that the aircraft is current 100 ft above your level, and descending relative to your aircraft.

Item g

The head of this white triangle indicates the position of your aircraft.

Item h

These dashed lines (3 of) are called "trend vectors". They show if the aircraft is turning, and if so in what direction. The first shows the expected heading in 30 seconds time, the second shows the expected heading in 60 seconds time, and the third the expected heading in 90 seconds time. The heading predictions are based on bank angle and speed.

Item i

Shows the active Flight Management Computer (FMC) flight planned route. It is shown in magenta (purple).

Item j

This is the TCAS symbol that indicates a "Surveillance" target. It is an open white diamond. If it gets too close it will be displayed as a solid white diamond, and is then referred to a "Proximate" target, like the one approximately 5 nm ahead of our aircraft.

Item k

This is called the "Vertical Deviation Scale". It is only shown when on descent, and displays graphically whether your aircraft is above or below the FMC descent profile. It is sort of like an ILS glideslope in that the magenta diamond represents the position of the FMC descent profile, and the centre of the scale represents the aircraft. Maximum scale deflection is +/-400 ft. In this case the FMC descent profile is about 350 ft below the aircraft, and so you are a little high. Some more expensive types of GPS receiver can also present a height deviation scale to better monitor your descent profile.

Item l

This is the active waypoint. In this case waypoint TIPSI. Only the active waypoint is shown in magenta.

Item m

This alerts the crew to the presence of TCAS "Traffic".

tem n

This shows the tilt on the weather (WX) radar . In this case 5 degrees up. Down tilt is show as a minus.

Item o

This describes the ETA at the waypoint currently being flown to (TIPSI). In this case 1320.1 UTC.

Item p

This shows the aircrafts current heading. In this case about 124 degrees magnetic. The instantaneous track (current track made good, TMG) is represented by the solid white line extending from the aircraft symbol to the compass arc, and is 120M. The difference between the aircraft heading and the TMG is the drift of 4 degrees left. Quite understandable as the wind is shown (item e) as being from the right.


I hope that this mini training editorial helps you in your studies. Further Airline Pilot training texts such as these are available at most pilot supply shops around Australia, or through the secure ONLINE SHOP at this website for those of you with plastic money. This editorial is infact a condensed extract from the training textbook on "Flying Glass". A list of approved distributors is available within the Avfacts site. ATPL training course information is also available within the Avfacts website.

 Finally, good luck with your studies, and remember ...

Pilots are the ultimate professionals - strive to be one !

Best Wishes

Rob Avery

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Marty says ... "Goodbye to GA".

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