Maximum safe Glideslope/Localiser deflection.

[Contacttower]

I'm currently studying for my IMC rating and there is and a little bit of the ILS which I don't understand. I've got the IMC Confuser to help me with my learning and in it there's a question which asks:

'which of these diagrams represents the maximum safe deflection from the glide path'

It has four diagrams all with varying degrees of deflection. My understanding was that the answer is half scale, ie 2.5 divisons of the verticle scale and that is the answer given. However the diagram printed for the right answer shows the glide path bar apparently only 1.5 divisions of the verticle scale. Why is this?

[low n' slow]

Could it be so that it's only the localizer that is on the "maximum safe deflection"? If the question is "which diagram shows the maximum safe deflection", it doesn't really mean that both the LOC and the GS need to be at the max. As long as one of them is at it's maximum permitted deflection and all the other diagrams are showing a lesser degree of deflection, this will make the given answer correct. Do you understand what I'm trying to get at or am I just making it worse. You've got the right idea, but it's a matter of question technique...

[dv8]

From Standards Document 24, CAA
Guidance to Examiners in Type Rating Skill Tests and Proficiency Checks
Tracking
All except precision approach ± 5°
Precision approach half scale deflection azimuth and glidepath

[Wingswinger]

The origin of the info: ICAO Doc 8168 PANS-OPS Sect 4 Ch5

5.5.5.2 The protection area assumes that the pilot does not normally deviate from the centre line more than halfscale deflection after being established on track. Thereafter the aircraft should adhere to the on-course, on-glide path/elevation angle position since a more than half course sector deflection or a more than half course fly-up deflection combined with other allowable system tolerances could place the aircraft in the vicinity of the edge or bottom of the protected airspace where loss of protection from obstacles can occur.

Ws

[Contacttower]

I've worked out the answer to my question, it's to do with how the ILS display is presented: The display in the picture has a centre dot, surrouned by a circle, then four dots downwards and upwards, the line showing 'maximum safe deflection' is between the first and second dots on the lower scale, however, including the circle around the centre dot the bar is indeed half way between the centre dot and the bottom of the scale, ie half scale deflection.

Thanks for the replies, I passed the exam with 100%.

[AirRabbit]

Well, congratulations on your success! But I think it may be important to recognize that for this particular discussion one would have to recognize whether the discussion was designed around the "maximum safe deviation" or the "maximum deviation acceptable for check ride purposes." Typically, the answer that dv8 provided, "All except precision approach ± 5°, and Precision approach half scale deflection azmuth and glidepath," is what most regulatory authorities accept as satisfactory for checking purposes. However, that doesn't necessarily mean that during a precison approach getting to a 1/2 scale deflection plus an indicator width or two or three would put you in an area that was unsafe. As far as I know, the only time you would be "unsafe" would be when either of the indicators (azimuth or glidepath) was at their maximum deflection - as you wouldn't be able to tell whether or not you had exceeded that actual distance from the center; or, if beyond, how far beyond. THAT would definitely show that you were at, or no longer within, the three precision approach path obstacle clearances ... left or right for azimuth or low for glidepath.

[Contacttower]

But I think it may be important to recognize that for this particular discussion one would have to recognize whether the discussion was designed around the "maximum safe deviation" or the "maximum deviation acceptable for check ride purposes."


I agree, I used the word "safe" because that was what my instrument flying textbook used, and that I take it equates to what would be acceptable in a check ride (half scale deflection but no more). Certainly I wouldn't expect to crash into anything just by going slightly beyond half scale deflection.