Weird autopilot question

[giladal]

This question includes a chart. Info might be needed is:

Straight in ILS min for cat C is 271(204) and vis of 1200m.
Rwy 33R threshold elevation is 67'
Airport elevation is 68'

You are performing an ils approach to runway 33R. (CAT C aircraft). Using your autopilot couples to the ILS, you receive the following MET data: Visibility: 8000m, BKN 2000". The maximum altitude loss for a malfunction of the autopilot under approach conditions is 40'. What is the lowest altitude at which you can use the autopilot.

I am totally lost about this one.

The answer they provide is 118ft.

[mad_jock]

I suppose it doesn't come with an option.

A....
B....
C....
D Who cares I would have it out at 1999ft when we got the runway visual.

[giladal]

It does have a few options, but I didnt write them as I just wanted the right answer copied down....

Some of this questions really frustrate me.

CG

[fly_antonov]

It' s not your fault, the problem is in the question and the answer.

Your ceiling and visibility are above minima' s, so decision height is not a factor.
All the weather and minima talk goes into the trash can.

Next, when there is an A/P malfunction, the malfunctioning A/P disconnects. In a fail-passive system that would mean that the pilot needs to take-over the landing if visibility permits or abort and GA. In a fail operational system, you can continue (because you have other A/P' s or guidance systems). I believe that the 40 feet loss would be for a failed, fail-passive system because in fail-operational you can continue without any altitude loss in case of a single A/P failure.

Your aircraft and pilot are certified for Cat 3 C but the A/P-aircraft combination can result in a 40ft altitude loss in case of malfunction of your A/P.
The lowest point on your approach where you can afford to lose your A/P is 40 feet above the point on the runway with the highest elevation (not the threshold, threshold elevation is irrelevant because its elevation is not necessarily higher than the elevation at TDZ).

To avoid being remembered as the unlucky dead pilot, you must disconnect your autopilot at 68 + 40 = 108 feet and continue your approach manually.
The 118 feet answer could mean that I am forgetting something but I think that it' s just a math error of the question bank editor.

A good thing to know is that there are approach precision categories (aircraft and ground ILS equipment precision/reliability requirements) and autoland capabilities (A/P precision requirements), both very distinct things.
An aircraft can be certified CAT 3 with manual landing capabilities (non-autoland) if the necessary equipment (aircraft/ground) is installed and crew properly trained.

In theory, you could take a Cessna 172 without A/P, put a Cat 3 C ILS system on it, add among other things a head-up display, flare and taxi guidance systems, anti-skid and extensive crew training and get approval for CAT 3 C manual landings.

[G-STAL]

Doesn't a coupled autopilot require a multiply of the minimums by 1.25?

40 ft x 1.25 = 50 + 68 = 118??

[fly_antonov]

I know that decision height must be at least 1.25 times bigger than your minimum A/P disengage height. The airport DH is given as 207 feet (which is more restrictive than the A/P off height x 1.25 for this exercise).

Here they ask for that minimum A/P disengage height and I do not know of any regulations asking for a safety factor. I would be very surprised if any ATPL GS books even mention anything about min. A/P disengage height.

[mad_jock]

There might not be in JAR but there very well maybe in FAA land