The 767, autoland and the ILS signal

[Broomstick Flier]

Hi,

While discussing autoland operations on the 767 with some colleagues we could not find a definite answer to the following scenario: performing an autoland approach to CAT 3 minima on a fail operational/AH setup, what would happen if, let say below AH (200ft RA) we loose both LOC and G/S signals.

Some reference sources indicate that the system would remain fail operational and the landing would be performed using IRU data for tracking and radio altimetre information for flare and touchdown, but we could not find this explicity stated on any of our Boeing manuals.

Such event would, according to our airline policy, mean a mandatory G/A anyway

Thanks!

[aterpster]

You won't lose those signals on a CAT III ground installation. Lots of redundancy. The G/S becomes unimportant below 150 or so, but you are not going to have decent rollout guidance without the LOC; at least that is how I remember it.

[Denti]

No real world experience, however we tested that in the simulator when we received our recurrent CAT IIIb training on the 737. With the loss of the ILS signal below 200ft the autoflight system remains in LAND3 status and will continue the landing, do the flare and touch down in the touch down zone. However if the wind changes during that time there will be absolutely no drift correction and the aircraft will drift accordingly. Which could lead to a very nasty experience. Both the G/S and the LOC FMA will turn amber, and for us it is a mandatory go-around because of the safety issue with the drift.

[Broomstick Flier]

Hi Denti,

Indeed earlier today we tested this on the sim (currently doing my initial on the 767) and it worked as advertised, ut as expected add the wind issue and it was not fun anymore. But it was virtual grass anyway :yuk:

Cheers

[scotbill]

How much wind do you expect in Cat 3 conditions?

[Duke of Burgundy]

Radiation fog isn't the only cause of Cat 3 conditions. Advection fog occurs in winds of up to about 15 knots. Then there are dust storms, sandstorms and blowing snow all of which are associated with strong winds.

[scotbill]

Genuine Cat 3 conditions are normally associated only with radiation fog.

[FlightPathOBN]

Denti,

Quick question, in regards to winds...is that the measured or input winds?

[ft]

OBN,
the automagics will compensate for winds as seen to remain on centerline.