HGS and CAT IIIB

[Levelmind]

Hi,

Correct me if I'm wrong, but as far as I'm aware the B737NG may perform a CAT IIIB approach, with fail passive, provided it has a HGS/HUD.

Now, if that is the case, the way I understand is that there's no autopilot rudder authority, and therefore, will the pilot manually have to apply rudder, like a "normal" approach?

Also, if a CAT IIIB approach is flown with the HUD (and fail passive), is the required RVR still 75m (as what seems to be the minimum RVR during HUD ops)? And, what will be the Radio Altitude minima? Is it still e.g. 50ft as during a CAT IIIA, or can it be flown with no minima, even if it's not equipped with fail operational?


Thanks in advance!

[FlyingStone]

You can't do a Cat IIIb approach without a fail-operational autoland system with rollout capability. HUD or no HUD.

[Levelmind]

Alright! Thanks for the answer!

What made me slightly confused, was that I read somewhere (can't quite remember where sadly...) that when using the HUD during a CAT III(A) APP, even if the autopilot fails you may continue the approach, which made me believe it sort of works like fail operational (clearly not though).

Lastly, while on the topic, I would just like to confirm what a Hybrid HUD approach means? Is it the fact that you may continue a CAT IIIA approach even if the autopilot fails, or does it mean something else?

[galaxy flyer]

The FAA says you can be approved for non-autopilot HUD only CAT III. SW does them and, I believe, AK. The FAA considers a HUD CATIII to be fail-operational. Maybe not in EASA.

http://fsims.faa.gov/wdocs/fsb/b-737_rev%2014.pdf

[Denti]

The 737NG is available with a fail operational autoflight system. I believe most US airlines go for the HUD imstead. However, in beautiful EASAland there was no advantage for my previous company on ordering the HUD, we bought the fail operational system which allowed full CAT IIIb operation with no DH and 75m RVR. Worked just fine and allowed single engine autoland to CAT IIIa minima as well. Only weird thing was the fact that the rudder servo was only available on approach below 1500ft AGL and during go around until another lateral mode was selected.

[Levelmind]

I was able to find the following in an EASA document: ’Fail-operational Hybrid Landing System: A system which consists of a primary fail-passive automatic landing system and a secondary independent guidance system enabling the pilot to complete a landing manually after failure of the primary system.
A typical secondary independent guidance system consists of a monitored head-up display providing guidance which normally takes the form of command information, but it may alternatively be situation (or deviation) information.’

So, if i understand it correctly, a hybrid HUD approach can be flown as fail-operational approach (even with a fail passive aircraft) but still only fly a CATIIIA since there’s no rudder channel?