EGPWS minima call-outs

[Britjet]

Hi Hardy

It depends on the type of CAT3 you are doing..These will depend upon the certification of the runway.
There is...
CAT3A, which typically has a DH of about 50R and requires a visual reference, usually at least 3 consecutive lights lights..(edge lights are allowed!)
CAT3B, which typically has a DH of about 20R and requires one centreline light.
CAT3B NO DH which means what it says! There is no requirement for the pilot to be visual at all!
If you land in 75m you probably won't see anything as the wheels touch and then as the nose lowers you will get a couple of centreline lights coming in.

That is why the ability of the airport to support a rollout is a requirement for this type of approach - you merely leave the autopilot in along the runway and only disconnect to taxi off (assuming that you can see a taxiway!).

There is a minimum visual requirement for a NO DH approach of typically 74m, but this is so the pilot can taxi off the runway and if necessary emergency vehicles can find the aircraft!

I *said* it was scary!!

[Britjet]

Yes, I think that many airlines use exact minima - that is presumably why the new mods on this panel support 1 foot increments..

Re TOGA - yes, you are correct Hardy - you can push TOGA at touch down and as long as you haven't  left it more than 2 seconds - up you go again!

It is quite permissable to press TOGA just before touchdown and the aircraft to touch the ground with height loss before it starts to climb away..

Brit

[Hardy Heinlin]

Hi Peter,

below 125 m RVR, wouldn't that be CAT IIIc?

Or is the CAT IIIb "No DH" you mentioned a Heathrow specific compromise between IIIb and IIIc?

In some airlines, 125 m is the minimum RVR for the 744.


Cheers,

|-|ardy

[Sylle]

Hi Hardy,

Extract from the Manual of all-weather operations

Category IIIA (CAT IIIA) operation. A precision instrument approach and landing with:  
 a) a decision height lower than 30 m (100 ft) or no decision height; and  
 b) a runway visual range not less than 175 m.  

 Category IIIB (CAT IIIB) operation. A precision instrument approach and landing with:  
 a) a decision height lower than 15 m (50 ft), or no decision height; and  
 b) a runway visual range less than 175 m but not less than 50 m.  

 Category IIIC (CAT IIIC) operation. A precision instrument approach and landing with no decision height and no runway visual range limitations.

Regards,
Sylvain

[IefCooreman]

Some info...

All minima callouts are the same, independent from the fact if a baro based minimum is used, or a radio altitude based minimum. The only 744 fcom I have seems to imply the same as the 777: the only callout that can change is the 500ft call, but this one is not based on the radio/baro choice, but on the fact if the aircraft senses an ILS or senses excessive glideslope deviation (so all non-precision approach cases).

The reason why CATIIIB/C approaches use no callouts has to do with the system design.

All ILS approaches from CATI to CATIIIA use the "fail-passive" design of the autoland system. The fail-passive design promises no excessive flight path deviations in case of a system failure somewhere. That's why you need the DA(H). The visual segment below this minimum value - besides checking if there is really a runway - allows the pilot to take over control in an acceptable way if the system would encounter a failure this close to the ground (this action is simply disconnecting the autopilot and initiating the go-around).

An ILS approach like CATIIIB and C use the "fail-operational" design of the autoland system. This design promises that the aircraft can handle a single failure completely on it's own and still land successfully without any intervention by the pilot. Therefore the pilot does not need the visual segment anymore, he won't need to disconnect.

The fail-operational design only works from a very low height, this height is called "alert height". Pretty sure this is 200ft for the 744, as it is for the 777. We use an "alert height" callout at 200ft RA (not an automatic callout, it is the right-seat pilot who does this), but I don't know if and what other companies use this method. The RA is turned away completely (below 0) to avoid the automatic callouts because they have no value. Below alert height many warnings are inhibited except a very limited few which still lead to problems for a fail-operational system (only 5 in total for the 777, all accompanied by a master caution warning).

Common procedure during approach preparation in low visibility is to first pre-set the baro minimum to CAT I minimum, then turn to radio minimum setting and set it to applicable low vis approach (DA or no DA at all). Above 1000ft, any problem will lead to a reversion to CAT I procedures by switching the minima selector from radio to baro. Nothing changes for the EGPWS minima callouts. Below 1000ft up to alert height, it is usually a go-around. Below alert height, only 5 problems can lead to aborting the approach, anything else and the autopilot lands without a problem. The ILS itself is considered highly protected because of specific ground procedures.

[Britjet]

Hi Hardy,

Yes, I guess CAT3B no DH would be CAT3C. I wonder what CAT3D does?
Maybe it goes straight to the bar???

[Hardy Heinlin]

Good evening,

does anybody know the re-arm altitude for baro minima?

For radio minima, which can never be higher than 999 ft, the alert is re-armed when the aircraft is above 1400 ft RA.

Baro minima, on the other hand, can be set as high as 15000 ft. This, plus 400 or 500, cannot be used as the system would never arm on short ferry flights or go-arounds.

The only way is to use the current baro minimum and add, say, 500.


Cheers,

|-|ardy