CAT II/III - why with reduced flap setting?

[galleypower]

Does anyone know the reason why low visibility approaches are generally flown with a lower flap setting? For the forward view, a higher setting would be more beneficial. Perhaps because of the GA at low altitude?

[Airbus_a321]

WHO TOLD YOU THIS :confused:

INSTEAD NORMAL FULL LANDING FLAPS

TO DECREASE PITCH AND SO GET A BETTER CHANCE TO SEE ANY VISUAL CLUES, IN CASE THEY ARE REQUIRED FOR THE TYPE OF APPR FLOWN

[Piltdown Man]

The reduced flap setting enables some aircraft to achieve the required 2.5% MAP climb gradient.

PM

[renard]

I'm with Piltdown Man.

Reduced flap gives better missed approach performance.

It is not to be unexpected to touch down during a missed approach initiated at 50RA with both engines working.

With full flap and only one engine working, it would probably guarantee it!

[A380 Jockey]

GP,
Most large,modern jets are flown low vis approaches with 'max landing flaps',so as to enable max forward/slant vis when and if the pilot breaks visual. The thrust available on these modern jets for their certified max landing weights is normally more than enough to meet the required go around climb gradient(s) required in the worst of conditions ie high temps,low pressure,high altitude and high terrain in GA path. This however may not ALWAYS be the case. Wherein,a lower flap setting IS used as normal procedure,on a field to field basis.
However,in the case of corporate jets and/or general aviation aircraft,this may not necessarily be the case. MOST airfields would have higher GA climb gradient than can be achieved by the smaller engines for their certified max landing weights. Therefore the need to NORMALLY use a lowr than optimum landing flap. Most importantly,the forward and slant visibility on a small corporate jet is much better than that of a larger passenger airliner,with both using their respective 'normal' landing flaps. This is because the angle of attack of say a 77W would be far higher than say a citation 5 in similar conditions for landing. Thus the need to enhance the forward/slant vis in larger jets.
Hope this helps..

[oz in dxb]

The B777 is allowed to use 2 different flap settings for Autolands.
Flap 30 for normal operations
Flap 20 for non-normal operations.
Not approved for flaps 25.

A normal operations go-around (from flaps 30) would result in flaps 20 for the go-around. Both engines would be operating so climb performance shouldn't be limiting.

A non-normal operations go-around (from flaps 20) would result in flaps 5 for the go-around.

Oz

[Spooky 2]

The prefered procedure (AC120-28D) is to use max flaps so as to lower the nose for better visibility for say a CATllla approach. Since There may be other issues that would make something less than full flaps an alternate choice.

BTW the 787 will be certified with flaps 20,25,30 and one or two engines operating for autoland.

[Denti]

The 737 is certified for flaps 30 or 40 autoland, F40 only if using both engines, F30 if doing an OEI autoland.

[Vertical Speed]

Denti said "The 737 is certified for flaps 30 or 40 autoland, F40 only if using both engines, F30 if doing an OEI autoland."

That only applies if you are lucky enough to be flying one of the 737NGs with the Fail-active,Cat3B capable autoland (i.e. "roll-out capable") MOST 737NGs only have the more limiting 2ch autoland system with no rudder channel and are not certified for single engine autolands. To the best of my knowledge the 737 (Classic and then NG) is the only airliner with only dual autopilot autoland ! All the other Boeings and all AB's have triplicated systems. Boeing themselves are very vague about what they actual did to the system to permit Cat3b landings! (As far as I am aware most JAA authorities have not permitted this(cat3b) upgrade to be used although they do allow the OEI autoland when needed.
Any further information on this subject will be avidly read!

[Denti]

There is no restriction from EU OPS about the number of autopilots. In fact the A320 has a dual autopilot system as well, not triple, and of course were are working under EU-OPS rules as well.

Boeing offers a choice between two different manufactures for the 737 autoflight system, Honeywell and Rockwell Collins. The Rockwell Collins autoflight system is allways fail operational capable, however you need to order some additional stuff like the autopilot rudder actuator and the ISFD which serves as the third attitude source to be able to use the fail operational stuff. Most airlines want to save a few bucks, especially for the maintenance of the rudder actuator and dont buy that which limits them to fail passive operation.

[hetfield]

On B727 F40 (full) was recommend for CAT II, for reasons Airbus_a321 already mentioned.

[Vertical Speed]

I stand corrected as regards the number of autopilots on the A320 series-- I had always thought it had three. (you can tell I've never flown AB types!)
However, ALL A320 series a/c do have a three channel a/p and the ability to do single engine autolands. From my own observations however less than 10% of all 737NG aircraft have the Fail-Operational autoland capability and the ability to do OEI autolands. The general advice has always been to use Flap 40 when conducting an autoland in marginal conditions as the chance of seeing the required lights at DH is much greater. Additionally any who, like me ,have conducted practise auto-lands in limiting cross-wind conditions will have come away with respect for the robustness of the 737 landing gear! (It is possible to apply some manual rudder to reduce the crab angle but this technique is officially banned)

[Cough]

V/S

The way I understand it is:

737 (classics) have two autopilots, able to monitor each other.

A320's have two autopilots, each with their own monitor channel.

737 : On a Cat IIIa approach the 737 Cl has an autopilot flying and the other monitoring. In the event of a failure, the system disconnects as there is then no integrity monitoring occurring for an autoland to take place.

A320 : In the event of a failure being detected hands over to the other autopilot. As this A/P has its own monitoring channel it can continue the approach to autoland. Hence this system is Cat IIIb. Each autopilot is capable of a Cat IIIa approach independently.