Why B737 only CAT IIIA?

[Olendirk]

Hey guys,

whats the reason why the 737NG can only fly CATIIIA approaches and Airbus can fly the CAT IIIB approaches?

Thanks!

OD

[Wizofoz]

Because the 737 (or at least most of them) only have two Autopilots and are therefore certified for dual-chanel, fail passive operations (EG Cat IIIA) whilst the 'Bus has three and are certified for Tripple Chanel fail-safe operations (EG CatIIIB).

I believe a third AP and CatIIIB capability is an option on current-build 737s.

[Capt Chambo]

Also the B737 autopilot(s) has no rudder channel, whereas the 'bus (and presumably everything else that can do a CAT IIIB) does.

[Nil further]

Sorry Wiz , the A320 family has only two Autopilots .
The Cat 3 status is to do with as you say the ability of those devices toconduct fail operational approaches .

Maybe someone here will provide all the technical stuff.

NF

[CaptainSandL]

From Feb 2003 the 737NG was offered with the fail operational (Cat IIIB) Collins enhanced DFCS. This has an optional rudder servo to give rudder controlled rollout and engine-out capability, although the EO facility is not autoland approved. Note the 737 still only has two autopilots, it is the rudder channel that makes the difference.

[FS-chick]

B737NGs may be operated to cat 3B but as far as I know moste operators only use them to cat 3A.
B737-200s and Classics can only be Cat 3A as there is no A/P input into the rudder which you need for rollout.


All of our B737-NG's operated to CAT3-a (no hud) and no rollout guidence.

[Wizofoz]

Note the 737 still only has two autopilots, it is the rudder channel that makes the difference.

Interesting. How does the system work fail-operational with only two autopilots?

[CaptainSandL]

Dunno, but don't take my word for it here are the official specs:

http://www.rockwellcollins.com/ecat/at/EDFCS-730.html?smenu=105

[FLCH-SPD]

I thought that CATIII operations require the autopilot to land the aircraft, how can this be achieved if there is no rudder input from the A/P? Is it assumed that x-winds are not usually a factor in fog?

[Wizofoz]

Thanks ASFCAP, it all makes sense now.

[Gullyone]

I thought that CATIII operations require the autopilot to land the aircraft, how can this be achieved if there is no rudder input from the A/P? Is it assumed that x-winds are not usually a factor in fog?
I was told that the "classic" 737 has a sort of pivoting main legs to absorb slight xwinds. That is why they always seem to be crabbing when taxiing as the gear would remain in its offset position until the next time it was retracted. Is the above correct?
Even the 1-11 had an autopilot rudder channel and managed cat3a (sometimes!)

[CaptainSandL]

FLCH-SPD,
737 Classic autoland x-w limit is 15kts & 737NG limit is 20kts (or 25kts with rudder channel). The aircraft simply autolands with the crab left on – just like in that crosswind video with the Portuguese subtitles that is doing the rounds! Once it has touched down the momentum seems to send the aircraft forward rather than in the direction of the gear, directional control is not a problem.

Gullyone,
The 737 gear does not pivot, although there is a bit of “play” in the links but not enough to increase any crosswind limit.

ASFKAP
I have never heard of dual-dual, I still have a lot to learn about this aircraft!

S&L

[InSoMnIaC]

in order the perform cat2 approaches with autoland on the b737 classic we have a max xwind limit of 5 kts. the autopilot can handle this quite well even without rudder

[Golden Rivet]

Sorry mate, Pallett beat you to it ;)

[CaptainSandL]

Thanks for the info, it might take me a while to digest all that.

BTW I thought avionics engineers were known as "fairies" - nothing to do with their private lives, I think the term came from the lights on Christmas Trees.

Similarly:
Electricians = Sparkies
Engines = Stokers
Airframes = Riggers
Cabin = Cushion stuffers

I am sure there are more.

[sureshav]

hi,

can any one throw some light on what are the sequence of operations a pilot does from start of the flight to end .. and what are the various systems that operate from start to end? for example what the does the pilot do to start the engine from where does the power come to start the engine?

looking forward for some help

[The Real Slim Shady]

Sureshav

Don't be offended, but in this day and age of heightened security you have no chance of having that question answered.

In fact, some nasty men with guns and big dogs might be round your place PDQ asking you questions!!

[panda-k-bear]

...but the bus does have lower minima for DH and RVR than the NG, though, doesn't it? Meaning that the bus is 'more' IIIB than the NG (if you catch my drift). I know what the minima are for the bus but what are they for the NG?