Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

'One click on the CAB bug please'?

Tech Log The very best in practical technical discussion on the web

'One click on the CAB bug please'?

Old 5th Jan 2002, 09:39
  #1 (permalink)  
Thread Starter
 
Join Date: Dec 1998
Location: The Sandpit
Posts: 34
Likes: 0
Received 0 Likes on 0 Posts
fish 'One click on the CAB bug please'?

I'm a B733 F/O and I'm seeing this called a lot by some of the newer captains on our fleet, in some cases being called for just as the gear comes up. What's the deal here?

As you know, pressing TOGA brings up the N1 and TOGA FMAs and the autothrottle goes to TOGA while pitch is controlled to target CAB+20 (V2+15-25). One click on the CAB bug resets it to 15kts higher than originally set, but I haven't seen FD commands in response to this increase in target speed (shouldn't we be seeing V2+30-40 in this case?)

So all in all, I can't see what we're achieving here. Maybe I've missed something. Any takers?
Friendly Pelican is offline  
Old 5th Jan 2002, 15:27
  #2 (permalink)  

Dutch Roller
 
Join Date: Dec 1999
Location: World
Posts: 76
Likes: 0
Received 0 Likes on 0 Posts
Post

Apperently I've also missed something the last couple of years; What is CAB??? <img src="confused.gif" border="0"> <img src="confused.gif" border="0"> <img src="confused.gif" border="0">
Track is offline  
Old 5th Jan 2002, 22:58
  #3 (permalink)  
Thread Starter
 
Join Date: Dec 1998
Location: The Sandpit
Posts: 34
Likes: 0
Received 0 Likes on 0 Posts
Talking

Ah, the johs of TLAs!

Command Airspeed Bug: I think I've also seen it called the cursor.

...three letter abbreviation
Friendly Pelican is offline  
Old 6th Jan 2002, 03:37
  #4 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 7,179
Received 93 Likes on 62 Posts
Post

I haven't played on the 733 for some time so I may be a little out of date ... but ..

(a) why would you want to bug up 15 knots anyway ? I am having difficulty seeing what purpose might be served ?

(b) why do something which appears to be quite unnecessary at a critical stage in flight .. such as just after lift off with the gear coming up ?
john_tullamarine is offline  
Old 6th Jan 2002, 12:19
  #5 (permalink)  
 
Join Date: Dec 2001
Location: Australia
Posts: 81
Likes: 0
Received 0 Likes on 0 Posts
Post

You certainly should not be calling for it just after lift off, but 'bug up one click' is nice at say 500ft, when you have done a flap 15 takeoff, want to go to flap 5 (to get rid of some drag) fuel)and leave the flaps at 5 for a while, as in a close-in 180 degree course reversal. No point in accelerating to some higher speed while heading in the opposite direction to your destination. Also in amongst rocks etc you might want to keep the turn radius down until near outbound track, so the 'bug up one click' does have its uses. Otherwise it wouldn't be an available feature?.
fruitbatflyer is offline  
Old 6th Jan 2002, 15:53
  #6 (permalink)  

Dutch Roller
 
Join Date: Dec 1999
Location: World
Posts: 76
Likes: 0
Received 0 Likes on 0 Posts
Post

Thanks, should have known I guess...

In Europe at most major airports you have to climb with T/O flaps setting and speed around V2+15 until reaching 3000ft, then clean up and accelerate. You are not allowed to climb with higher speeds before 3000ft due to noise regulations. Do you have the same regulations?

Regards, Track
Track is offline  
Old 7th Jan 2002, 04:55
  #7 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 7,179
Received 93 Likes on 62 Posts
Post

.. is it not standard, though, to set a white bug at the scheduled V2 + 15 ?

I still cannot see any need for the apparently non-standard and unnecessary extra action ?

[ 07 January 2002: Message edited by: john_tullamarine ]</p>
john_tullamarine is offline  
Old 7th Jan 2002, 07:01
  #8 (permalink)  
 
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 5,787
Received 110 Likes on 54 Posts
Post

You used to see it in Ansett on ICAO A noise abatement procedures in Sydney.

In the take-off mode for the flight director, the FD will command MCP speed (bugged at V2) + 20 knots (after achieving sufficient climb that is).

As the ICAO A noise abatement requires that you maintain V2 + 10 to +20 until 1500 ft, and max V2 + 20 to 3000 the flight director is set correctly.

Although the manual bugs on the ASI are set, with one at V2 + 15, the speed tape only shows the magenta mark at MCP speed (V2). Asking for "Bug one click" resets the MCP Speed bug on the speed tape to the target speed you are looking for, matching the command you are recieving from the Flight Director.

As the climb to 3000' takes a while, it is a good thing to have the speed tape bug and flight director commands agree, on other take-offs it isn't necessary, as you usually select a speed mode before the FD hits the V2 +20 limit in any case.
Checkboard is offline  
Old 7th Jan 2002, 08:49
  #9 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 7,179
Received 93 Likes on 62 Posts
Post

That makes good sense, provided that it doesn't infer a breakdown in normal panel scan patterns ?

Being something of a dinosaur, I always found the speed tape to be a bit of a pain in the 733/4 setup .... but, then again, I guess that one ought to view that installation as a developmental stage of the evolution of data presentations.
john_tullamarine is offline  
Old 7th Jan 2002, 21:58
  #10 (permalink)  
Thread Starter
 
Join Date: Dec 1998
Location: The Sandpit
Posts: 34
Likes: 0
Received 0 Likes on 0 Posts
Post

Thanks everyone.
Perhaps I wasn't too clear when I asked the question, but we seem to have got there in the end...
Airmanship aside, (yes, I agree it's less than ideal, but it's LHS prerogative; yes I understand close-in turns and reflapping in the turn, do them twice a day, but generally at a nominated airspeed, not 'one click'; etc etc - I wasn't really looking for a pissing contest <img src="wink.gif" border="0"> ) all it seemed to achieve was synchronisation between FD pitch command and target airspeed on the ASIs, as you'd expect in other modes. What I wanted to know, down deep, is this: apart from making the picture look nice, does it achieve any real benefit; what about the engine failure mode, is command pitch still v2+15, presumably so, but what monsters lurk behind?
Thanks again

(Edit for the usual, and far too frequent spelling srcew ups <img src="smile.gif" border="0"> )
(D*mn: missed one!)

[ 07 January 2002: Message edited by: Friendly Pelican ]

[ 07 January 2002: Message edited by: Friendly Pelican ]</p>
Friendly Pelican is offline  
Old 8th Jan 2002, 06:08
  #11 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 7,179
Received 93 Likes on 62 Posts
Post

I shouldn't think that there are too many significant traps provided the pilot still flies the base pitch attitudes and maintains a sound instrument scan ...
john_tullamarine is offline  
Old 8th Jan 2002, 07:26
  #12 (permalink)  
 
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 5,787
Received 110 Likes on 54 Posts
Post

As you haven't selected a flight director vertical mode, the flight director is still in take-off mode, engine failure operation will not be affected.

Engine failure for the flight director commands V2+20 in anycase, after achieving that speed.

Incidently, should you just reach over and engage an autopilot (above 400'), then the bug does the same thing - it jumps to the V2+20 knot position to match the Flight Director, but in addition the pitch mode changes to LVL CHG and pitch mode FMA is MCP SPEED. "Bug one click" keeps the FD in take-off mode, with take-off power.
Checkboard is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.