Sign in to follow this  
Guest Ian_Riddell

Alt Sel Bug and C-chord set off by FMC restriction

Recommended Posts

I noticed this before and after the latest patch....In VNAV climb and prior to TOC, I decide at the last minute to set an FMC altitude restriction at the next waypoint. The predicted altitude for that waypoint is below cruise altitude. My MCP Alt is set at, say, 32,000'. I set the altitude restriction at 30,000'/FL300. When the aircraft passes through 29,300', I hear the Altitude Alert C-Chord Aural and see the Alt Sel Bug sitting on 30,000'. It's very difficult to resync the Alt Sel Bug with MCP Altitude after this.Anyone else experiencing this?Thanks.Cheers.Ian.

Share this post


Link to post
Help AVSIM continue to serve you!
Please donate today!

I don't get what you mean by *resync* the bug? What I think I see here is that you set a restriction on the FMC @FL310 for ROBIE, the aircraft shows the constraint on the PFD and has reached FL310 and correctly leveled off, FMA shows FMC SPD (correct), what will happen here is the moment that you reach ROBIE, the FMC will command a resume CLB to your set CRz ALT of FL320. THe *Bug* will shoot to 320 on the PFD once you reach ROBIE...[h4]Best Wishes,Randy J. Smith [/4]P M D G 7 3 7 NG[/h4]http://img.villagephotos.com/p/2003-8/196432/mineimage.jpg [h3] Realism on the horizon AMD XP 2200 |MUNCHKIN 512 DDR RAM |ECS[/b ][i] K7S5A MB[/i] |GF2 MX 32 MEG and still runs GOOD!|WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"[/h3]

Share this post


Link to post

Randy,Quick question I am at work and cannot get at the FMC manual. When look at speed/altitude constraints on the legs page they are usually in a large FONT. If I click the LSK next to the constraint the font becomes smaller. Now once on route example KSFO to KLAX flying the SID out I noticed that when I clicked on the constraint I was heading to the font becomes smaller and the altitude constraints change along the waypoints of the SID right down to the STAR into KLAX...namely the CIVET into 24L.Can you elaborate more on the speed/altitude capabilities of the FMC prcedurally from the crews perspective. When they file their flight plan and changes occur are they updating the FMC with new constraints ro just setting the MCP and utilizing LVL Change?Thanks ahead of time. Tony Ascaso RN

Share this post


Link to post

Large font setting are hard set.Small font setting are dynamic, not locked in stone.If you are inserting large font settings in your legs page, you are breaking the cardinal rule.Speed is fixed in climb mode. (climb/1/2 thrust)Speed is fixed in cruise mode. (cruise thrust)Speed is fixed in descent mode. (no thrust)Since you have fixed speed (1 of 3 variables), fixed location (navaid/fix/waypoint), fixing the 3rd variable (altitude) makes for a puzzle for the FMC to have a heart attack over.Only place you should see Large font on your legs is during SID/STAR legs. Other times, use your MCP and FMC Cruise ATL settings. You'll find life is much easier without Vnav going nuts on you.Ray

Share this post


Link to post

"I don't get what you mean by *resync* the bug?"On all the Boeing aircraft I'm familiar with, Randy, the bug (magenta spectacles) on the altitude tape will show what is selected in the MCP window... no exceptions. It has nothing to do with the FMC. The Altitude alert, too, is always linked to the altitude selected in the MCP window also and has nothing to do with the FMC. If the 737NG is different, I would be very, very surprised (despite all that I've learnt about the 737 in recent weeks).Anyway, before I reached ROBIE, I tried to change the MCP altitude to see what would happen.... the bug moved, but it was always 1000' away from the altitude in the MCP window.Hope this makes sense?Cheers.Ian.

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this