737ngx can I prevent aircraft from turning the wrong way on to a reciprocal heading?

[leftseatguy 0]

Hi chaps,

 

The other day I was flying the PMDG 737ngx 800 from EGKK to EGCC, to land on runway 05L using STAR DAYNE2A and transition DAYNE. This approach leads to crossing MCT at 3000 feet on track 233 degress, then turning left onto 188 degrees and then turning right through 180 degrees to intercept the localiser for 05L. I was flying with LNAV and VNAV at the time, and the aircraft decided to turn left instead of right to pick up the reciprocal 008 degrees. The reason was fairly obvious, the system cannot know which way to turn when going on to a reciprocal heading!

 

I guess it might have been better to have disconnected from LNAV and flown manually or to go into MCP HDG mode, and set the heading to just less than 008 to force it to turn right? What would the normal practice be, perhaps to fly the whole approach in MCP HEADING and MCP SPEED mode, or fly it manually?

 

I would be interested in any comments.

 

Best wishes,

 

Dennis Hickman

[kevinh 409]

As I understand it you can code a STAR to tell the aircraft which way to turn from one leg to another. If it turns the wrong way in this case it may be the STAR is coded wrong or has left it as automatic (so the AP has to choose).

 

PMDG don't provide the navdata, including the SIDs and STARs, so that part of things isn't their fault. You could examine the STAR code (it's just text) and see whether a turn direction is specified and if so if it is correct).

[vololiberista 205]

If you are heading is say 046 then your reciprocal is 226. But you want to turn onto a heading of 180.

With a normal HSI turning the hdg knob directly to 180 will make the a/c turn right. So the only way to turn left is to select a hdg greater than 226 and wit for 180 to come round past the 6 o'clock position and then select that. So in your example this is how the a/c will behave unless you can programme a forced turn.