I have been flying a SID recently that basically requires a 180 turn shortly after takeoff, so for the first couple of waypoints I have entered a 230 kt speed restriction to stop the turn being too wide.

 

However, after getting my clearance from ATC I am cleared above the SID's altitude constraints, so I select the altitude in the MCP and press Altitude Intervene to clear the various (built in) height constraints, but it also clears my manually input speed constraints.

 

Is this how the real FMS behaves or is this a fault ?

Afaik this is real. Every press of atl intv removes the next speed restriction. Try using lvl chg instead..anyone, feel free to correct me!

 

J

Once this occurs, Glen, activate SPD INTV and input the 230 into the SPD window and that will reimpose that restriction until you complete your turning waypoints. At that time de-activate the SPD INTV and the aircraft will accelerate to climb speed. You can use the LVL CHG as suggested by Jake or simply remain using Vnav with SPD INTV. Both methods should produce the results you desire.

Jake, Craig

 

Many thanks

 

Good advice using Level Change and Speed Intervention, that would certainly bypass the problem until the turn was complete. I have also noticed a couple of times if VNAV is engaged and I have to climb say 1000 ft from one SID waypoint to the next, the aircraft can easily climb at 5000 fpm - that should send the trolleys crashing down to the back of the aircraft :Doh:

 

Another good reason to control the initial climb with Level Change and/or VS until we are a bit higher.

Or just don't press ALT INTV unless you are already leveled off and it won't clear them out. It will only clear out the next restriction if you push ALT INTV while you are still climbing. (i.e. Not in ALT HOLD or ALT ACQ)