View Full Version : Descent mode with turbulences

23rd Apr 2006, 17:26
When Turbulence is present and the airspeed indicator move fast from ( ej: 250 to 240 ias or something so ) What descent mode do you prefer to use ? FLCH, VS, or manual mode
With FLCH you will find that the aircraft pith down or up very fast trying to maintain the preselected ias.
With VS and A/T on, the aircraft will maintain the preselected VS and the preselected ias. This will lead to a constant increase and decrease in power. Perhaps it could be a possible to fly without A/T on, but I am not sure.
In manual mode you will fly attitudes and its perhaps the most confortable option for such scenario.

In the md83 you will find the TURB mode. If you select it, this will happen:

--Vertical speed window is now: degrees AND or ANU ( attitude nose down or up ) you select then the attitude you want, and it will be flown by the autopilot
--Automaticaly it will revert to HDG HOLD mode, then you can change to HDG, is only to prevent overbanking with heavy turbulences


Empty Cruise
23rd Apr 2006, 18:43
Your AFM and/or OM should contain a section on adverse weather, including turbulence. Normally, you want to fly a speed just below Va, since this will give you optimum protection from both stall & overstress.

On the 737, we fly .73 (.76 for the NG) / 280 KIAS - however, when mass is below 50T, we reduce this to 250 KIAS.

I know naught about the MDs, but it sounds a s though the TURB-mode you describe is more minded on being a reactionary mode rather than a preventive mode, i.e. if the turbulence becomes so serious that maintaining speed/atitude might overload the aircraft - we have a (almost corresponding) setting called CWS, where the AP maintains the attitude we have set. Also we have to disconnect the A/T and set turbulence N1 manually - so it sounds as a smart idea to have it all combined in just one push-button.

However, we wouldn't go to CWS/ Turb N1 untill we actually hit turbulence of some severity - up to that point, we'd just fly .76/280/250 with A/T engaged & normal lateral & vertical modes. For descend, if we feel it prudent to limit the vertical speed & have the engines spooled up, we'd use VS & MCP SPEED.

Hope this helps - cheers fm