In my current outfit we fly the A320 and most of our CPT push the Mach/Speed pushbutton when we face turbulence conditions when flying in cruise. Do you guys, flying the Bus or any other jet, do the exact same thing?
I am a newbie but I don't feel it to be safe to fly IAS at (say) FL390... On the A320 you should fly 250KIAS up to FL200 and 275/.76 above that altitude. Once you pass the speed crossover why should you fly IAS?... The fundament this decision on (by flying selected IAS) the temperature will not be an extra factor on the change of Mach Number and therefore th flight will be smoother and more safe...
Any thoughts on this one? Did I manage to explain myself?
Thanks in advance.
Last edited by C212-100; 3rd Nov 2011 at 17:59.
Reason: Change in title
I've done it before, but it has nothing to do with trying to smooth out the ride.
With "ALT CRZ" displayed on the FMA, the airspeed is allowed to fluctuate quite a bit. This is usually a good thing. But in turbulence, the speed will often go to the low limit, then thrust will surge, until you are up against the barber pole when thrust will decrease dramatically. THAT is actually unsafe. Pulling speed and selecting IAS forces the autothrust to maintain speed much more accurately.
Some 6 years since I flew the A320, but can't you enter a new Cruise Level in the FMGS which will then change 'ALT CRZ' to 'ALT' which also changes the A/Thr responsiveness, which is the fundamental issue?
I agree that your technique would work, and have indeed used it myself!
I was just pointing out an alternative method.
I remember trying to make a night curfew at TXL, and gettting direct to the OM from near the UK east coast. As I recall the A320 is very speed twitchy at M0.80 and above (Mmo 0.82), so we did use both techniques (ALT + IAS hold) which worked well - the Ground Engineer had left the destination assuming that we would have to divert to SXF, but we touched down with about 3 seconds to spare! Well worth the extra fuel burn.
"The soft altitude mode engages when the aircraft reaches the FCU altitude set as the cruise flight level (entered in the F-PLN or on PROG page).
The soft altitude mode corrects minor deviations from the Mach target by allowing a ±50 ft variation from the CRZ FL. This feature improves fuel efficiency and passenger comfort and minimizes the changes in thrust."
So by either going ALT (as opposed to ALT CRZ) or Speed from Mach you are effectively 'stiffening' up the response to BOTH Speed and Alt?
The speed control I can get, but surely a 50ft margin in turbulence might make it more comfortable and less stressful for humans and machine?
Interesting. I knew about soft alt but not about the speed/mach pb.
The other day a line trainer on the A320 changed to IAS for keeping the A-thrust more responsive in the turbulent weather. Later I was checking the FCOMs, to confirm that the A-thrust is more responsive, but I can not find anything about this in there.
I believe that Soft Altitude Hold is only active when in Alt Crz and Managed Speed and I think it needs to have transited to Mach.
When you fly short relatively low sectors (below FL300) e.g. LHR MAN or even AMS the 320 doesn't go into Mach, it stays in speed (although in managed speed). Therefore Soft Alt Hold is not active with Alt Crz and Managed Speed.
In TX it is useful to simply pull the speed knob and use selected Mach, this takes it out of Soft Alt hold and the A/T will target the speed much better. Select .76 if it gets worse. In Soft Alt the speed can wander around a lot before the a/c reacts.
I press the mach/speed button sometimes as well to nudge the speed back a few knots rather than a whole mach number.
my 2 cents answer... the alt crz mode allows 50' change in altitude before actually moving any a/thrust setting this is to relieve stress on the airframe and its contents... you can disconnect it as well, setting the recommended turb n1... meaning like 'don't touch anything in turbulence unless really needed'... at higher altitudes fly mach i/o ias as a result of air compressibility... meaning less a/thrust response for deviation unit,... thus more comfy ride. happy landings all
Happy 4th of July to the USA people. We are now watching the fireworks across the bay with our grandkids. The 10 yr old girl has been blowing the conch shell for 15 minutes so came to the den to let my ears relax. Nice display of something very important to our people. I think it made a difference to the world too, but most won't admit it. WWll was a bit of a toss up in my opinion. Maybe without our support it wouldn't have ended up like it did. Now back to aviation talk.