My question is about climb performance on the Airbus A318 especially with PW6122A engines.
Certain times with heavy weight and crossing high altitutes have a negative rate of climb using managed climb. The rate of climb is very inconstant (200,300 ft/min)
I would like to know is is a good technique use a select V/S with select high speed to keep the aircraft wit constant rate of climb (example: M 0.80 with select V/S +- 500 or 600 ft/mn). Sure that we have to use this in high altitutes, when the climb performance is not to good.
This is a good technique? It's recommended? Not recommended? Anybody knows if have something about it on FCTM?
Selecting a V/S higher than the a/c will climb in VNAV(managed climb??) is a very, very, bad idea.
The risk is that you'll select a V/S rate of climb greater than the a/c is capable of flying. Even with CLB PWR the aircraft will slow. Get too slow and you'll be unable to climb. Get slow enough and you can be forced to descend to regain speed or get so slow that the plane gets into slow speed buffet. It can get very ugly.
Aeons ago, a DC10 out of Italy for the States I think, arrived with some 8' of elevator missing. On the climb the crew twice encountered severe turbulence.
I believe the subsequent investigation revealed that VS had inadvertently been selected and infact the aircraft encountered pre stall buffet, twice on the climb, resulting in the shedding of a significant amount of elevator.
Sure if select a high vertical speed (1000, 1500 ft/min) the aircraft will try to keep the rate and the speed will drop fast. But, if we select the rate of 500 ft/min and select the speed to M 0.80 for example, to keep the airplane with constant rate of climb. What do you think about it?
Airbuses would have to be the lousiest bloody climbers above F200 at medium to high weights. They're prone to reducing climb rate to neg esp if encountering a sudden or increased t/w. To maintain a positive rate (and not scare ATC) keep the managed speed and use 500fpm, and maintain a damn good eye on your speed. If you do get distracted for any reason get the PNF to look after it.
The problem with that technique is that if you encounter an updraft or increasing headwind, the thrust will decrease to not exceed 500 FPM/bug speed. You will be limiting yourself to 500 FPM. Why not just let the airplane climb at it's optomized speed and ensure thrust remains at max climb thrust? If it won't maintain at least 500 FPM, advise ATC like you're supposed to. Odds are there will be no conflict and you can continue a slow climb. If not, ATC will have you do an intermediate level-off. This job is easy - don't make it harder than it has to be.
...but in my company, on the E-145, we normally use VS to the FL370 (max FL). The plane slows down during the climb, and we let it accelerate once we level off. Of course, we keep a sharp eye on the ASI during the climb.
I know it's contrary to popular wisdom to use this procedure, but what would be the danger of doing this in an Airbus? Mind you, we do not have autothrust.
The method of using v/s 1000ft/min & Mach 0.8 was used widely in my old company as the A320s we had always struggled to reach cruise levels. It was particularly used when having to make levels by Casablanca airspace when leaving the Northern Canary islands. The reason - the FMGS did a really poor job of maintaining speed (it chased it) and the aircraft made a good impression of a nodding dog. This technique would ensure you were level 10-20nm earlier than the managed mode would allow. It also kept a positive climb throughout. The alternative - enter the hold before entering Casa airspace.
There is no procedure in the FCOM to reflect this method but likewise there is nothing to say not. The risk is obvious but nothing that an attentive crew should worry too much about.
It's an acceptable method in our company. The A320 does tend to chase its speed too much when reaching cruise level at high weight, resulting in fluctuating vertical speeds, which can at times become zero or even slightly negative. My colleagues and I regularly use small V/S values (300-500 ft/min) while selecting a higher Mach (to make sure we get climb thrust) in the final stages of the climb. This gives you a more comfortable and steady climb while letting the speed vary slightly rather then letting the A/P chase the speed while the V/S fluctuates (too much). It's obvious that you should stop reading the newspaper or filling out the sudoku while doing this and keep a very close eye on the IAS to avoid an A/P mode reversion! Anyway, normally the A/P should revert to open climb and get back to it's original speed if you mess up.
Here's what our FCTM says:
".... If the crew selects a high V/S, it may happen that the aircraft is unable to climb with this high V/S and to maintain the target speed with Max Climb thrust, for performance reasons. In that case, the AP/FD will guide to the target V/S, and the A/THR will command up to Max Climb thrust, in order to try to keep the target speed; but the aircraft will decelerate and its speed might reach VLS. When VLS is reached the AP/FD reverts to OP CLB and the aircraft accelerates to initial target speed. Whenever V/S is used, pilots should pay particular attention to the speed trend as V/S takes precedence over speed requirements."
Last edited by sabenaboy; 14th Mar 2012 at 14:15.
Aren't you gentlemen missing an opportunity to hand fly your machines? If your system is not smooth in the way it handles the aircraft, you can probably do better, i.e. be more efficient.
In the old days, it used to be real fun nursing your heavily loaded/underpowered aircraft up to altitude. If you became accomplished at doing this yourselves (manually), you would understand better why your system is having difficulty.
E145 is climbed using VS or FLC because that keeps you within modes that contain under & over speed protection. When I flew that machine, we were banned from using pitch because a crew one day made a mistake, and forgot that pitch does not afford such protection.
On my current type, we use pitch hold, because it's a docile machine to fly and pitch gives stable, repeatable results well within safety margins. We are banned from using VS in the climb, because of the possibility of commanding more than the aircraft is capable of giving, and there being no speed protection in such a mode.
Best advice, use either your operators, or the aircraft manufacturers guidance and try to understand the reasons behind their logic.
As has been said above, if the aircraft is incapable of going up, or staying up, then your likely pushing your luck.
And anyway, the big boys can tell you all about step-climbing!
Aren't you gentlemen missing an opportunity to hand fly your machines?
I'm all in favour of handflying the big jets, but the rules say: "Autopilot should be engaged within RVSM airspace for cruise an d flight level changes." (In Europe all FL's between 290 and 410 included are rvsm)
V/S is a great mode. Smart pilots will use it when appropriate.
I use it exactly as BizPilotBrazil describe it, only in the 737 (although I am probably a couple of thousand feets higher, that he is in his Airbus ). Better to let the airspeed fluctuate a little bit, than huge variations in vertical speed. Thousands of other pilots use it like this as well. No, it is not described in the manual, but come on, do we need a procedure for every technique available?
It's also great for acceleration in a light 737. Eg. when passing FL100 with an acceleration from 250 to say 290 climb speed, VNAV will shove the nose down to less that 500 fpm climb, followed by a pull up to say 3-4000 fpm when reaching climb speed, with such a G-load that you have to use muscle staining techniques to stay conscious! A better way is to set 300 knots (to prevent reduction from climb thrust) and V/S 1800 - 2000 and when passing 285, press VNAV. That way it will accelerate gently and and transition nicely (and with moderate G-loads) to a steady climb speed. Passengers are happier, and we are not flying fighter aircrafts.
Vice versa, it also works great for deceleration in descent, when you are not interested in almost leveling off to reduce, as Level Change does. Set a low speed to keep idle thrust and V/S for the desired deceleration vs. descent rate.
Another great use of V/S is if kept high during descent (for terrain or ATC issues) and coming coming a lot above the path. You already have the speed dialed back to clean maybe. Instead of level change and increasing speed, just dial in initially 5000 fpm V/S. The aircraft will get the nose down and accelerate much faster than with Level Change (thrust will stay at idle due to the low speed set). Gradually reduce the V/S when approaching VMO to stop the acceleration. Chances are that you now almost reached you next cleared level, so now you can press Level Change and watch the speed coming back again and vertical speed reducing to TCAS friendly levels. Great for step downs descent over mountainous terrain. Using this technique, you you will get down a lot faster that with any other autopilot mode and in the 737 you can catch up several thousand feet above the path, with out using speed brakes.
...oh and yes, for it to be safe, you actually have to look at the instruments and watch what the aircraft is doing, like a competent pilot always would.
There's nothing forbidden about it but airmanship and common sense should preside here - if you are climbing at m0.8 in managed climb and get a negative v/s then go to open climb with selected speed and notch off a couple of decimals of mach. If need be take it back to green dot. Then, if something distracts you, you aren't going to DIE. Yes the protections are good, but bad habits form and a later conversion onto another type without protections could result in a nasty accident. You get the same result at the end of the day, but through a safer way of achieving it.
Another example would be glideslope from above, I can't stand guys who select 100 feet.