Boeing aircraft decent technique?

[cva1077 68]

Ah-I think I misunderstood your other post, sorry. I thought you meant that V/S was "a thing of the past" for all contexts, not just descents.

 

Again my apologies.

[777simmer 117]

No need to applogize :-)

 

You picked up how I feel about V/S correctly ;-)

I am a bit biased against the use of V/S unless needed.

 

I hope PMDG is able to intergrate Lnav/Vnav approaches in their B777 as well.

It is really cool stuff you know.

With Lnav Vnav you can fly any non precision approach just as accurate as an ILS.

No wind correction required, no timing required, no V/S required calculation required, no busting of intermediate approach altitudes, etc - real nice :-)

 

 

[kevinh 409]

Not really, in one of the ACA 777 vids(can't remember which) they used v/s(500fpm) for the step climbs because VNAV and FLCH are too aggressive for short maneuvers like an s/c.

On the 777 (also 747 and 767) VNAV and FLCH shouldn't be too aggressive. FLCH is designed to increase thrust gradually to achieve the altitude change in a set time, while maintaining airspeed. Something like four minutes, but I'm relying on my hazy memory here, not facts I can check. So for small altitude changes thrust may not need to reach the CLB limit. So it behaves rather like V/S, but with full speed protection. For larger level changes this nominal climb rate is unachievable so the thrust eventually reaches the CLB limit and the vertical speed is then whatever can be achieved.

 

VNAV is different in that it is aiming for a flight path angle, rather than a climb rate, but thrust changes to get to CLB or IDLE are still slow, not sudden.

 

In the 737 and 757 LVL CHG (FLCH for 757) thrust goes rapidly to CLB, and in that case the change may be too aggressive for small changes in fllght level.

[MatthewL 16]

I actually got a chance to enter the cockpit (not during flight of course ) and had a quick talk to the pilots. My most common question is, what will be the technique for climb and descent? I usually always get the answer "VNAV"."V/S" seems to be used for small altitude corrections.

 

Of course corrections can be made using Alt Intervention (ALT INTV)

[777simmer 117]

Yes thats about right if you dont want to get into the lengthy discussion we are having here :-)

 

What do you mean by corrections?

You mean a climb/descend?

 

Altitude intervention is for Vnav only.

Not for FLCH or V/S.

And Vanv can be (and usually is) used for all normal climbs and descends, both small and large (for exceptions see earlier posts)

 

You really need an FMC manual to see what the Altitude Intervention feature does in each situation as is can give you:

During cruise - initiate the descend now feature (same as FMC descend now line select key)

During FMC climb/descend mode - it can erase one altitude restriction each time you push it.

After an intermediate level off (departure/arrival)- a further climb/descend to the new set altitude.

During cruise - a step climb or step descend to the new cruise altitude while at the same time updating the FMC to the new cruise level so that performance calculation (fuel/time predictions) are correct. (something that FLCH and V/S do not do automatically)

[jsscvtx 0]

Below is an interesting link I found about VNAV.

 

It is a NASA report from 12/1/2000. It's a pretty interesting read, and after skimming a bit of it seems the report is about it's difficultly of use for a pilot, the flaws from an engineering standpoint, and how or what could make it better.

 

Again I have just skimmed it, so my brief summary above is just what I picked up from skimming.

 

Here's the link.

 

A Cognitive Engineering Analysis of The Vertical Navigation (VNAV) Function

 

http://human-factors.arc.nasa.gov/publications/20051025130903_VNAVCogEngJournal1TM.pdf

 

John

[cva1077 68]

FLCH is designed to increase thrust gradually to achieve the altitude change in a set time, while maintaining airspeed.

 

I always thought that FLCH was to get the max Vertical Speed while holding airspeed without having to fiddle with the vertical speed wheel.

[777simmer 117]

 

 

I always thought that FLCH was to get the max Vertical Speed while holding airspeed without having to fiddle with the vertical speed wheel.

 

Wel,l you are half correct.

FLCH is a speed mode. It holds airspeed or Mach.

On the 737 this mode will use idle Thrust during descend and climb thrust during climb.

On the 777 this mode will vary thrust to reach the desired altitude after two minutes if it can. If it can not then it uses Idle or max climb thrust just like the 737.

(I noticed I made a typo in my earlier post. Its two minutes, not one!)

 

If your goal is to get max vertical speed (go down like a brick or up like a rocket you would:

During descend - use idle thrust and increase airspeed up to limit airspeed by pitching down (can be achieved with FLCH or V/S mode with a little trick) and/or use of speed brakes.

During climb - use max climb thrust and decrease airspeed down to a safe but slow airspeed by pitching up. (can be achieved with FLCH and V/S mode with a little trick)

[cva1077 68]

On the 737 this mode will use idle Thrust during descend and climb thrust during climb.

On the 777 this mode will vary thrust to reach the desired altitude after two minutes if it can.

 

Ok, I'm more familiar with the 737 at the moment, I just assumed that all Boeings acted the same-obviously not! Thanks for clearing it up.

[scandinavian13 5,192]

During climb - use max climb thrust and decrease airspeed down to a safe but slow airspeed by pitching up. (can be achieved with FLCH and V/S mode with a little trick)

 

Not quite. Max angle, sure. Max rate, not really.

[777simmer 117]

 

 

Not quite. Max angle, sure. Max rate, not really.

 

You are touching on a new subject.

Max rate vs max angle of climb is not what I was talking about when I replied to this:

quote

"I always thought that FLCH was to get the max Vertical Speed while holding airspeed without having to fiddle with the vertical speed wheel."

unquote

 

I was just saying that the main purpose of FLCH is not to get max vertical speed but to climb/descend at a fix airspeed.

And continued to explain how FLCH can be used to increase/decrease rate of climb/descend.

Of course you can only increase/decrease speed so much, so naturally this method can only make you dive like a brick or climb like a rocket for a very short time.

This method is very often used to comply with ATC insteuction to reach a certain altitude quickly (expedite) for traffic reasons.

 

Neither max angle nor max rate speed will give you the enormous V/S increase (like 6000ft/min) that I had in mind when discussing how to get max V/S.

But like I said, its only for a little while that you can maintain these V/S

 

Max angle and max rate speed are realy not used that much in modern aircraft like the B777.

You would usually fly ECON (cost index) and then if required by ATC (expedite climb/descend) use the described FLCH method (or even better, you can use Vnav speed intervention).

 

 

[scandinavian13 5,192]

You are touching on a new subject.

 

Fair. It's a new topic and not directly related to what we're discussing, but I didn't find your generic explanation very accurate.

 

If your goal is to get max vertical speed: [...]

During climb - use max climb thrust and decrease airspeed down to a safe but slow airspeed by pitching up. (can be achieved with FLCH and V/S mode with a little trick)

(emphasis mine)

 

Best rate of climb (the best climb given a period of time -> best change in altitude / time -> best change in feet / minute -> best vertical speed

 

The only time a climb would have a goal of slow airspeed would be best angle, because the slower speed means you're moving forward less over the time it takes to climb. Sure, FL CH is a speed hold via pitch mode, but I doubt a pilot would pull the speed back to "slow but safe," as econ speeds (which are often close to vy because you want to get up to cruise ASAP) are on the higher side of the envelope. Slow speed is not at all a goal.

[Avantime 81]

Note there are things called continuous descent approaches or CDAs (for noise abatement reasons, you'll bust your clearance if you level off) and with those it's almost always VNAV all the way down as only VNAV obeys crossing restrictions in the FMS. FBW Airbuses have a choice of managed descent (Airbus term for VNAV) or FPA. If you don't have VNAV however you'll need to use V/S combined with periodic range checks by ATC and a bit of mental arithmetic. 

 

People don't like V/S because it doesn't provide speed protection (except FBW Airbuses I think) and you can actually stall or overspeed the aircraft as the autopilot tries to maintain the commanded V/S.

[777simmer 117]

 

 

People don't like V/S because it doesn't provide speed protection (except FBW Airbuses I think) and you can actually stall or overspeed the aircraft as the autopilot tries to maintain the commanded V/S.

Correct. Which is why I am a little biased against the use of V/S as a default descend mode.

If you need it fine. But not as a standard descend mode.

[777simmer 117]

 

Not quite. Max angle, sure. Max rate, not really.

Fair. It's a new topic and not directly related to what we're discussing, but I didn't find your generic explanation very accurate.(emphasis mine)Best rate of climb (the best climb given a period of time -> best change in altitude / time -> best change in feet / minute -> best vertical speedThe only time a climb would have a goal of slow airspeed would be best angle, because the slower speed means you're moving forward less over the time it takes to climb. Sure, FL CH is a speed hold via pitch mode, but I doubt a pilot would pull the speed back to "slow but safe," as econ speeds (which are often close to vy because you want to get up to cruise ASAP) are on the higher side of the envelope. Slow speed is not at all a goal.

Yes you are correct, and you know your theory well.

I worry that my replys are too long and complicated already and maybe more than most fellow simmers require, so I did not realy want to go into a lesson on jet aircaft climb/descend performance.

 

I was just giving examples from the praxis.

Try to imagine you are climbing to FL350, passing through FL200 with 310kt.

Now imagine ATC asking you to expedite your climb untill passing FL240 (because he has oposite traffic that he wants you to be above before you get too close)

 

Whatever Vy or Vx is, it does not matter!

 

What if Vy is 330kt?

Are you going to accelerate first so that you can then climb at best rate?

Oviously not because during acceleration you loose so much climb performance that you can never comply with ATCs instruction.

What if Vy is 300kt?

You roll back speed from 310kt to 300kt?

This will give you a small climb boost, but again (maybe, most likely) not enough to comply with ATCs instructions.

 

I can assure you that reducing airspeed is excactly what pilots do.

Use Vnav speed intervention and roll the speed back to 270kt for example.

You are trading kinetic energy (speed) for potential energy (height) and you will be passed FL240 in no time even though you are not at Vy or Vx.

 

ofcourse thereafter you want to continue again with ECON climb speed (so accelerate again) because as you said you want to get up to your cruise altitude asap (and 270kt is probably not going to achieve that on the long run) for best ecomomy.

 

You see what I mean?

You are talking performance over a longer time.

I was talking about a short climb/descend performance boost.