An old chestnut, use of VS in climb?
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Boeing logic, at least on B744, is that it wants to reach a new altitude in 2 minutes.
So a climb of 2000' in FLCH ot VNAV will give you a ROC of 1000'.
If you are cleared to climb for example 10.000', it will still want to reach the new level in 2 minutes time, and will go to THR REF. , max. Reference thrust as computed by the FMC.
So for small altitude changes, V/S is never needed, the aircraft will give a sensible climb rate anyhow.
So a climb of 2000' in FLCH ot VNAV will give you a ROC of 1000'.
If you are cleared to climb for example 10.000', it will still want to reach the new level in 2 minutes time, and will go to THR REF. , max. Reference thrust as computed by the FMC.
So for small altitude changes, V/S is never needed, the aircraft will give a sensible climb rate anyhow.
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It's a nice way to keep the ride comfortable, predictable and to care about the crew in the back pushing trolleys from the rear to the forward galley. A sudden increase in headwind will make a heavy trolley a pain if not impossible to push if in lvl change (737) 6-7,5 degrees is usually pretty good. I don't care what the econ-beancounters say.
The V/S mode is a good way of controlling the pitch instead of going CWS. If you take a look at the wind at different altitudes prior and keep an eye on the ship you'll be fine and know what to expect but you have to be careful.
The V/S mode is a good way of controlling the pitch instead of going CWS. If you take a look at the wind at different altitudes prior and keep an eye on the ship you'll be fine and know what to expect but you have to be careful.
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TypeIV: I'm with you, but.....
Flying stone: use V/S, if you know what you're doing, otherwise stay with the speed modes (OP CLB / LVL CHG ).
On B767 (mid-90's) an FMC performance improvement package was introduced that improved the intelligence of FLVL CHG and gave less than full power for small climbs. It's what we had been doing via V/S. There was no stall protection in V/S and perhaps someone had wet their knickers at high level.
Going to B733 (late 90's) LVL CHG did not have this nice feature, so we used V/S for the many 1000/2000' step climbs & descents we received in NATS airspace. Then HOT discovered this technique was in use and banned it because crews had not been trained in it. Well, those of us enlightened LTC's had trained it, but it was not company-wide, so forbid it. It had too many risks?????
Now in NG there is mode reversion and low speed protection, and LVL CHG does not have the intelligent mode, so the enlightened ones gave the cabin a smooth ride during all those step climbs. However, that too became heresy, and it is VNAV or nothing until V/S is required for ALT ACQ in RVSM or high risk RA areas.
All caused because there were not too many 'who knew what they were doing'.
I think the DC-10 got caught out in a Boeing trap. They were in a climb in the DC-10 VNAV mode and went into ALT ACQ mode; they then were re-cleared higher, before ALT HLD, and selected MCP ALT to higher level. The climb mode then reverted to V/S and no-one noticed. Or something like that. It was an unawareness of the mode change, but the attitude, speed & V/S should have been a clue.
Flying stone: use V/S, if you know what you're doing, otherwise stay with the speed modes (OP CLB / LVL CHG ).
On B767 (mid-90's) an FMC performance improvement package was introduced that improved the intelligence of FLVL CHG and gave less than full power for small climbs. It's what we had been doing via V/S. There was no stall protection in V/S and perhaps someone had wet their knickers at high level.
Going to B733 (late 90's) LVL CHG did not have this nice feature, so we used V/S for the many 1000/2000' step climbs & descents we received in NATS airspace. Then HOT discovered this technique was in use and banned it because crews had not been trained in it. Well, those of us enlightened LTC's had trained it, but it was not company-wide, so forbid it. It had too many risks?????
Now in NG there is mode reversion and low speed protection, and LVL CHG does not have the intelligent mode, so the enlightened ones gave the cabin a smooth ride during all those step climbs. However, that too became heresy, and it is VNAV or nothing until V/S is required for ALT ACQ in RVSM or high risk RA areas.
All caused because there were not too many 'who knew what they were doing'.
I think the DC-10 got caught out in a Boeing trap. They were in a climb in the DC-10 VNAV mode and went into ALT ACQ mode; they then were re-cleared higher, before ALT HLD, and selected MCP ALT to higher level. The climb mode then reverted to V/S and no-one noticed. Or something like that. It was an unawareness of the mode change, but the attitude, speed & V/S should have been a clue.
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737 (not only NG, but classic as well) has this annoying "Performance Limit Reversion" feature that immediately reverts from V/S to LVL/CHG if airpeed is less than 5 kts than commanded speed and it's not increasing. So no way to select a higher speed to keep the climb thrust and play with V/S to keep approximately the economy climb speed, like you can on the Bus.
What I found out that on 737, CWS P is a very nice way of controlling a heavy aircraft approaching the cruising level - provided you can explain to your colleague in the cockpit that this is an actual mode of the autopilot that can be used in normal operations. And it even reverts to ALT ACQ, like from other modes.
What I found out that on 737, CWS P is a very nice way of controlling a heavy aircraft approaching the cruising level - provided you can explain to your colleague in the cockpit that this is an actual mode of the autopilot that can be used in normal operations. And it even reverts to ALT ACQ, like from other modes.
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If the aircraft is pitching up and down a lot in LVL CHG (up to a point where the aircraft levels off momentarily), than it's better to use V/S. But if you don't want thrust to vary, the idea would be to select a higher-than-normal speed (to keep A/T in climb thrust) and just maintain approximately the desired speed with V/S.
To put in in other terms, let's say you are flying in LVL CHG with speed 280 kts selected and the airspeed is varying a lot, which causes the aircraft to pitch up and down, while having ROC between 1500 and 0 ft/min. The solution could be to just select speed 300 kts, V/S 800-1000 ft/min or so and you'd have a safe speed and a constant climb.
As far as I can remember, you can't do this in a 737 unless your airspeed is strictly stable or increasing, otherwise it reverts to LVL CHG.
To put in in other terms, let's say you are flying in LVL CHG with speed 280 kts selected and the airspeed is varying a lot, which causes the aircraft to pitch up and down, while having ROC between 1500 and 0 ft/min. The solution could be to just select speed 300 kts, V/S 800-1000 ft/min or so and you'd have a safe speed and a constant climb.
As far as I can remember, you can't do this in a 737 unless your airspeed is strictly stable or increasing, otherwise it reverts to LVL CHG.
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As far as I can remember, you can't do this in a 737 unless your airspeed is strictly stable or increasing, otherwise it reverts to LVL CHG.
nor if your SOP's do not allow you to use such apparently sophisticated mind boggling dexterity.
I used to do it on B757 when approaching a cleared level and expecting a further climb shortly afterwards. It avoided a large thrust reduction followed by reapplication of climb power. It caused both a large change in noise down the back and a sense of driving through a puddle. Quite disturbing in the last few rows.
Another use I made of V/S was to avoid levelling off and the accompanying disturbing sensations when ATC said expect '"further climb in 3mins" after passing a now visible TA traffic above. Both required a brief simple explanation to F/O, some of whom picked up the tips.
Some operators forbid such heretic deviation from rigid SOP's.
nor if your SOP's do not allow you to use such apparently sophisticated mind boggling dexterity.
I used to do it on B757 when approaching a cleared level and expecting a further climb shortly afterwards. It avoided a large thrust reduction followed by reapplication of climb power. It caused both a large change in noise down the back and a sense of driving through a puddle. Quite disturbing in the last few rows.
Another use I made of V/S was to avoid levelling off and the accompanying disturbing sensations when ATC said expect '"further climb in 3mins" after passing a now visible TA traffic above. Both required a brief simple explanation to F/O, some of whom picked up the tips.
Some operators forbid such heretic deviation from rigid SOP's.
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My point was if you're below selected speed and decelerating what the hell are you doing up there.
I think you can do what you suggest otherwise how would you accelerate at all?
I think you can do what you suggest otherwise how would you accelerate at all?
