An old chestnut, use of VS in climb?

[oicur12.again]

An old debate I know and certainly less important on modern gen aircraft, but......

22 years of flying 320/330 and I have never used VS in climb at high altitudes. I figure if the ROC drops off to near zero in op clb or clb then its natures way of suggesting you are aiming to high. I have always figured maintaining speed is more important than maintaining a specific roc.

BUT my new airline, MOST pilots have been using VS to maintain a certain roc, usually about 500 fpm, in the last thousand feet or so before level off (320, heavy) and letting the speed wander back occasionally towards VLS.

I find this odd particularly in light of the fact that that the company has issued a notice cautioning flying too high over terrain where mountain wave activity has caused several serious "loss of altitude" events.

Am I missing something here? Do many other operators condone VS up high?

[172_driver]

If you mind the ship, no problem?

A sudden wind shift and you can see pretty exciting rate of climb even at high altitude - at least with the 737NG autopilot. V/S brings about a smoother ride if you tame the horse a bit. Also ATC requests to pass a level within so many minutes can be achieved by trading speed for ROC, within limits of course.

To answer your question, there's no SOP in place here that prohibits its use. So yes, I use it occasionally.

[Goldenrivett]

We use VS with caution at high levels to prevent the machine chasing 1 or 2 knots with large variable rates of climb just before ALT capture.
It's necessary to use VS intelligently so as not to loose speed. In order not to have thrust reductions (should we momentarily exceed the required speed) we select a higher speed in the ATS window.

300 ft per min ROC approaching Optimum ALT is not unusual.

[Chesty Morgan]

Yep, quite often you get people slagging off the "****e" climb performance and useless autopilot when it's pitching up or down to achieve the speed target. It just shows a lack of understanding.

I use VS quite often and something I like to do is display a page on the FMS which has climb N1 on it. As long as you're at that N1 with speed stable or increasing or below the N1 then it shouldn't be a problem. Still got to mind the shop though.

[BizJetJock]

The problem is that everyone who ever stalled or nearly stalled an aircraft using VS at high altitude thought there was no problem as they were perfectly capable of minding the shop...

As the OP says, if the a/c doesn't want to climb then you're probably asking too much, either in altitude or climb speed.
As for being concerned at the RoC varying with a few knots speed change, I've never understood why this is seen as a problem. The attitude rarely varies more than a degree or two.

[Jwscud]

Some aircraft have VS as the only sensible pitch mode in the climb. You learn fairly rapidly to actually fly the aircraft. Another example of automation makes you lazy.

A major harp-tailed loco recently issued a memo to the effect that use of VS in the climb was streng verboten except when rocketing towards a cleared level as I imagine they have had the odd low speed event related to it.

Such events are indicative of very poor mode understanding, monitoring and common sense, despite the sensible mode reversion logic built into the 737 which should protect you.

In flying bizjets solely in VS and without autothrust up to 490 we never had any speed events (in an OFDM monitored operation) whereas it seems to happen with depressing regularity in more automated airliners, albeit with a far larger fleet and higher sector numbers.

[RVF750]

I guess 737 jockeys would use it more as the thrust is a tad more sporty up high than on the Minibus...

To be honest, my last operator stipulated a maximum ROC in the last 2k and 1k before level off which made changing to VS almost always necessary to comply with. Then again the flights were often 1hr sectors so even with a full house you could almost always shoot straight up to FL390 or even FL400 with no issue.

In my humble experience, The "Airmanship" element is to watch out for AOA as once you let the speed decay past a certain point you simply cannot accelerate back to the FMC climb speed without stopping the climb. On longer sectors, I prefer a good margin either side of the current speed to eeking out the last drop of efficiency according to the box...

[oicur12.again]

“…if your ROC drops to near zero in CLB or OP CLB, I'd think that may be nature's way of saying your speed (selected or managed) is a tad high….”

Yes, of course. Obviously speed would be reduced first to achieve the best ROC, that’s what I have always done.

“As for being concerned at the RoC varying with a few knots speed change, I've never understood why this is seen as a problem.”

Exactly, kind of what I am getting at. There is a lot less “shop” minding needed if you accept a variable ROC with a fixed speed. Distractions approaching top are generally something out of your control.

[Capn Bloggs]

Such events are indicative of very poor ...... common sense
That would be the commonsense that dictates "don't climb in VS", so you will never have a low speed event? ;)

[galaxy flyer]

Having flown mil heavies w/o anything other than pitch and bizjets that actually prohibit climbing at high Mach/levels in FLCH, I fail to see why VS is so much of a problem. You are FLYING the plane, pay attention. Once the VS starts getting below 1000 fpm and above 400, I just set 300-400fpm and watch the speed--if it won't hold the Mach, stop climbing and ask for lower level.

GF

[Check Airman]

My current company doesn't prohibit using VS, although there's something in the manual that basically says "pay close attention if using VS".

At my previous airline, 90% of the pilots used VS in the climb. I never liked it. The AP had a pitch hold mode which I'd prefer to use. Never a dull moment though, as I had a CA criticise my use of pitch hold mode, as I was "more likely to stall at altitude in PITCH mode than VS mode" :D

[beardedclam]

I am VERY new to the bus, 300 hours on type, and have most certainly been taught to us VS as we struggle the last 1000 or so feet. I have been taught, to avoid flying like a dolphin, to select a sensible VS and glue my eyes to speed and VLS. (It does vary skipper to skipper.) There have been memos stating the obvious, advising us to be careful......

[Chesty Morgan]

Never a dull moment though, as I had a CA criticise my use of pitch hold mode, as I was "more likely to stall at altitude in PITCH mode than VS mode" :D

I hope you asked him for an explanation.

[FullWings]

I’ve been witness to a fair few interesting events brought on by the use of V/S at altitude. This is not to say it can’t be done but why increase your workload and the possibility of something undesirable happening?

Approaching optimum flight levels and above, buffet margins are considerably reduced and you need speed stability much more than anything else. Excess thrust is also reducing and in many latitudes you are often close to the tropopause, where there can be marked fluctuations in wind and temperature over a short vertical distance.

Modern aircraft, especially FBW ones, are very good at masking the reality that your flight envelope has shrunk considerably compared with lower down and the ability to climb/manoeuvre is similarly restrained. What they can’t do is override the laws of physics and if you get too slow, you’re going down again, whether you like it or not. LoC.

If you want to get to your cruising altitude in the shortest time, keep climb thrust on and fly the aeroplane-recommended optimum speed/mach until the level is reached. Using V/S, if the speed goes above optimum, thrust reduces and if the speed gets low, drag increases. Both effects will increase the time and distance required to get to cruising level.

[Wageslave]

It seems sensible to me to restrict vs mode to occasions when you have a thrust margin capable of maintaining speed. At high altitude in the climb loss of speed is likely to compromise your buffet margins, not a sensible place to be using VS imo, and in descent a high r.o.d. in vs can cause overspeed. Who cares if the aircraft porpoises a bit on the final climb? Does it matter? It probably indicates you are aiming for a level close to the aircraft's limit (esp airbus) anyway, and then surely you need to protect your speed above all.

My general rule of thumb was if there was spare thrust (in either direction) vs was available, once thrust was at or close to either max or idle it wasn't the right mode.

OK, sometimes you'd use vs to bump a sluggish airbus up the last 100ft or so at the expense of a couple of knots, but not much more.

[dream747]

Many Skippers have advised me that it is okay as long as we understand what we are doing and conscious of the risks and really mind the shop especially at higher altitudes.

However from experience, looking from the risk management perspective, as long as everything is normal when both pilots are aware and consistently paying attention to this matter it's really no big deal. What about times where you are presented with problems or ECAMs etc, it is very easy to let that slip off your mind when both of you have your mind on the issue at hand. I know it says Aviate first, but I've been caught myself with us busy with the ECAM and asking for weather deviation etc, and the speed meanwhile is dropping as the aircraft is climbing away.

[Jwscud]

As ever, the key is monitoring and airmanship. I regularly fly with colleagues who start paperwork or generally non-pertinent stuff before top of climb rather than monitoring. As galaxy flyer says, flying. An aircraft with VS only acquaints one closely with the required technique and pitfalls. One's hand is almost permanently twiddling the VS knob at high altitude and it becomes second nature very quickly.

If one is used to flying an aircraft with VNAV/FLCH/OP CLB, for the majority of the time reacting to minor speed trends is not required so you become desensitised to it. Then, when using VS close to OPT/MAX, a wind shift or lack of attention can cause problems.

Like anything, it is a tool to be used when appropriate, rather than banned by SOP-heavy airlines.

[Calmcavok]

If you've ever been sat behind a pair of GE90s when a step climb has been initiated in VNAV, then you'll know about it! I don't know what is more startling, the roar as they spool up or the feeling of doom as they return to cruise thrust and you have the sensation they've both failed!

Of course every situation is different, but my preferred option is a carefully monitored step climb using v/s, usually around 500fpm. Keeps the noise down in the back allowing pax, and crew in the bunks, to snooze undisturbed. If the situation warrants VNAV though, then VNAV it is.

[FlyingStone]

In UK, you have to advise ATC (according to AIP) any time rate of climb/descend is less than 500ft/min, which happens quite often in CLB/OP CLB.

My opinion: use V/S, if you know what you're doing, otherwise stay with the speed modes (OP CLB / LVL CHG ).

[megan]

http://www.fss.aero/accident-reports/dvdfiles/US/1979-11-11-US.pdf

Precis: November 11, 1979 - an Aeromexico DC-10 entered a sustained stall while climbing through 29,800 ft, to its assigned cruise altitude of 31,000 ft, over Luxembourg, Europe. The flight crew failed to monitor their flight instruments, so they did not immediately recognize the plane was in a stalled condition. Instead, they blamed the heavy buffeting on the #3 engine, which they shut down, while continuing to hold the nose up. The plane continued to descend for one minute in a fully stalled condition, until the pilots lowered the nose and began a proper stall recovery procedure, which was completed at 18,900 ft. The #3 engine was then restarted, the declaration of emergency canceled and the flight continued to Miami, Florida. Ground inspection revealed four feet missing from each of the outboard elevator tips, including the balance weights. The NTSB concluded the sustained stall buffeting produced a dynamic structural overload on the elevator, which resulted in the failure of the elevator tips and balance weights. Further, the NTSB concluded the autopilot had improperly been placed in vertical speed mode. That forced the AP to keep increasing the angle of attack, to maintain the preselected vertical speed number, because maximum available engine thrust declined (normally) with increasing altitude. That in turn, caused the airspeed to fall below the stall speed of the aircraft.

[Icelanta]

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.

[TypeIV]

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.

[RAT 5]

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.

[FlyingStone]

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.

[Chesty Morgan]

FS, why you would be using VS is you're below selected speed and decelerating anyway?

[FlyingStone]

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.

[RAT 5]

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.

[Chesty Morgan]

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?