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Spin737

V/S or Not to V/S - That Is the Question

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Rather than blather on in someone else's thread and drift away from their topic, I thought I'd start a (re)new thread.

 

Here's where it started:

 

Position: There are good reasons to use V/S. Preventing TCAS TAs, smoothing turbulence, preventing large pitch and G variations, smooth transitions to G/S, etc.

 

There are cons to it - specifically that you need to be more observant than with some other modes, but it is largely - and incorrectly - believed that there is no stall protection. You can overspeed in V/S, but you can in VNAV as well.

 

V/S is not a very good mode for climbing to cruise.

 

So if you don't want to use V/S, don't. Your passengers will be less comfortable for it. But don't say Don't use V/S! and don't say RW pilots don't use V/S, because that's simply not true.


Matt Cee

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Am I missing some posts that triggered this topic?


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

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Am I missing some posts that triggered this topic?

Same question I had!


Regards, Jeremy Chesney

 

 

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So if you don't want to use V/S, don't. Your passengers will be less comfortable for it. But don't say Don't use V/S! and don't say RW pilots don't use V/S, because that's simply not true.

Well yes. We use VS just about every flight. Usually on approach during vectors to avoid early level offs, but also it is much more responsive than LVL CHG for managing deceleration too. However, once near bugged speed, VS is very conservative and will generally hold the speed about 10kts faster than bugged so becomes less useful when speed control is paramount in the terminal.

 

Traffic Advisories would be very common without VS but still happen from time to time. Prompt action is required and as VS is responsive it works very well. If a TA is anticipated with a high RoC then VS +1000 fpm always works. Low stop altitudes on departure are due to traffic, so steaming into 4000' with CLB thrust and a 2500 fpm RoC is not advised!

 

VS at higher altitudes is quite risky. The aircraft is very sensitive to temperature at altitude and can quite simply stop climbing before resuming just as the buffet bars are closing on the bugged speed. Climbing in VS at altitude risks some kind of low speed situation (might be nowhere near stall, but well within the lower amber bar) which can be extremely difficult to recover from without leveling off at best or even descending, even with two engines at MCT.

 

Here's a scenario. You get windshear in cruise at FL370 traversing the bottom of a jetstream. The aircraft max alt is FL373. The speed suddenly increases with the trend arrow well within the overspeed barbers pole. What to do?


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VS at higher altitudes is quite risky. The aircraft is very sensitive to temperature at altitude and can quite simply stop climbing before resuming just as the buffet bars are closing on the bugged speed. Climbing in VS at altitude risks some kind of low speed situation (might be nowhere near stall, but well within the lower amber bar) which can be extremely difficult to recover from without leveling off at best or even descending, even with two engines at MCT.
Good point. This is much more of what our training has been on for the past year.

 

If I use V/S at altitude, it's going to be for smoothing out the level off. If a -700 is climbing at 1500fpm, up in the 30s, I'll slowly roll the V/S down to 500fpm for the level off.

 

 

 


Here's a scenario. You get windshear in cruise at FL370 traversing the bottom of a jetstream. The aircraft max alt is FL373. The speed suddenly increases with the trend arrow well within the overspeed barbers pole. What to do?

 

Speedbrake. Don't yank the T/Ls to idle. Note the max Mach so the mechanics can do their inspection/sign off. Have some more coffee.


Matt Cee

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Speedbrake. Don't yank the T/Ls to idle. Note the max Mach so the mechanics can do their inspection/sign off. Have some more coffee.


Good one. Definitely don't idle the thrusties. In reality, once the speed starts to fall away without thrust things will get exciting pretty quickly! Even without any drama, bringing them back to idle in the cruise near max alt will almost certainly result in having to descend to get the speed back!

The speedbrake thing I have debated myself. Although there are no specific limitations, in an overspeed at a high mach speed I don't really know what aerodynamic effect the speedbrakes will have. The airflow over the wings could be near supersonic and air behaves very differently.

 

I think I would avoid speedbrakes but as you say, just accept the overspeed with reduced thrust.
 


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Our AOM recommends the use of partial speed brakes in the event of an overspeed during cruise at high altitude, like you would encounter in a mountain wave situation.  The procedure is leave auto throttles engaged.  If auto throttle correction is not satisfactory deploy partial speedbrakes until a noticable reduction in airspeed is achieved.  When speed is below VMO/MMO retract the speedbrakes at the same rate they were deployed.

 

Then as Spin said, have some more coffee.


Joe Diamond

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Our AOM recommends the use of partial speed brakes in the event of an overspeed during cruise at high altitude, like you would encounter in a mountain wave situation. The procedure is leave auto throttles engaged. If auto throttle correction is not satisfactory deploy partial speedbrakes until a noticable reduction in airspeed is achieved. When speed is below VMO/MMO retract the speedbrakes at the same rate they were deployed.

Actually, what you say is what is in our FCTM too, so best answer funnily enough goes to you and not to me :wacko: However, the FCTM does highlight the risks of leaving A/T engaged, but I guess their determination is that leaving the A/T in is less risky than manual control. The Reduced Engine Response Times Bulletin makes interesting reading too.

 

If the airplane experiences a sudden increase in airspeed that causes the
autothrottle to reduce thrust, manually guard the thrust levers to maintain
a minimum of 60% N1, if possible. Ifthrust is reduced below 60% N1, a
significantly longer time will be required for the engines to spool up if the
time at idle thrust is less than 60 seconds


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Actually, what you say is what is in our FCTM too, so best answer funnily enough goes to you and not to me :wacko: However, the FCTM does highlight the risks of leaving A/T engaged, but I guess their determination is that leaving the A/T in is less risky than manual control. The Reduced Engine Response Times Bulletin makes interesting reading too.

Ours does include a comment that the thrust levers may advance slowly to achieve cruise airspeed and if necessary to push them up more rapidly.


Joe Diamond

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