View Full Version : 737 Level change/Vertical speed modes

20th Sep 2007, 20:09
Hi wondered if anyone could spare some time to explain when you would use level change over VS mode or VS mode over level change mode?
I have a basic understanding that level change will pitch the a/c to maintain speed with either increased or decreased thrust.And VS mode will adjust thrust to maintain selected rate of climb/descent.
Trying to build my knowledge of 737-300 automation.Thanks for any help:)

20th Sep 2007, 20:28
When you want to go up or down quickly then use level change. When you want more control when going up or down slowly then use v/s.

If you use a high rate of climb in v/s, then the autothrottle will go to max climb thrust, but the speed will decay. So much so that you can approach the stall. You can also do a similar thing when going down with too high a rate of descent and end up approaching Vmo.


20th Sep 2007, 21:15
So much so that you can approach the stall. You can also do a similar thing when going down with too high a rate of descent and end up approaching Vmo

Using V/S doesnt it automatically convert to LEVEL CHANGE before reaching STALL and Vmo ?????


20th Sep 2007, 21:58
Depends on the version of the automatics. We have very different 300s in our fleets and some convert from VS to LVL CHNG when the speed differs by only 5 kts from the selected speed, others quite happily fly into the shaker or clacker. So dont trust it, you have to be aware what the plane's doing and keep yourself in the loop.

In general V/S is more controlled in terms of at what place do i want to be at what level, but can be more uncontrolled in terms of speed. During climbout i usually use LVL CHNG except for small altitude changes or the last 1000 ft, during descent i usually start out using VS and revert to LVL CHNG when the speed becomes controlling, for example given speeds in a terminal area while still keeping an aye on actual VS of course since we have SOP limits about VS in certain altitude differences above the MSA.

21st Sep 2007, 04:06

In theory, there is a speed protection mode with V/S on the B737Classic/NG, but in reality, as Denti said, it is not to be trusted on the B737 Classic.
If you fly the Classic, you will no doubt know that it has a very weak autopilot system and that it gives you "surprises" from time to time, especially related to speed. Be aware of that, anticipate it and you'll be fine.

21st Sep 2007, 10:51
I have a basic understanding that level change will pitch the a/c to maintain speed with either increased or decreased thrust.

Pipertommy: The basic principle behind LVL change is; for climb mode, A/T gives you climb N1. For descent mode, A/T gives you idle N1.

(However for descent mode, after the thrustlever is closed, A/T goes into arm, which gives you manual control of the thrustlever, and A/P pitch goes into Speed mode. You can then reduce your V/S by changing the thrust)

If you want/need a specific ROD then you normally would choose V/S.

Capn Bloggs
21st Sep 2007, 14:42
I don't fly the 737, but Level Change and VS are Level Change and VS. When near the ground (less than 5000ft AGL) be wary of quite steep descent angles if using Level Change and say Speedbrakes (or other drag creators). I have seen some VERY steep angles resulting in very high rates of descent, all because George (the A/P) is chasing the speed with the nose attitude.

VS is much neater, as it allows YOU to control the rate of descent (as mentioned by Denti your company may even have descent rate rules down low) to a manageable level. In VS, if you get too slow, the A/T will kick in to keep it where you want it. IMO, it's better being under control sinkrate-wise and using power than the nose going down and down and down at idle as the jet tries to chase the speed.

21st Sep 2007, 16:01
Thanks everyone for taking the time to reply!!I now have an understanding behind the two modes :ok:
Thinking back to the IR training,when in the descent from a flight level to another level,i`d reduce power pitch down to maintain speed and accept the ROD = sounds like "level change"
And then from the hold outbound towards the base turn i would nominate a vertical speed ="Vertical speed" mode.
Thanks again.

21st Sep 2007, 18:15
Bloggs is of course with the danger of high rates if using level change. However i've seen far more often that people try to press on down with a certain VS, say 2500 fpm while being speedrestricted by ATC wich then means you gonna overspeed for sure over the given restriction. That is the reason why i use at low speeds level change, with low speeds there is normally no chance in hell to get too high rates.

But as always, be aware of the performance and what condition youre in.

21st Sep 2007, 18:36
Toggling the different modes, is the name of the game. Use whatever mode you like in order to keep your speed and profile within the "box".
Kinetic energy+potential energy = Total energy
Aerobatics taught me alot in that respect. Great fun too.

21st Sep 2007, 18:59
We also use level change in the event of the autothrottle being unserviceable, or switched off in the case of an engine failure. The reason is the autopilot or flight director will control the speed and you can then vary the rate of climb or descent using thrust. If you use VS with no autothrottle it's harder as you have to monitor the speed much more closely.

During descent once the thrust levers are closed, the autothrottle (if now working) will go into arm mode allowing you to manually re-position the levers so you can add thrust if you want to reduce the rate of descent, a slightly crude vertical speed mode!

22nd Sep 2007, 15:02
Caution...in VS the 737 will NOT look at the speed, just chase the required VS....it will take you all the way to the barber pole trying to get the required VS. In Level Change it will hold the set MCP speed, the vertical speed can be increased using speed brakes or increasing the IAS.

AS was said in another post, going between the two can be the best option..it all adds up to mode awareness.

22nd Sep 2007, 15:41
Great!Thanks.Understand each mode is used to create the climb/descent profile that PF requires. :ok:lot clearer