Lvl Chng and V/S
 

I Know that this must have been discussed to the bones but can anyone tell me the exact diffirence between V/S and Lvl Chng, esp in aircraft behaviour.

Thanx

Simple. Level change closes the thrust levers (or opens) and speed is MCP/ FCU speed, VS gives priority to vertical speed, thrust levers adjust accordingly to maintain the VS, not the MCP or FCU speed. On some A/c there is no stall or overspeed protection when in VS mode, (757 example)

LVL CHG = fixed power mode, idle in descent and climb thrust in climb
VS = variable power mode, power as required to maintain requested vertical speed.

On B737, VS mode reverts to LVL CHG when the requested vs cannot be maintained and aircraft performance is exceeded by the requested VS.

Not quite so simple. Upcoming posts will explain!

Hi fastcruise,

In FLCH / LVL CHNG / OPEN CLB / OPEN DES etc. then Elevators are controlling speed.
In other modes (VS, ALT, GS etc), thrust is controlling speed.

Not quite, think about that. what would happen if the elevators commanded nose down pitch and the thrust levers remained at cruise power! The VS mode commands pitch to hold selected vertical speed and engages the autothrust is speed mode to attempt to maintain the selected airspeed. The LVL Change mode co-ordinates pitch and thrust to make automatic climb or descent at selected airspeeds. In climb, thrust will be limit thrust, (N1) in descent, thrust levers will be at idle. If a speed mode is engaged when LVL change is selected, this becomes the target speed, if a speed mode is not engaged at the time of selection of LVL change, the existing speed becomes target speed.
I have not mentioned reversion modes or protection, in an attempt to answer the original question!

Thanks everybody
Is it possible to understand that in LVL CHNG the thrust is CLB/-1/-2 for climbs and Idle for descent, pitch is controlling the speed as selected in the MCP. However, in V/S mode the pitch controls the V/S and thrust tries to maintain MCP speed as far as possible.
So Lvl Chng should be the mode providing more exact control than V/S through, large magnitude of descents. Can someone dwell upon the reversions, we had a case of one pilot descending in V/S and exceeding the Vmo in the B737NG.

VS vs LVL CHNG
 

If you fly 737 CL in LVL CHNG mode, you just adjust MCP speed and the aircraft keeps with thrust closed just changing PITCH.

In VS, you have to watch the speed, because it is not protected by respective "pitch reduction" as it is in LVL CHNG. If you have XX rate of descent and AT closes levers, increasing of the VS will unevitably increase your speed and you will need to produce some drag or reduce VS again to keep speed it under Vmo/Mmo.

I used LVL CHNG often when I was starting my 737 flying, but then I found VS more controlable, more flexible for many situations, so it is my primary descent mode:-). If you choose VS for descent, rate is usually lower than in LVL CHNG - that means you start descent earlier and you have space to INCREASE the rate if requested by ATC. Level change is kind of "on the edge" no thrust, high speed - so significant increase of rate upon ATC request is not always possible without drag devices, which on the other hand reduce the pax comfort.

Only if you make it so. FLCH flies you at the speed in the MCP window.

Fastcruise, this is a very common problem, even when in VNAV, it can be avoided to an extent by putting in the descent winds and hence a more accurate TOD, if speed is 10kts above FMC speed, it will revert VNAV Speed VNAV PATH. LVL change is essentially the mode VNAV will use as it assumes thrust is idle at TOD. If you get the msg " Drag required" use the speedbrakes or you will get " overpseed disconnect" and CWS reversion. One technique I find useful is to allow the a/c to start the descent in VNAV or level change and when the speed stabilises select VS to maitain the ROD, allowing for the pitch reduction in the last 1500ft as preferred by ATC. As mentioned, you can easily go through VMO using VS, which is why it is better for small changes when momentum cannot build up. Of course if you use LVL change above Mach transition, remember to manually revert to IAS below Mach change over or you will rip the wings off!

Level Change doesn't have to be done with thrust in idle.
When in Level Change, and with autothrottle engaged, you can manually keep an N1 or FF that you need, eg. in Icing conditions or heavy precipitation. :ok:

You are right, but flying in heavy rains is not daily condition:-) Only once during the whole summer charter season, I had to use manual N1 increase to 45% with A/T and LVL CHNG because of the heavy precipitation:-).

Of course you can manually adjust the thrust levers, but the level off prediction arc will be inaccurate. In heavy pcp or ice, I just switch on eng AI and let it sort out the low idle as it was designed to do :ok:

On the Douglas/Boeing 717, Level Change causes the autopilot to use pitch to control airspeed, because either idle, or climb thrust is selected. Whereas V/S causes the autopilot to use thrust to control speed within the limits of idle to climb thrust, and pitch to control the desired RoD or RoC.

To the original poster, what type of aircraft are we talking, as not all a/c's are the same??

Limitation
 

TO Kirks gusset
Your way is right, but if you read Boeing books, they recommend 45%N1 in heavy precipitation, and some airlines have it is obligatory procedure next to the engine AI:-)

Min N1 in or near MOD or HEAVY rain, hail or sleet: 45%

But this is already out of the topic.

Talk about making something complicated.

The key point here is that pitch attitude can only be used to control one parameter.

This is fundamentally true regardless of the control mechanism (raw data human pilot, manual flight following a Flight Director, or autopilot engaged)

In level change the pitch attitude is controlled to maintain a speed (IAS or Mach). Speed lower than target? Pitch down. Speed higher than target? Pitch up.

In Vertical Speed the pitch attitude is controlled to maintain a vertical speed (fpm). Vertical speed more positive than target? Pitch down. Vertical speed more negative than target? Pitch Up.

That's the fundemental difference. Everything else is type dependant.