Jump to content

Sign in to follow this  
Guest diskus

V/S or VNAV SPD

Recommended Posts

Guest diskus

Rewatching some of the Virgin 19, 747 DVD. I noticed the pilots flew some early parts of the climb using vertical climb speed settings rather than vnav. any thoughts as to why one would choose this over the other?ThanksMike

Share this post


Link to post

Hi MikeI believe they had to get to 6000ft by London 16DME. I think VNAV would have got to 6000ft earlier than they needed to (i.e. before LON 16DME), so Captain Carter decided to make a more gradual climb with V/S mode. They were also cleared of a speed restriction - V/S mode could aslo have been handy because VNAV may have climbed more slowly in order to accelerate to climb speed due to the loss of the speed restriction.CheersRudy


Rudy Fidao

Share this post


Link to post

Almost certainly due to an ATC restriction or SID contraint as already stated. From an operational efficiency and safety point of view VNAV would have been more suitable obviously. Some dude has done a type rating course article on it recently on this forum, you may have heard of it. :-)


Mark Adeane - NZWN
Boeing777_Banner_BetaTeam.jpg

Share this post


Link to post
Guest D17S

VNAV SPD will get you the maximum climb available at your targeted airspeed. The problem V/S addresses is that the 744 can be a real rocketship at light weights. A VNAV SPD climb can be a bit much for a sensitive load of Pax or a restrictive ATC environment. You can use V/S to provide a kinder, gentler climb. Be careful though. There is no low speed protection in V/S. As you select a greater and greater V/S, the nose will rock up to get that ROC. Thrust will then increase to maintain your targeted airspeed. The problem comes in when you run out of thrust. At some point, thrust will no longer be able to maintain speed at your selected ROC. Thrust will hit the EICAS thrust limit and go no more. The AFS blindly keeps pulling the nose up to maintain the selected ROC as the airplane slows down . . . Thrust is "at the stop", but to no avail. As you slow down the AFS pulls in even mode nose up to maintain the ROC. This can get ugly. As you finally hit the stickshaker and start to fall out ot the sky, AFS will finally wake up and put in his relief pitcher, FLCH SPD. Now we start that Falling Leif maneuver as the airplane tries to save itself.If you want to try this, watch thrust. In your V/S maneuver, thrust will increase as you dial in more and more V/S. Watch it like a hawk. Thrust must NEVER touch EICAS thrust limit. Go for a test flight. It's the best way to see all this. In a VNAV SPD climb, press V/S. You are already at EICAS thrust limit with thrust. V/S will capture at the existing V/S. Note this number. Now dial in a higher ROC. The ROC will increase, but can you see the airplane start to slow down? Now dial in a lower V/S. See thrust start to back away from EICAS thrust limit as your decrease the ROC? V/S can be used, but only if you are aware of what's going on. You might not think of it, but thrust is the Big Dog to watch when you are V/Sing. This dog can bite.

Share this post


Link to post

mike,if memory serves me well, they switched to v/s because of a tcas alerting them of traffic. ("Traffic, Traffic").i believe they switched to 600 fpm from vnavtomas


Tom James

Share this post


Link to post

>Rewatching some of the Virgin 19, 747 DVD. I noticed the>pilots flew some early parts of the climb using vertical climb>speed settings rather than vnav. any thoughts as to why one>would choose this over the other?>>>>Thanks>>>MikeHi Mike, Alan switched to SP V/S when the TCAS gave an aural traffic alert. If you listen to that segment, you'll hear "Traffic Traffic. I believe pilots do this because there are times when TCAS will alert the crew, if for example traffic is too close from above, TCAS will give an aural alert "Increase Decent Rate." The pilot would adjust the V/S on the MCP until clearing the traffic. When clear of the traffic, TCAS will give an aural alert, "Clear of Conflict." A nice device to have, isn't it. Ken.

Share this post


Link to post

The reason VSPD would have been selected was because of the TCAS alert. Reducing the Rate of Climb lessens the chances of a resolution advisory (RA) occurring. Vert SPD is a perfectly acceptable, and often used mode, as long as you are aware of it's limitations. What Sam says is quite correct. The other thing to remember is the IAS/MACH crossover.....often forgotten :)CheersSteve


Cheers

Steve Hall

Share this post


Link to post

Yes, keeping the ROC below 1500 fpm should prevent TCAS RAs, you would use V/S to acheive this.Also the company tries to encourage the use of VS or FLCH for small step climbs, like 6000 to FL 80 on departure,I guess because its less agressive for TCAS and passenger benefits.Jon

Share this post


Link to post
Guest RollsRoyce

Here I go offtopic...stupid, but I have to ask.Whilst we have a few real life pilots in this thread, can you (or any other) gentlemen tell me if at light loads, probably during short hops in the 744, given the aircraft's propensity to climb rapidly and notwithstanding derated thrust, will a quicker climb result in increased probability of passengers (including crew) suffering from ear-aches?(Due to the quick(er) change in pressure)If so, could be one factor of what you all keep referring to as "passenger comfort".TIABilal

Share this post


Link to post

Bilal,good question,it shouldnt do no, as its(the pressurisation) all smooted out and computer controled. there maywell be a slightly higher cabin climb rate than with a slower aircraft rate of climb but its still a lot better than any turboprobs and indeed most smaller jets,not enough to hurt onless you have a head coldJon

Share this post


Link to post

I agree with Jon regarding the pressurisation. Compared to some twins like the 767 or 777 the attitude and ROC of the 747 (even light) in the climb is relatively low.The whole idea with transporting passengers is to provide as smooth a flight as possible. Some people are particularly sensitive to flying and any slight change in noise etc makes then quite anxious. By doing gentle climbs or descents etc the deck angle is not too steep and also the thrust changes smoothly and slowly.A case in point is after TO. When the aircraft reaches the altitude where climb thrust is selected the Auto throttles reduce power very quickly (especially if going to a derated climb like CLB2). This can be very disconcerting to passengers as they might think that there is a problem with the engines. I cringe when I watch guys just let the AT's whip the thrust back with a huge change in noise and an abrupt pitch over too. It is much better to place your hand over the thrust levers and slow them down. You will feel the clutches working but that is fine. This way you can control the rate of thrust reduction and also smoothly pitch over to accelerate. Same with a low altitude level off (say 2000ft). If I know this is going to occur I try to give a PA to the pax to let them know to expect an abrupt power reduction shortly after TO. In a freighter... a different story.CheersSteve


Cheers

Steve Hall

Share this post


Link to post

Hi Ken,I may have and forgot about it sorry,been flat out with work this time of year due to lack of crews and people going into over time.had a bout 3 nights in my own bed since the end of May. I`ll try and find it in over the next few days or resend itcheersjon

Share this post


Link to post

>Hi Ken,>>I may have and forgot about it sorry,been flat out with work>this time of year due to lack of crews and people going into>over time.>had a bout 3 nights in my own bed since the end of May. I`ll>try and find it in over the next few days or resend it>>cheers>jon>Hi Jon,That's okay. I know how it is to work and not get enough sleep. I'll resend it so you won't have to look for it. Thanks, Ken.

Share this post


Link to post
Guest RollsRoyce

Thank you gentlemen!Bilal

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    2%
    $640.00 of $25,000.00 Donate Now
×
×
  • Create New...