Question About VSpeeds

[Guest michaelandre]

Sam,Great description! only one question at this stage: are you not in an unbalanced field situation the way you proceed?It is my understanding but correct me if I am wrong, that:If the take-off mass is the maximum that the runway and weather conditions allow, then there is only one value of V1. If the mass is less, then a range of V1 speeds is possible, and for each type of aircraft there is a way of calculating that range of speeds.A "balanced field" is a runway which had the "stop" distance equal to the "go" distance. The stop distance is the runway length plus any paved stopway. the go distance is the runway length plus any clearway.So, you don't unbalance the runway by changing the speeds or am I wrong?Michael

[Guest tmetzinger]

>Thank you all for your detailed contributions.>Now, why do I get the same V speeds for short and very long>runways, everything else held constant?Possibly because the PMDG FMC doesn't take the selected runway length into account (I'm pretty sure it doesn't in the 737). For instance, you could select a flaps 1 takeoff at MDW in a fully loaded -800. You are NOT gonna get off the runway unless you've got a massive headwind (which as pointed out earlier shouldn't be taken into account).I believe the FMC in the PMDG only calculates the balanced-field vspeeds, and it's up to you to determine if the runway length is adequate.A program like Tomaflex or the virtual dispatcher one of the members was working on would take all the factors into account.Best wishes,

[zubart 5]

A Balanced field is a minimum field length. You can not have one inch of runway less.But...Yes, you can unbalance V1 with excess runway available. If the actual runway available is longer than the balanced field length then there is a range of V1 available. Selecting a V1 other than the balanced V1 is called unbalancing.Unbalancing may be used to increase a Vmcg limited takeoff weight or a brake energy takeoff weight. Decreasing V1 can be used to reduce V1 below the maximum brake energy speed.there is a upper a lower bound on V1 speed. The upper bound on V1 is the rotation speed Vr, because no takeoff may be rejected after rotation. The lower bound on the V1 speed is V1mcg which corresponds to an engine failure at Vmcg(minimum control speed ground).Floyd

[Guest tmetzinger]

Your method is the one used when pre-calculated data for the airport and runways is available.I believe my method is used when that data is not available.Your method is much quicker, and answers the question "how much stuff can I put in this airplane and still use the active runway"?My method answers the question "which runway (if any) will work for this already-loaded airplane?"Nowadays with electronic flight bags/performance computers one can work the problem either way depending on what the known values are.I remember watching a video once of some cargo company, and they were explaining to the client that they had to have their load aboard the aircraft no later than 8 AM or they couldn't lift it as they'd be temp limited after 10 AM.

[Guest szp]

yeah, i've heard the temp problem is always a big issue in asia.