13 minutes ago, Wothan said:

I did read the FCOM pages, but still...

Quote from a Type rating instructor:

"V1 speed is the tool used by pilots of transport category jet aircraft to ensure adequate stopping distance remains if the pilot elects to stop the takeoff due to an engine failure, fire or other event which may limit the ability to safely fly. The key component of V1 is the balanced field length which is the distance to accelerate to V1 and then stop if necessary using max braking, engines at idle and spoilers deployed. If the Balanced Field Length computed is equal to runway available then the Takeoff Distance is said to be Field Length Limited. In some cases the computed balanced field length may exceed runway available and in such cases either the weight must be reduced, a different flap setting used or another longer runway used for takeoff.

To determine V1 the pressure altitude, temperature, runway length, runway slope, wind, condition of runway (dry, wet or icy) must be known and the aircraft takeoff weight and flap setting for takeoff. Above V1 the pilot must continue the takeoff or risk going off the end of runway attempting to stop."

So for proper V1 calculations, runway length and conditions must be known.

All these parameters are available in the FMC, either by pilot input or from the navdatabase (Runway length, filed elevation).

 

FinnJ

I think although the numbers and parameters are in the FMC by some sort of input the FMC isn't designed to do that sort of calculation hence airlines using EFB etc to get that information calculated.

2 minutes ago, akabam said:

I think although the numbers and parameters are in the FMC by some sort of input the FMC isn't designed to do that sort of calculation hence airlines using EFB etc to get that information calculated.

Yes the V1, Vr and V2 suggestion by the PMDG FMC is kind of a cheat I think, and the numbers should come from the EFB (or part of the flightplanning on none EFB equipped aircraft).

But when the V-speeds can be calculated right now in the FMC - also for Assumed Temperature, TO1 and TO2, I cannot see why this doesn't work in the EFB.

Offcourse proper V2 calculation doesn´t take obstacle clearance into account as long as the data for this isn´t available, but thats not different from the other PMDG airliners.

 

FinnJ

44 minutes ago, Wothan said:

Yes the V1, Vr and V2 suggestion by the PMDG FMC is kind of a cheat I think, and the numbers should come from the EFB (or part of the flightplanning on none EFB equipped aircraft).

But when the V-speeds can be calculated right now in the FMC - also for Assumed Temperature, TO1 and TO2, I cannot see why this doesn't work in the EFB.

Offcourse proper V2 calculation doesn´t take obstacle clearance into account as long as the data for this isn´t available, but thats not different from the other PMDG airliners.

 

FinnJ

Cause I think like they said that doesnt take into account, runway length, obstacles etc. If they did then there would be no need for a EFB whatsoever. 

42 minutes ago, akabam said:

Cause I think like they said that doesnt take into account, runway length, obstacles etc. If they did then there would be no need for a EFB whatsoever. 

How can they then calculate V1 at all ?

V1 is the decision speed, meaning that You cannot get past that speed / calculated point, without running out of runway, if doing a rejected take off.

To do this You need to know RWY length etc.

 

FinnJ

Edited September 27, 20187 yr by Wothan

1 hour ago, Wothan said:

How can they then calculate V1 at all ?

V1 is the decision speed, meaning that You cannot get past that speed / calculated point, without running out of runway, if doing a rejected take off.

To do this You need to know RWY length etc.

 

FinnJ

Assume there is sufficient runway.  The FMS does not use runway length therefore it follows that the decision speed is not going to be runway limited.  I don't know what's hanging you up.. but even if you have an infinite runway you will still have a decision speed based on minimum engine out control speeds.

1 hour ago, downscc said:

Assume there is sufficient runway.  The FMS does not use runway length therefore it follows that the decision speed is not going to be runway limited.  I don't know what's hanging you up.. but even if you have an infinite runway you will still have a decision speed based on minimum engine out control speeds.

Nothing is really hanging me up..

I´m just trying to figuring out how those V-speeds are calculated and why the EFB calculations are incomplete.

I just thought that part of calculating V1, was to take runway length into consideration, due to how it´s described by some professionals like this qoute from a Type rating Instructor, which made me believe that Runway length is a major contribution for this calculation:

" To determine V1 the pressure altitude, temperature, runway length, runway slope, wind, condition of runway (dry, wet or icy) must be known and the aircraft takeoff weight and flap setting for takeoff. Above V1 the pilot must continue the takeoff or risk going off the end of runway attempting to stop."

But sure I might be all wrong.

 

FinnJ

18 minutes ago, Wothan said:

But sure I might be all wrong.

You are not all wrong, just incomplete.  V1 is not only dependent on runway length but is also the minimum speed that the pilot can takeoff with an engine failure, and that must be greater than Vmcg.  Take away the runway information and now V1 is basically Vmcg and an engine failure above V1 means you must takeoff, and below V1 you must abort.  Adding runway information, using the EFB, might adjust V1 if the runway is too short to abort at V1 and stop in remaining runway.  This leads to the reasons why one seldom uses derates for quad jets and usually uses derates for twins.  Vmcg (velocity of minimum control speed on ground) is a key factor.

See also https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=23&cad=rja&uact=8&ved=2ahUKEwi4x83A9tvdAhUBa60KHYCgDEAQFjAWegQIAhAC&url=https%3A%2F%2Fskybrary.aero%2Fbookshelf%2Fbooks%2F493.pdf&usg=AOvVaw0X0qgaRLqK4cILKfddKV1N

Thanks Dan for the link.

I think I now know the difference from the FMC calculated V1 speed and what to expect from the EFB calculations considering derates and assumed temp thrust reduction.

Looking forward to the updated EFB once ready.

I was afraid the 747-8 lacked something in this area compared to the 737, 777 and 747-400, but what's missing here is just an added feature, that quite isn´t ready yet - no problem for me.

 

FinnJ

I'm slightly confused by some of the assertions on this thread.

Firstly:

V1 is the maximum speed at which you can stop, yes. But don't forget that it is also the MINIMUM speed at which you can safely GO, and this is just as important as the STOP case.

With reduced thrust the aeroplane will accelerate more slowly, therefore if an engine fails at V1 more distance will be required to accelerate to Vr, lift off and achieve the 35ft screen height at V2 by the end of the TODA. Therefore all things being equal, one generally might expect V1 to reduce somewhat with reduced thrust (strictly, increased temperature/reduced density) and indeed this is what one sees if one looks at a takeoff performance table.

Of course the FMC knows the runway length -- it is in the navigation database and you can see this when you look at the APPROACH REF page, for instance. This means that the FMC is capable of producing a basic balanced field calculation for V1, taking runway length alone in to account.

However, particularly significant in the determination of the maximum permissible take-off weight and V1 for a particular runway, however, are obstacles off the end of the runway as the net engine-out takeoff flight path must clear these. Therefore it may be necessary to 'shift' the decision point and lift-off points closer to the start of the runway by reducing the maximum permissible takeoff weights and V1 speeds. This is a relatively complex calculation which is specific to each runway and and requires large amounts of data for each runway, and it is this data which the FMC lacks and why the FMC generated V1 is (from a real-world point of view) close to useless. 

Vr and V2 are very closely tied to aircraft weight and the FMC calculated Vr/V2 speeds take in to account the actual minimum takeoff speed requirements.

For manual takeoff performance calculations in the B744, only V1 is provided in the tables for each runway and the Vr and V2 speeds from the FMC would be used. The FMC will also advise of the V_MCG and V_MCA values and limit the minimum V1 and V2 as appropriate.

Runway length in the FMC is ot taken into account.

I tried at two different airports with different runway length (very short ones). but at same field elevation (EKBI and EKVJ).

The FMC calculated V-speeds where the same, and after reaching V1 there was no way to safely stop before reaching the runway end.

 

FinnJ