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comparing V speeds

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Member
Registered: Jul 2014
Posts: 170
Location: Antwerp
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
This is quite a light weight and you will inevitably see some spread, but BA performance tables give a V1 of 150 (dry), and a V2 of 156.
There should be no argument about the V2. At these weights (unaffected by VMCG limitations) V2 will be the Flap30 Vref, which is 156 or possibly 155 for an RB 211 aircraft.

I really cannot see any reason why there should be a significant split in the V1 and Vr figures as quoted in some cases. That makes no sense on a balanced field calculation for this long runway.

Using TO2 and assumed temperature together forces the EPR to look better ( I would expect about 1.55 at this weight) but I think that the mixed use probably invalidates the comparison..

I think that BACARS has the Vr (and hence the V1) a little low..but I like the fact that they coincide for this runway.

I have found TOPCAT figures to be quite a long way off the mark on other runways - there seems to be a bias towards a low V1 which I don't really understand.

Peter
« Last edit by Britjet on Mon, 30 Mar 2015 21:49:26 +0000. »
Member
Registered: Aug 2014
Posts: 60
Hi Ivo,

V Speeds are calculated by cross referencing the assumed temperature against the pressure altitude. This gives you a separate table to look in against weight to calculate the VSpeeds.

Since PSXPerf, TOPCAT, BACARS, PSX are all entering into the first table with different temperatures and/or % derates it means the subsequent table they find VSpeeds won't be the same one, hence the difference in speeds.

For example if I force PSX Perf to use the TOPCAT +71 that gives me speeds of 142, 149, 157 which are fairly near it's own 142/146/156.

Also, after speaking to Britjet, it looks like all the calculators work out V1 based on the lowest speed, rather than looking at balanced fields and seeing if the V1 can be higher. I'll do some research into that and see if it can be incorporated.

I won't say whose figures I believe the most :twisted: as that would be very biased!
Member
Registered: May 2012
Posts: 158
Location: Farnborough, UK
Ivo,

The plan will be for BACARS to transition to use PSX Perf Figures once Triple7 has finished fine tuning them. They seem to take into account alot more information than I am at the moment.

For now BACARS VR and V2 will match very closely to the PSX FMC, with V1 coming from TOPCAT for now.

Cheers
Gary
Member
Registered: Jul 2014
Posts: 170
Location: Antwerp
hi Britjet, Triple7 and Gary,
Thanks for the explanation .
I am sorry for my ignorant questions, but not to ask is not to learn.

A while back I saw on youtube a clip, where the captain entered the V speeds from his flight plan, or performance dispatch papers, in short, from a paper sheet in to the CDU, the boxes where dashed, thus meaning there was no runway selected to my knowledge.

Now I am a bit puzzled here,

1, this V speeds are runway specific, so if for instance they are calculated for 27R and it changes to 27L, assuming the latter is shorter in this scenario, they are obsolete no?

2, If a runway is entered and the performance page is filled in, and the FMC gets altitude and pressure information, at least I suppose so, the V speeds are calculated by the FMC, if this is the case in real life on every 747.400, why is there then a need to overrule these settings and manually put in different ones? Assuming of course that the calculated figures from flight dispatch are different.

all my assumptions are based on the data I get from different performance planners, in reality maybe there is no discrepancy , and no need to overrule , this however brings me back to my second question, If the FMC can do the job, why input manually ?
ivo
:mrgreen:
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
There is quite a bit more to the calculation than just the runway - TODA, for example, (Take-Off Distance available) and the TORA (Take-off Run available) may well not be the same as the FMC database.

In addition local obstacles and terrain needs to be taken into account for the climbout - the performance has to be checked through several different phases - for example - to 50 feet, the end of gear retraction, to 400 ft, to thrust reduction, to the end of flap retraction etc..

So the FMC alone cannot check all these factors.

Peter
Moderator
Registered: May 2009
Posts: 5140
Having dashes in the V speed fields doesn't necessarily mean the runway is not entered.

In real life, some airlines have disabled the automatic V speed computation in the FMC. Qantas, for instance.

In PSX, the computation is not disabled so that all PSX users can get the data in case external data sources are not at hand. It's easy to ignore the computed data; just overwrite them. Therefore, it's not specifically disabled in PSX, just like the PVD, or gasper etc.: if your airline says "don't use it", just don't use it :-)


Cheers,

|-|ardy
Member
Registered: Jul 2014
Posts: 170
Location: Antwerp
thanks Peter and Hardy for your answers.
ivo
Member
Registered: Jun 2009
Posts: 87
Location: KMEM
Keep in mind that rarely does Balanced Field Length give you the highest max allowable weight off a runway.

Optimum V1 and Max V1 do that.

As Britjet says, TORA and TODA can allow an airline to unbalance the field, and thus lift a higher gross weight off the runway. Balanced Field Length might have been nice in the 707 days, but if you want to lift the max weight possible, you unbalance the field and don't use the speeds out of the FMC...

The FMC calculates balanced field length v-speeds for a flat runway at sea level, which you may or may not have. The FMC also doesn't consider obstacles in the departure path, which might require you to use a shorter takeoff distance to be able to achieve the required clearance over the obstacle. TOPCAT considers obstacles based on the method in the Boeing FPPM - obviously real carriers use the data out of the Airplane Flight Manual or the Boeing BTM Modules.

To learn the painful details of takeoff weight calculations, google "Boeing JTPM" When I went to Boeing for Perf Engr Training back in 2010, we lived in that manual.
Moderator
Registered: May 2009
Posts: 5140
DougSnow wrote
The FMC calculates balanced field length v-speeds for a flat runway at sea level, which you may or may not have.

Are you sure? I think the wet/dry and the slope/wind entry fields are available as per company option code, but the pressure altitude and OAT should be available from the ADCs in any case. There is a certain preflight phase during which an automatic V-speed deletion may occur when the actual, sensed OAT changes so much that it affects the FMC settings. I guess this applies to pressure changes as well. So the FMC must know the current sensed pressure altitude and sensed OAT.


Cheers,

|-|ardy
Member
Registered: Jun 2009
Posts: 87
Location: KMEM
That is true Hardy, for example US carriers all have warnings in their FCOMs not to use the FMC v-speeds as they are calculated based on a balanced field length (which US carriers for example don't use - they all unbalance the field for takeoff).

In the UA FCOM it says "FMC-generated V-speeds use the balanced field concept and are not authorized."

At UA and DL, they can send a request via an ACARS to their processor and it returns with all the data they need for the takeoff, speeds, flaps, etc. It reads the latest METAR and uses that for the conditions. At UA, no procedure uses the WIND/SLOPE fields of the TAKEOFF REF page.
Moderator
Registered: May 2009
Posts: 5140
I think my question wasn't clear enough.

I didn't mean to ask if the balanced field length is used.

I meant to ask if you are sure that the FMC refers to "flat runway" and "sea level"?

Balanced field - Yes.

Flat runway - Depends on whether a slope can be entered (optional feature).

Sea level - I think it takes the pressure altitude from the ADCs.


|-|
Member
Registered: Jun 2009
Posts: 87
Location: KMEM
https://www.dropbox.com/s/3dk2ai8g3578mza/Photo%20Apr%2004%2C%2011%2024%2014.png?dl=0

The B744 FCTM says this:

"747-400
Proper takeoff speeds (V1, VR, and V2) are based on takeoff weight, flaps setting, thrust rating and assumed temperature, ambient temperature, QNH, wind, runway surface condition, and performance options. The FCOM and FMC computed takeoff speeds (if enabled) are only valid for dispatch performance based on balanced field length, no improved climb, the most forward CG limit, and dry runway. Wet runway takeoff speeds may also be available for airplanes with wet runway takeoff performance in the AFM."

The speeds displayed are the QRH speeds. The page also says that slope and winds will vary the calculated V1. So if you don't enter any slope or winds, the speeds you enter will match the QRH, which is based on BFL. Press Alt is important so it must use the press altitude from the ADC.

Looking in the current generic Boeing FCOM, it really doesn't say much more than that. I remember when I went to Boeing for Perf Engr training back in 2010, they commented that the printed speeds in the QRH and FPPM are based on the balanced field, and that if you used an Optimum V1 policy (like my previous and current carrier do), or alternate forward center of gravity (like I wanted to start at my previous airline), to never use FMC speeds.
« Last edit by DougSnow on Sun, 05 Apr 2015 12:29:03 +0000. »

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