Any simple way to calculate flaps, derate etc?

[rondon9898 84]

21 hours ago, skelsey said:

Because aviation is expensive! 🙂

On a serious note - don't underestimate the cost to Boeing of generating that data in the first place, which will have involved significant amounts of flight testing for the data collection, as well as a lot of work from performance engineers, plus (if you're wanting the data needed for proper calculation) obstacle and terrain databases that need to be maintained, certified and kept up to date. Then there's the cost of certifying the data in the first place. None of this is cheap... the thing about real data is that it has to be right or else people die, and the process for ensuring that it is right takes a lot of expensive people a lot of time and the use of a lot of very expensive equipment (like flying real 747-400s).

Obviously in a sim the same standards do not need to be applied and there are tools like Topcat that use basic FCOM/QRH data to do the calculations to what I would suggest is probably a good enough standard for sim use... but people seem to complain that that's not good enough, so...

When you put it like that, Simon, the cost seems a little more justified, but I still can’t help feeling that access to information about Boeing aircraft that, outside of our hobby, would be massively abstruse, seems very, very steep. It’s not as if PMDG are asking for information that would require Boeing to employ specialist engineers to provide; it’s info that Boeing must know for safe operation of their aircraft.

That said, there aren’t many people to whom I will bow to superior knowledge - you are one of them, Simon. 

[skelsey 765]

1 hour ago, rondon9898 said:

When you put it like that, Simon, the cost seems a little more justified, but I still can’t help feeling that access to information about Boeing aircraft that, outside of our hobby, would be massively abstruse, seems very, very steep. It’s not as if PMDG are asking for information that would require Boeing to employ specialist engineers to provide; it’s info that Boeing must know for safe operation of their aircraft.

That said, there aren’t many people to whom I will bow to superior knowledge - you are one of them, Simon. 

Ha - that's very kind of you if a little unnerving!!

I guess the issue from Boeing's point of view as much as anything is that the data isn't free (or even cheap, as Jason alludes above) even to airline operators and as far as Boeing are concerned, PMDG are just another customer (and not a high-spending one at that). It's precisely because the data is so essential to developing a takeoff performance application that it is so valuable - the actual calculations themselves are not particularly onerous if you have the information.

In an industry where the price of even a simple button can easily be in the hundreds of dollars (for example, when you turn on the fuel pumps on the 747 overhead you're pushing several thousand dollars' worth of pushbuttons, the autobrake selector switch alone retails at close to a thousand dollars, and an MCP would easily set you back a good half a million) $400,000 is a fairly ordinary number in the scheme of the operating costs of a fleet of 747s... and unlike an airline which will probably only be operating one variant/engine type and from a limited number of airports, PMDG would need data for three different engine variants and tens of thousands of airports worldwide.... in that regard $400k may actually be quite cheap!

To answer the OP's original question about calculating takeoff performance -- I'm not on my sim PC so I'm unable to check the PMDG FCOM but are the field length limited takeoff weight charts supplied in the Performance Inflight section? (They've been removed from the operator-specific FCOM I have so all pilots have are the runway-specific tables in the performance manual).

If so, it is possible to perform a basic reduced thrust calculation (if you're not worried about the engine-out net takeoff flight path and any obstacles you might encounter) manually and with a little practice it doesn't take long at all -- back in the days of the Level-D 767 when BA didn't have the 767 on their computerised performance calculation system, I used to do a manual takeoff data card before every departure which looked like this:

Looks complex but more simple than it seems. You need the basic environmental data (OAT, QNH, wind component), the length of the runway and your actual planned takeoff weight.

The boxes at the top are for 'positive' and 'negative' corrections -- 'positive' corrections being things which increase your performance compared to the tabulated figures and 'negative' corrections being those which will reduce it (so for instance, you would need to check whether the table assumes packs on or off, anti-ice on or off etc).

In the example above, you can see that on the 767-300, each knot of headwind equates to a 100kg increase in takeoff weight, so 800 has been entered in the H/W COMP box.

The tables are all based on ISA (i.e. standard pressure and temperature). Lower pressure = reduced performance, so because the QNH of 997hPa is below 1013 a figure of -3200kg (200kg per hPa above or below 1013 - for the 767-300 with RR engines) has been entered in the -ve corrections box.

The tables assumed packs off, so because I was using packs on for the departure I've also entered a further -2500kg to account for this in the -ve column (a standard correction provided with the performance data, again for the 763).

This gives us a total of 800kg in the +ve column and -5700kg in the -ve column. Add these together and we get a total of -4900kg

Adding -4900 to the performance limited takeoff weight for the runway length and conditions of 190,400 (found by selecting the table with the appropriate flap setting and runway length in the FCOM and reading across to the actual OAT) gives a regulated takeoff weight (RTOW -- i.e. the actual performance-limited maximum takeoff weight for the conditions on this day) of 186,500kg. However, this is still above above the maximum structural takeoff weight of 181,400kg so that's what's been entered in the RTOW box. This, happily, is above our planned takeoff weight of 154,200kg.

Because of this, we have margin to reduce the thrust. We do this by taking our actual takeoff weight of 154,200kg and applying the corrections in a reverse sense (because in this instance the aircraft is going to behave like it is 4,900kg heavier than it really is). So we add 4,900kg to 154,200kg to give an assumed takeoff weight of 159,100kg.

Now all we have to do is look at the FCOM table to find the OAT at which our maximum takeoff weight would be 159,100kg -- 49 degrees C. This is our 'assumed temperature' which we will enter in to the FMC. We also read off the V speeds from the table and you're golden (though, again, this does not take in to account obstacles in the takeoff flight path -- not likely to be a problem all engines operating but may get interesting if you have an engine failure somewhere that is not flat). 

Dan's method of increasing the OAT in the EFB is essentially doing the above through the EFB software rather than with pen and paper.

[Jason_Peters 16]

9 hours ago, downscc said:

I think a peek into the complexities involved in aviation are a good thing, but at the end of the day when we are not focused on problem solving and just want to enjoy simulated flight there are work arounds that you would never do real world.

For ATM calculations, run your takeoff calcs in the EFB with wind, temperature, and flap setting and note the result.  Then change the OAT to an assumed temperature up to 60C.  I usually start with 40C and increase or decrease by 5 C until I get acceptable takeoff settings.  Now you have found a suitable ATM value for simulation purposes.

 

Dan, I sure hope you didn't lose my point in translation!  We agree!  For the sim world these simpler work arounds work great!  The point I was making is that users should learn to accept these types of workarounds, rather than demanding infinite realism down to the minutiae.  Otherwise, I'll hand em' my 400 page performance manual full of spaghetti charts and tables and they can have at it.

Also, you'd be surprised at some of similar work arounds we older FE's will use to scrape together some data on the quick in the real world! Ha!  

 

4 hours ago, skelsey said:

If so, it is possible to perform a basic reduced thrust calculation (if you're not worried about the engine-out net takeoff flight path and any obstacles you might encounter) manually and with a little practice it doesn't take long at all -- back in the days of the Level-D 767 when BA didn't have the 767 on their computerised performance calculation system, I used to do a manual takeoff data card before every departure which looked like this: 

 

Simon, that was a great explanation for folks to use!  The use of tables, or Tab-Data as we refer to it, in newer performance manuals sure makes things a lot faster and the data card much shorter.  The E-4B (747-200) performance manual still uses spaghetti charts for everything... much more time consuming.

 

[downscc 1,327]

15 hours ago, Jason_Peters said:

Also, you'd be surprised at some of similar work arounds we older FE's will use to scrape together some data on the quick in the real world! Ha! 

Probably not, one of the things I learned during my military career was to plan, plan and plan and then when called to action improvise like heck.

[skelsey 765]

18 hours ago, Jason_Peters said:

The E-4B (747-200) performance manual still uses spaghetti charts for everything... much more time consuming.

Ouch! Yep, I bet. Just been reading up a little on the E-4B -- certainly looks like a pretty interesting (and impressive) bit of kit!

[berts 97]

On 12/31/2018 at 4:18 PM, Jason_Peters said:

This is not an easy task, and what PMDG has managed thus far is nothing short of outstanding.  I'm all for maxing out the available realism and procedures of my desktop sim, but at the end of the day it's just that, a desktop simulation.  It's intended to give you a highly realistic peek behind the curtain at what it's like to operate one of these magnificent machines, nothing more.  Perhaps some of these complex tasks are better left to our imagination...

I wholeheartedly agree with you. 

If anyone wants to delve into the finer details of the -8's Performance A type calculations I suggest they either talk to Boeing or think about training for a Commercial Pilot's licence and obtain a B747-8 Type Rating.  Assuming PMDG's EFB is able to perform all of the calculations for us, it should be a relatively straightforward task to load the FMS and EFB with the relevant airfield, aircraft and metoeorological data, use the EFB to complete the takeoff, landing as well as weight and balance calculations and then simply accept the resulting Performance figures for use in the QOTS II. 

[Wakner 14]

Take a look at QSimPlanner - might be useful until PMDG figure out a cost-effective solution. Cheers.