Basic Empty Weight and Performance

[martin.s]

Has anyone noticed that when you open the Saab 340 .acf in Planemaker, the Basic Empty Weight is set unrealistically low?

The setting in Planemaker for BEW is 12,655 lbs. I did a Web search and found a Saab 340A sales brochure listing the average BEW as 18,800 lbs. I also found a Saab 340A for sale listing BEW of 17,615 lbs. The problem with having such a low BEW in planemaker is that it gives the impression the aircraft can take an unrealistically large payload.

Since MZFW = 25,700 lbs

Max Payload = 25,700 - 12,655 = 13,045 lbs! According to the brochure Max Payload is actually closer to 6,900 lbs. So why has the Planemaker BEW been set so low? After some experimentation my best guess is that it is an x-plane "tweak" to allow actual aircraft performance in the sim match the charts.pdf data. I actually modified the .acf, correcting the BEW to 17,615 lbs, but having done so, got nowhere near the performance figures in the book.

So the trick with this aircraft seems to be to accept the fictitious planemaker BEW as a necessary tweak (leave it at 12,655 lbs), but treat that as BEW of 18,000 lbs for planning purposes. So if you add 30 passengers and bags (about 6,900 lbs) and 3000 lbs of fuel, planemaker will report total weight as:

12,655 +  6,900 +  3000 = 22,555 lbs

However the aircraft will actually perform in the sim like:

18,000 + 6,900 + 3000 = 27,900 lbs

I have seen some posts mentioning the Saab barely scraping into FL200 at 100-200 FPM. I got similar results with my modified .acf where BEW was set at the more realistic17,615 lbs and X-Plane Weight & Balance & Fuel reported a "total weight" of 28,000 lbs with 6,900 lbs payload and 3,000 lbs fuel. Going back to the original .acf with the same payload and fuel I got performance very close to what is published in charts.pdf.

So I am documenting my findings here in case it helps anyone new to this outstanding aircraft. Don't add too much payload. When Planemaker reports a total weight of 22,500 lbs, read that as 28,000 lbs and you should not be surprised by bad performance. If anyone has got different results and thinks my findings are incorrect, please add to this thread.

Edited May 22, 2016 by martin.s

[Goran_M]

I won't dispute your findings...yet.  
Before I address this further, could you show me your information by way of a pdf or link.

 

[martin.s]

The source of BEW/OEW can be found in these links

http://www.saabaircraftleasing.com/prod/datasheets/340brochure.pdf
(See page 17 of pdf for OEW and Max Payload)

http://exclusiveaircraft.co.uk/aircraft-sales/saab-340a-for-sale-
(OEW listed as 17,615 lbs)

My initial attempts at accurate trip fuel planning in ISA conditions using the Trip Fuel table on page 72 (charts.pdf) were always inaccurate. The trip took significantly longer and used more fuel.

The reason for this may be shown by the ISA SPEEDS, NORMAL RANGE table on pg 80. At 28,000 lbs is ISA conditions at FL200 I was expecting the published KIAS/KTAS of 180/244, but in the sim I was only getting about 156 KIAS (with engine temps close to Red Line and props at 1330). In other words, the model performed like it was heavier than 28,000 lbs requiring a higher AoA in level cruise and therefore 24 KIAS slower than published.

The other performance figures I am still trying to reproduce are the Climb Performance (Dist/Time/Fuel) to a specific Flight Level. pg 91-93. I don't have any numbers yet.

The LES SAAB 340A is an excellent aircraft and I don't expect the XP model to be perfect knowing XP has its own limitations. I also don't usually spend so much time with the performance charts, but on this ocassion my inability to use the published charts for reasonably accurate fuel planning got me more interested in the performance figures.

If anyone is able to load the plane up to 28,000 lbs and get the published 180 KIAS at FL200 in ISA, please let me know.

 

[JGregory]

Martin,

We are in the process (and have been for some time) of evaluating and adjusting the flight model and performance of the Saab.  We appreciate your input.    However, at this time, I wouldn't spend a lot of time on this as the work we are doing will certainly change any conclusions you may come to. 

[martin.s]

I recently read on the LES Facebook page that v 1.4 (bug fixes) for XP10 is close to completion. I am just wondering if we can expect any changes to the performance numbers (as discussed in this thread) in v1.4, or is that planned for a later release? Thank you.

[Goran_M]

8 hours ago, martin.s said:

I recently read on the LES Facebook page that v 1.4 (bug fixes) for XP10 is close to completion. I am just wondering if we can expect any changes to the performance numbers (as discussed in this thread) in v1.4...

Yes.  

[SinusJayCee]

Hi all,

I'm new to the LES Saab and came across the same problem. I also noticed that the max weight in XP is set to 27300lbs, which should be 28000lbs according to the Systems.pdf.

Are there any news or new insights on that topic?

Should we set the payload according to our planning and pretend to have the additional 6145lbs on board? Or should we add this value to our payload to get the correct performance?

Thank you in advance for your help!

[Goran_M]

I'll look into the weights a little more and run a few tests, then come back with a possible fix.

[SinusJayCee]

Thanks for you quick reply and for looking into it!

[SinusJayCee]

Did you find anything out?

During my last two flights, I tried to collect some evidence by comparing the performance in the sim with the expected performance from the charts. In particular, I evaluated the expected distance to reach the cruise flight level.

The first flight was with 25,000lbs TOW to FL140. The second flight was with 27,000lbs TOW to FL210. In both cases, I used a medium climb profile and I set the weight such that the values in XP match the planning by adding the missing 6145lbs to the payload. During the first flight, the performance matches more or less the expected values. Except for 1-2NM more, the cruise  FL was reached within the correct distance to the airport. In contrast, during the second flight, I needed a much longer distance. Furthermore, the plane was quite hard to fly but not unflyable.

I hope that helps you.

[SinusJayCee]

Today I loaded the plane to a TOW of 28.000lbs according to the X-Plane Weight & Fuel menu and I suffered a stall during approach somewhere between 140kt and 160kt with flaps 0. Since this is way to high for almost any bank angle, I tend to say that the payload has to be set correctly while ignoring the TOW in order to achieve the correct performance.

I'd appreciate feedback on this regarding the experience of you.

*edit*

I did some further evaluations. In particular, I choose different total weights and flap settings and evaluated the stall speed with zero bank angle. In one case, I set the total weight according to X-Plane. In the other case, I set the total weight using payload and fuel (i.e., add 6145lbs to the XP total weight to get the pretend value). For all flap/weight combinations, the stall speed matches more or less the charts (Charts.pdf, p. 30) when the setting the total weight using payload and fuel, while it is about 15-20kt to high if it is set according to XP total weight.

This is strong evidence that setting the weight using payload in fuel while ignoring the total weight reported by XP results in the correct performance.

Edited September 23, 2018 by SinusJayCee