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codechris

PACKS off take-off question

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I want to get FSipanel, but right now, it's a huge amount of money to me. But, soon I will. It looks fantastic

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I ran though the flight in Topcat selecting "optimum" Flaps and it gives me a full throttle flaps 25 take off, which I've never seen. I normally manually select Flaps 5 in Topcat and everything is good. But, I called in to question if this would be a scenario which would warrant a packs off takeoff?

 

As Spin and Steam pointed out, this is a performance issue. Look at it from this perspective. Takeoff performance is a balance between atmospheric conditions, field length and climb restrictions. Some aircraft do a rough job at the balance while some finesse the process. My first heavy aircraft, C-141B, had one takeoff flap setting. 75% was the takeoff/approach setting with 100% being landing flaps. The plane was great at short field performance, but sucked in climb performance. The DC10 finessed the process with it's variable flap wheel setting. I could set any flap for takeoff between 5 and 25(i.e 5, 7.4, 10.7, 18.2, etc). The DC10 fully optimized max weight and flap settings based on conditions, field length and climb requirements. More flaps get you off the runway quicker, but the drag caused by the flaps kill your performance up to accel height. less flaps require more runway distance, but less drag allows a higher climb angle and climb rate. For example, the C141 would get off the runway quickly but suffer in the climb unless it was light weight. In the DC10, i could easily estimate flap settings based on runway length and obstacle/min climb rate. Long runways, greater than 10,000ft, resulted in flaps 15 to 5 depending on length. The aircraft takes advantage of the runway length to achieve a higher rotate speed and cleaner profile. This resulted in a higher pitch for V2/V2 + 10 and higher climb rate. Close in obstacles and high min climb rates resulted in the same. Shorter runways resulted in flaps 15 to 25 flaps depending on length. The aircraft took advantage of the flaps to get off in a shorter distance which resulted in a lower pitch due to the flap drag at V2/V2 + 10.

 

Even in the Gulfstreams I fly(5/550), flaps 10 gets you better climb and flaps 20 gets you off the runway quicker. The 5 series is so over powered that flaps 20 is the standard unless dealing with high climb gradients like KASE and LOWI.

 

Packs off takeoffs in the jets I flew only resulted in a extra of 2000 to 4000 pounds of weight, which only became a factor when needed for holding or alternate fuel. In that case it made the difference between needing a fuel stop which impacts your crew duty time. It sucks to have to crew rest just because you didn't have enough fuel to fly direct,but it happens sometimes in my ops. Usually when faced with this situation we stage a crew at the fuel stop for crew swap to keep the jet moving.     

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Packs off takeoffs in the jets I flew only resulted in a extra of 2000 to 4000 pounds of weight, which only became a factor when needed for holding or alternate fuel. In that case it made the difference between needing a fuel stop which impacts your crew duty time. It sucks to have to crew rest just because you didn't have enough fuel to fly direct,but it happens sometimes in my ops. Usually when faced with this situation we stage a crew at the fuel stop for crew swap to keep the jet moving.     

 

Part of this point comes down to the way PFPX deals with "random payload" because I imagine it is completely random, and not random given your aircraft, weather and airfield situation. So, in this case of high temperature and short runway, it doesn't care. An airline would care and adjust, and this is where I have to not be so random and tweek the numbers to be suitable. So, BAW001 which is London City to Shannon, then on to JFK, which in real life is something life 23 business class passengers, I guess PFPX would still give me a load far above that when hitting random load. But I admit that asking PFPX to do that would be far outside it's remit, hence needing to get involved. With Vatsim, since I usually fly in to a London airport which is busy almost all of the time, hold times or extra fuel helps. Maybe that's my bad fuel calculations, I don't know. But a bit extra helps.

 

So, to you're point that it only allows more weight suggest to me that I have to get involved. However, unless we state that the margin numbers of runway  that Topcat provide are wrong, then running a series of numbers (as me a flight simmer) to find the best margin of runway, is the best way to go about it. I don't care about profit margins for a flight, as an airline would, however to add realism I should try and do more then a 0 PAX takeoff which solves the whole issue. I do admit that we're speaking of "real life" and "sim life" and I can happily do what I wish and still conduct my flight. A ZFW at bearlly anything would not affect a flight since we're we don't have a CFO worrying about my flight. However, we, or most of us at least, at aiming for some sort of realism.

 

Also, a flight this short, I think it comes in around 1 hour and a bit, you can't fuel stop and make any profit. Again, who cares in the sim world, but I can't see an airline doing that (maybe they would?).

 

So, if we stick with random payload, and adjust as necessary, then move on to the numbers and say "given our payload, runway, and weather, if a bleeds off takeoff gives us that, then so be it". If I run the numbers through Topcat and bleeds off gives me 300ft margin, but bleeds on gives me 200ft. I'll take the 300ft, since the numbers are tiny as they are.

 

But, I admit. I am doing this with a very basic understanding and only 1 hour of flying time in a C172. However, thank you for your input, that has been very helpful and I'm very thankful for you're real world experience. I hope I don't come across as arguing with you, I'm not. I'm just trying to join two worlds of real life and sim life and how I can take this and mirror it in lots of other situations  :)

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BAW001 which is London City to Shannon, then on to JFK, which in real life is something life 23 business class passengers, I guess PFPX would still give me a load far above that when hitting random load.

 

It's random within the limits you set when you set up the aircraft in PFPX: so if in the aircraft database you set pax capacity for G-EUNA/B to 23 (or whatever the layout is) you won't ever get a load higher than that (you might want to restrict the cargo capacity as well if it regularly gives you, say, 15 pax but 10 tonnes of cargo which is putting you over the limit). I don't think you can currently cap the loads between city pairs though, so you're right in saying you may have to tweak the payload in some cases.

 

However, you can quite easily go in to TOPCAT and generate some "maximum" payload numbers for certain airfields/runways which you can then note down for future reference. Just select your aircraft type, go to the Takeoff page, select a flap/thrust config etc then click "Runway Table". This will give you the performance limit weights for that runway at the specified flap setting and at a range of different temperatures & wind components. So, for instance, looking at LSZB rwy 14, with Flap 25 and bleeds on the limit weight at 20C is 64.2 tonnes: whilst this number fluctuates depending on the temperature etc., it's a good starting point: keep a note, and next time you know you need to keep the TOW to less than 64.2 tonnes. Allowing for ~6t of fuel to London, that gives you a maximum payload of ~15 tonnes. If on the day it's warmer than 20C, or perhaps your payload is 15.2 tonnes, you may have some tricks up your sleeve (such as bleeds off) to enable you to lift that little bit extra weight.

 

 

 

If I run the numbers through Topcat and bleeds off gives me 300ft margin, but bleeds on gives me 200ft. I'll take the 300ft, since the numbers are tiny as they are.

 

Worth mentioning that there is a school of thought which says: the calculations have big margins built in already. For a start, reverse thrust is not taken in to account in the performance calculations for a dry runway, the stopping distance is based on brakes worn to the legal limit, tyres worn to within 20% of the legal limit, only 50% of the headwind component but 150% of any tailwind, first action taken up to 2 seconds after V1 (not that this should be considered as usable when making a decision to reject), plus an addition margin (I think 10% for twins?) on top of the demonstrated accelerate-stop distance. 200ft is an absolute worst-case scenario that would require everything to go against you, for your stopping performance to be significantly degraded compared to the book and for you to have mishandled the aircraft. Therefore, there is no need to add your own margins on top of the existing margins: especially if doing so might involve an unusual or uncommon procedure with the potential to introduce other problems later on.

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It's random within the limits you set when you set up the aircraft in PFPX: so if in the aircraft database you set pax capacity for G-EUNA/B to 23 (or whatever the layout is) you won't ever get a load higher than that (you might want to restrict the cargo capacity as well if it regularly gives you, say, 15 pax but 10 tonnes of cargo which is putting you over the limit). I don't think you can currently cap the loads between city pairs though, so you're right in saying you may have to tweak the payload in some cases.

 

My fault, yes you are right. In that case I would have a pre-preared aircraft setup in PFPX with that PAX limit. So yes, you are right there is a way to deal with that in PFPX. I was wrong there.

 

 

 

Worth mentioning that there is a school of thought which says: the calculations have big margins built in already. For a start, reverse thrust is not taken in to account in the performance calculations for a dry runway, the stopping distance is based on brakes worn to the legal limit, tyres worn to within 20% of the legal limit, only 50% of the headwind component but 150% of any tailwind, first action taken up to 2 seconds after V1 (not that this should be considered as usable when making a decision to reject), plus an addition margin (I think 10% for twins?) on top of the demonstrated accelerate-stop distance. 200ft is an absolute worst-case scenario that would require everything to go against you, for your stopping performance to be significantly degraded compared to the book and for you to have mishandled the aircraft. Therefore, there is no need to add your own margins on top of the existing margins: especially if doing so might involve an unusual or uncommon procedure with the potential to introduce other problems later on.

 

I didn't know that was taken in to consideration. To be fair, now I think about it, I ask myself "what does margin mean in Topcat" is that margin to abort up to V1, or margin after rotate, which is what I thought it meant. But that comes down to, I presume, not reading the documentation enough. Which, in the end, makes me wonder how much of us are taking too much on face value. That's a personal observation though, not a fault on Topcat.

 

So. if I take you're comment, then a 79ft margin, which I've seen in my margins, could be deemed close, but fine. And striving for extra feet is probably a lost cause. As long as it's more then 0ft (maybe more then 0 but you get my point) then we are safe

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I ask myself "what does margin mean in Topcat" is that margin to abort up to V1, or margin after rotate, which is what I thought it meant.

 

No problem! I had to double-check it myself.

 

It's ASDA - ASDR: in other words, the amount of runway you would expect to have remaining in front of you after a reject from V1 (in the worst-case scenario outlined above).

 

 

As long as it's more then 0ft (maybe more then 0 but you get my point) then we are safe

 

Exactly!

 

The numbers in the performance manuals are derived from test flights: but the regulators recognise that not every pilot is as skilled as the Boeing (or whichever company) test pilots that achieved those figures, and that the test pilots weren't recording the figures on a dark and stormy night somewhere on the edge of civilisation whilst operating their sixth sector of the day after a week of 3am starts! For that reason, almost all the numbers that you see in performance manuals have quite large margins added to them, as well as adding an extra factor intended to keep you safe even if you (moderately) mishandle the aircraft.

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For fun, I did a lszb to lszh flight. Loaded the NGX to MZFW + 4100kg fuel. Takeoff Weight 65.9 tonne.

 

Bleeds and packs on and wing and cowl tai on (as a test). Ambient temp 30 degrees C. Can't remember QNH. Wind 8kt HW. 26K thrust. 737-800W SFP.

 

V speeds were generated from the NGX FMC for flap 25. Think they were 128/131/140? Can't remember.

 

Programmed an engine fire 5 knots before V1.

 

Let's just say there is no room to spare after an RTO. Remember; I was expecting the engine failure. Imagine the response time if I weren't expecting it. I would've ended up on the grass.

 

Out of LSZB I would impose a payload limit, turn bleeds off and thrust bump to 27K.

 

I don't know what climb perf is like at the same weights with an engine failure at/after v1.

 

Is there any payware for these airports?


Brian Nellis

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I also re-did it with a thrust bump, weirdly I never even noticed that option. But I agree, getting the most you can from your aircraft would be the best way to get out of there. Being Switzerland you would expect that anti-ice can be required in the winter.

 

That being said, looking for scenery I noticed there was a file for "runway extension" so as far as understand the runway is slightly longer in real life then the FSX default. That was freeware, also found this

 

http://www.shop.aerosoft.com/eshop.php?action=article_detail&s_supplier_aid=11837&s_design=DEFAULT&shopfilter_category=Flight%20Simulation

 

However, I'm sure I saw payware scenery for Switzerland which covered more then this airport. Don't quote me on that though

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Takeoff Weight 65.9 tonne.

Bleeds and packs on and wing and cowl tai on (as a test). Ambient temp 30 degrees C. Can't remember QNH. Wind 8kt HW. 26K thrust. 737-800W SFP.

 

FWIW, I just ran those figures in TOPCAT for fun. It won't let you go in that configuration (in fact, in any configuration at that weight) -- albeit I don't know what thrust rating the TOPCAT data is using. The maximum RTOW I could come up with was around 64.8T.

 

 

 


Being Switzerland you would expect that anti-ice can be required in the winter.

 

Bear in mind in the winter the cooler (cold) temperatures will increase your performance: 30C at the airfield elevation of 1500ft is around is ISA +18, resulting in a density altitude of around 3600ft. With an average winter temperature of around 0C, density altitude will be around 400ft, which will help offset the impact of TAI usage (though probably not completely).

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High speed RTO practice in the 737-800W SFP 26K.

 

RW14/26K/F25/27C/1013/10kt HW/Engine Bleeds ON/WAI ON/CTAI ON/65.8T TOW - 131/132/140 you can juuuuuust make it both engines operative. With a severe damaged engine below v1 you overshoot.

 

RW14/WET/26K/F25/27C/1013/10kt HW/Engine Bleeds ON/WAI ON/CTAI ON/55.8T TOW - 119/120/131 easily make it both engines operative. With a severe damaged engine below v1 you make it.

 

Somewhere in between I guess there is a healthy median. For some weird reason, when on the take off roll down RW32 IAS jumps 10knots as soon as THR HLD is displayed on the FMA, GS increases gradually as expected.


Brian Nellis

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That's a good point, I hadn't even factored in wet or contaminated runway. In those cases I presume you would have to use even less payload.

 

I had to lower down to around 100 passengers, can't remember what the cargo load was, but this was in the dry. With a wet runway, if you had to reduce passengers some more it could make the flight not cost worthy. Not that this is an issue for me obviously but it is something I thought of.

 

If we take you're two examples and try to find a happy medium it may end up be that the use of Bleeds OFF would either give a bit more payload, or more runway in which to stop. Myabe that would come down to weather conditions at the time and there is no hard answer on it.

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That's a good point, I hadn't even factored in wet or contaminated runway. In those cases I presume you would have to use even less payload.

 

I had to lower down to around 100 passengers, can't remember what the cargo load was, but this was in the dry. With a wet runway, if you had to reduce passengers some more it could make the flight not cost worthy. Not that this is an issue for me obviously but it is something I thought of.

 

If we take you're two examples and try to find a happy medium it may end up be that the use of Bleeds OFF would either give a bit more payload, or more runway in which to stop. Myabe that would come down to weather conditions at the time and there is no hard answer on it.

+1. Why don't they have a like button on this forum.

Brian Nellis

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Unfortunately only for the whole thread at the top of the page.

 

Thanks for all the help though, it's been fantastic

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That's a good point, I hadn't even factored in wet or contaminated runway. In those cases I presume you would have to use even less payload.

 

Funnily enough, you might be surprised. The performance requirements are different in the wet: whereas on a dry runway you have to reach a 35ft screen height at the end of the TODA, in the wet you only have to reach 15ft. You can also take reverse thrust in to account for the stopping distance.

 

Counter-intuitively, in some circumstances you can even find it's possible to get more payload (particularly in the wet) by using a derate: if you are Vmcg limited, a de-rate gives you a potentially lower V1 and thus possibly more payload.

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Counter-intuitively, in some circumstances you can even find it's possible to get more payload (particularly in the wet) by using a derate: if you are Vmcg limited, a de-rate gives you a potentially lower V1 and thus possibly more payload.

A lower V1 would be desirable at a short field, but how would the derate impact Vr and V2 if at all?

Brian Nellis

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