PACKS off take-off question

[Stearmandriver 2,205]

To be honest, I can't give exact parameters because I don't know them either. From a pilot standpoint, we use the bleed and flap configuration that our performance data from central load planning says to. Bleeds off = a little more available thrust, a higher flap setting gets you off the runway sooner and so helps with short runways and close in obstacles. sometimes you'll see artificially inflated v speeds that use more runway but launch you with more energy, resulting in a steeper climb that can help clear obstacles farther out in the take off profile.

[codechris 9]

Ok thanks, that helps.

 

No obstacles at Bern, however a short runway so I will use Topcat as stated earlier and just work out what gives me the best run way length. I saw some runway margins around 80m but bleeds off and I can get 200m. I'll take the 200m.

 

I'm not a pilot or a dispatcher, but sounds good to me

 

Thank you for contributing

[kevinh 409]

There are other airports with short and narrow runways like Queenstown (NZQN). It's runway is 5830'/98' (1777m/30m). It is 47m longer than the one you're proposing to takeoff from. 737's operate in and out of there.

 

But most airlines flying to these small runways usually have SFP and 26K thrust as a minimum.

 

With regard to BLEEDS off (not PACKS off, because the pack switches don't move from AUTO), it's up to you.

 

I wouldn't do an unpressurised takeoff if the APU was working ok. But my appetite for risk is different to yours. I wouldn't risk another Helios.

 

PS

The airport would need special assessment for suitability. It would then might regulatory approval. Pilots would get special training.

Maybe they don't operate 737's out of LSZB because pavement limitations. If worst comes to worst, just limit the amount of weight you leave LSZB with, like they do in the real world.

Helios wasn't caused by packs or bleeds being switched off and your after takeoff checks should ensure packs are on. Not really a risk.

[codechris 9]

Packs have nothing to do with this Kevin as we have learnt in this post, it's bleeds. I was wrong in quoting packs when infact I meant bleeds. This was a misunderstanding on my part. I quote from Helios

 

"After the aircraft was returned into service, the flight crew overlooked the pressurisation system state on three separate occasions: during the pre-flight procedure, the after-start check, and the after take-off check. During these checks, no one in the flight crew noticed the incorrect setting"

[Copper. 78]

Helios wasn't caused by packs or bleeds being switched off and your after takeoff checks should ensure packs are on. Not really a risk.

You're right on both counts. In the interest of brevity, I didn't and won't elaborate.

 

I would still consider a bleeds off, unpressurised takeoff a threat and brief it as such because the only guard against forgetting to reconfigure air would be the after takeoff c/l, until you get the horn; the final guard.

 

At that stage (intermittent horn), if you're unlucky enough to get to that stage, the "tea and biscuit" conversation is the least that would occur as a result, hence why is consider it a threat for briefing, and why i'd opt for SP 2.5 if the APU was available.

 

That's just me. I'm not a pilot. Your thoughts?

[Spin737 102]

Bleeds OFF is pretty much the last resort. The computer is looking to optimize the N1 hat still allows runway and climb performance.

 

For places like LIH, we're runway limited so it's a F25 takeoff. If 27k BUMP still doesn't get us the performance, then it's add BLEEDS OFF.

 

Some places might be climb limited, so F25 won't help. If we're at max power, then it's usually F5 BLEEDS OFF.

[codechris 9]

Hmm....makes judging, as a non pilot, situations like this near impossible unless you have someone with a pilot experience to tell you.

 

As I said, Bern doesn't have a climb issue, just a short runway and right now, a high temperature. I know when it comes to 1830z Sunday, it's going to be between 30 and 35 degrees,. So maybe F. 25 isn't needed, or even Bleeds off, just as long as I can hit V.Rotate in time. But, as a non-pilot that's difficult to know.

 

Maybe that's the point. This comes out of either experience of yourself as a pilot, or airline experience. The second you will never get in the sim world, the first you can get but maybe with some dead virtual passengers in the process

[Copper. 78]

Stearmandriver and Spin737 are both pilots for an airline. Real 737 pilots

[codechris 9]

I just meant there are lots of these things that come up, and it's hard to ever know what the right answer is as a non-pilot, and even then, answers vary. I just used it as an example of it being near impossible for me to know the best answer.

 

But I appreciate the responses from real pilots it's great to see and read. But, after everything that's been said all that calls out most to me is run the numbers in Topcat and see what works in my head. I'm not working in dispatch for an airline so it might not be the best answer, but what works for me is good

[Stearmandriver 2,205]

It's worth remembering too that all this data is calculated based on worst case scenario - engine failure at V1. So realistically, just use max thrust and flaps 15/25 if you really want to be safe, and don't even mess with bleeds off. An engine is probably not going to quit. (I'm certainly not advocating this attitude in real life.) But you aren't really "missing out" on an experience by doing it this way; like I said, RW airline guys aren't figuring this out themselves, based on experience or anything else. Typically, it's computed by a computer. Possibly, it's computed by a pilot or load planner using tables... But regardless, none of us are making decisions. Those have already all been made by Boeing engineers; we just use what they tell us the day's combination of factors requires.

 

I agree, btw, with whoever said they'd regard a bleeds off takeoff as a threat and brief it accordingly. I'd consider ANY non standard takeoff config a threat. I'm not worried about forgetting to turn the bleeds back on (no way you wouldn't notice an unpressurized climbout), I'm worried about remembering to turn them off in the first place. Or about briefing a flaps 25 takeoff and then slamming the handle into the 5 gate like you've done a thousand times before. Habits are powerful things, and if you consciously develop good ones, they are valuable things as well... buy they DO mean that any required actions that run contrary to your habits automatically become threats. It's just too easy to fall back on ingrained habits. Of course, the way to trap this kind of error is to also consciously develop good checklist discipline. Don't just read/respond out of habit, but force yourself to really consider every item. I know that sounds really obvious, but on the line, doing the same things day in/out, it's easy to get complacent and sloppy if you let yourself.

 

Anyway, my two cents.

[Chrjs 61]

btw, i actually wouldn't compare a E190 to a B737, there are really two different categories of airliners. THe 190 has a MTOW of around 110,000 lbs, the 738 around 170,000.

 

But as you can see, 738 are operating out of Bern, even though they are BBJs and most certainly much lighter than a regular charter flight.

 

 

However, you can easily compare a 738 to a A420, and they are quite common in Bern

 

 

 

So as long as you limit your weight and check the numbers with Topcat you should be fine. I have used flaps 25 quite few times (Santo Dumont!) and there was never a problem.

[jonesrob 96]

As an outsider coming across this thread, I just want to say how interesting and useful I found it - good question and excellent responses, and I've learnt a bit. Thanks folks.

[kevinh 409]

Hmm....makes judging, as a non pilot, situations like this near impossible unless you have someone with a pilot experience to tell you.

Indeed, and that is because they have the training and experience to do that. Fortunately this is a simulation so if you make a misjudgement it won't have any consequences, unlike the real world.

You're right on both counts. In the interest of brevity, I didn't and won't elaborate.

 

I would still consider a bleeds off, unpressurised takeoff a threat and brief it as such because the only guard against forgetting to reconfigure air would be the after takeoff c/l, until you get the horn; the final guard.

 

At that stage (intermittent horn), if you're unlucky enough to get to that stage, the "tea and biscuit" conversation is the least that would occur as a result, hence why is consider it a threat for briefing, and why i'd opt for SP 2.5 if the APU was available.

 

That's just me. I'm not a pilot. Your thoughts?

My thoughts are that as a non-pilot flying in FSX I'm not briefing anyone, though no doubt many simmers actually do. I don't feel the need to carry the role play that far.

 

Bleeds off takeoffs are, or at least were, fairly routine on 747s at high weights. So I don't regard the concept as inherently dangerous. The fact is the Helios accident which everyone thinks of was a perfectly normal bleeds on takeoff and still it went wrong.

 

Specifically in the NGX, the only reason I would make an unpressurised takeoff (as opposed to a bleeds off takeoff with the APU supplying a pack) would be to overcome the N1 bleed correction logic errors in the NGX simulation.

[codechris 9]

FS2Crew will deal with the bleeds after take-off if I select a bleeds off takeoff, so technically I don't need to worry, however I know that isn't for everyone.

 

Also I've never thought of flight sim as role playing but in a way it is!

 

btw, i actually wouldn't compare a E190 to a B737, there are really two different categories of airliners. THe 190 has a MTOW of around 110,000 lbs, the 738 around 170,000.

 

But as you can see, 738 are operating out of Bern, even though they are BBJs and most certainly much lighter than a regular charter flight.

 

However, you can easily compare a 738 to a A420, and they are quite common in Bern

So as long as you limit your weight and check the numbers with Topcat you should be fine. I have used flaps 25 quite few times (Santo Dumont!) and there was never a problem.

 

Thanks for confirming, very useful. And you're right, I should have looked up the numbers to get a better idea. The people who wrote the event page had said these types of planes are ok which was helpful from the outset http://www.vatsim.net/events/capital-sunday.

 

One thing that had come up is this question of weights, and because I'm generally flying out of places where length of runway is so long I can just click though PFPX at a rather rapid pace, hitting "random payload" and not thinking any more has never proved an issue. However, in this one it's conceivable that I cant be so lazy, especially if I was flying a lot further then London. You could argue that some people wouldn't want to go that deep in to planning and dispatch but it's good to know. I think there is a rather funny irony in that we have all these tools to gain as much realism as possible, but we end up not being realistic because we're far too involved in the planning!

 

And yes, the most that will happen should it go wrong is I embarrass myself on Vatsim and give a few people a good laugh as my plane can't rotate early enough. I think I can live with that on my concious

[skelsey 765]

As I said, Bern doesn't have a climb issue, just a short runway and right now, a high temperature. I know when it comes to 1830z Sunday, it's going to be between 30 and 35 degrees,. So maybe F. 25 isn't needed, or even Bleeds off, just as long as I can hit V.Rotate in time. But, as a non-pilot that's difficult to know.

 

When you do your performance calculations you'll either get an answer or you won't! If you're running a performance calculator such as TOPCAT, just run the performance with bleeds on. If you get a set of speeds, the takeoff should be fine. If you don't, try with bleeds (air conditioning) off. Likewise with F25.

 

It's easier to get a feel for the impact of different factors on performance if you're used to doing the performance manually, because your starting point (before you even get as far as finding the V-speeds and assumed temperature) is to find a maximum RTOW (Regulated Takeoff Weight) for the runway and conditions.

 

To get to that, you start with your MTOW for the runway length. There's then a list of standard corrections for QNH, temperature, headwind/tailwind component and other items such as packs off, anti-ice on, MEL items and so on. This works as a credit/debit system: so for each knot of headwind component, you credit extra weight (100kg in the 767, from memory), whereas a tailwind would be negative, and so on. You then add up each side to get a total postive and total negative figure, find the difference and add/subtract from your MTOW as required.

 

From memory, packs off offered a +2500kg credit in the 767: so it's a fairly significant performance increase, but you have the option: either reduce your payload by 2500kg, or perform a packs-off takeoff.

 

Of course, in a real airline, as has already been mentioned, the fleet only flies to a limited number of places and therefore anywhere on the network that is performance-limited (in whatever way) will have already been noted and there will be guidelines in place in the Route Information Manual (for instance, there might be a payload limitation imposed on the route or there might be guidance that above X degrees C or at weights greater than Y a bleed/packs-off takeoff is required, or that a particular flap setting must be used (GIB is a good example on the BA network where Flap 3 is "strongly recommended" in the Airbus, for instance) -- so in normal ops it's unlikely to be a surprise to the crew.