killbilly
Vocals, Lyrics, Triangle, Washboard, Kittens
Would like to know more about this as a new 145FO. What worked?
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- Thread starter ZapBrannigan
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Would like to know more about this as a new 145FO. What worked?
You know things are going downhill when the CA is making the PA announcement hawking the AA credit card and all its bennies for signing up. Like he is today.
is that one of the new changes?
ZapBrannigan
If it ain’t a Boeing, I’m not going. No choice.
Here’s an interesting one. We have an app we use to determine holdover times. It uses current weather, visibility, fluid type, etc to produce a holdover time and then includes a nice timer function.
Yesterday it came back with heavy snow. No holdover time. With a little note at the bottom that says (paraphrased), “No holdover time. Do cabins check within 5 minutes of takeoff.”
So let me get this straight. For light snow, we measure with a micrometer, and we calculate a holdover time down to the minute… but for heavy snow they’re like “F it. Send a dude back there to peek out the window that’s covered with precip and deice slime and if he’s mostly sure that the fluid is still working, blast off!”
Seems so sketchy to me. But that’s what we did.
I remember deicing in LGA back in the 90s and when the holdover time expired we got back in line for another spray. What happened in the last two decades that now it’s ok to cabin check and go? Or am I misremembering the circumstances?
Yesterday it came back with heavy snow. No holdover time. With a little note at the bottom that says (paraphrased), “No holdover time. Do cabins check within 5 minutes of takeoff.”
So let me get this straight. For light snow, we measure with a micrometer, and we calculate a holdover time down to the minute… but for heavy snow they’re like “F it. Send a dude back there to peek out the window that’s covered with precip and deice slime and if he’s mostly sure that the fluid is still working, blast off!”
Seems so sketchy to me. But that’s what we did.
I remember deicing in LGA back in the 90s and when the holdover time expired we got back in line for another spray. What happened in the last two decades that now it’s ok to cabin check and go? Or am I misremembering the circumstances?
SlumTodd_Millionaire
Most Hated Member
I believe the reason is that heavy snow is considered too difficult to predict a holdover time, so the only safe way to do it is an actual check, because no amount of time without a check would be considered safe.
roundout
VNAV monitor
y’all still gonna push your mic boom out of the way and pick up the hand PA even on planes where there’s a PA pushbutton that allows you to make PAs through the boom mic?
roundout
VNAV monitor
Yes. At least at my place there’s no holdover time for heavy snow (and some other stuff). The only way to take off legally is to do an exit row inspection. If the FO can’t determine the condition of the wing, you can’t take off.
I’m going to guess and say better fluids and better testing methods.
roundout
VNAV monitor
No
Stone Cold
Well-Known Member
The best part of those holdover times is moderate snow at night magically become heavy snow, with no holdover times. According to the notes, you’re not supposed to use the tower or ATIS measurement of snow at night, so there are no options. It gets fun in Anchorage this time of year.
I’ll just leave that conversation there without more comment.
killbilly
Vocals, Lyrics, Triangle, Washboard, Kittens
I wonder if you use the same app that we do.
I'm curious about the heavy snow thing, too....they preach the Clean Wing/Airframe Concept here so perhaps there's no HOT since snow can pile up on top of a treated wing faster than the anti-icing fluid can fight it? Which sort of makes sense? But I've never had to deal with that. Yet. I'll let you know. I suspect I'll have some first-hand experience before too long.
L-16B
Well-Known Member
Shhhhh that PA button is a secret. Still gotta use the hand PA.
L-16B
Well-Known Member
Haha I’ve never seen a captain push the credit card sale. I cringe when the FA’s do it and we are running like 3-4 hours late. Gotta know the audience.
L-16B
Well-Known Member
If I had to guess OE trips will be some of the few complying with the new changes.
First, you allow copilots to make PA's, next they'll be sleeping with our wives! WHERE DOES IT END?!
BobDDuck
Island Bus Driver
@BobDDuck is correct as usual, because he’s a super smart dude. @ppragman is also a super smart dude and pulled a mathematician moveand approximated a linear relationship on things that are actually curves, which was an intuitive assumption and cut to the chase of how the variables were related. You guys don’t need my help but I thought about what I might be able to type that would be value added and include some of the extra variables in the drag equations without getting too in the weeds.
When I first thought about this problem I skipped ahead to drag counts and parasitic vs induced drag and the relationship between CL vs CD at different airspeeds…
View attachment 68826
…and then I realized I think @BobDDuck ’s point can be made by looking at some graphs and just using the lift equation to relate the Lift force (L) back to the non-dimensional lift coefficient (CL) and then using the CL vs alpha curve to back out the required angle of attack (alpha), so let’s do that.
Recall in level un accelerated flight Lift must be equal to Weight and Thrust must be equal to Drag. L=W, T=D.
Let’s think about @mikecweb ‘s MD-11on an empty repo flight vs at max gross weight. Empty is W1, max gross is W2.
On both flights the lift force produced by the wing has to be equal to the current weight, so empty lift is L1 and max gross lift is L2 because L=W.
Onward to the lift equation:
View attachment 68827
We can call everything in red dynamic pressure (q) and say q = 1/2*rho*v^2. S is the wing area (usually in ft^2). The cool thing about this equation is that since CL is what’s related to angle of attack we don’t actually have to care about dynamic pressure or wing area and we can divide both those things out to get the lift coefficient by itself:
L = 1/2*rho*v^2*CL*S
But L = W, so
W =1/2*rho*v^2*CL*S
W = q*CL*S
CL = W/(q*S)
So for our two weights:
CL1 = W1/(q*S) (empty) and
CL2 = W2/(q*S) (max gross)
The MD-11 wing is going to have a relationship between angle of attack and CL that looks something like this (and you could take a section of it and throw it in a wind tunnel and verify experimentally and your plot would look similar to this):
View attachment 68828
So in other words as you increase angle of attack your CL increases linearly up to the hump, which is CL max, after which lift starts to drop off and your wing stalls.
You can use the sketch above to translate CL back to angle of attack, so knowing that CL1 at empty weight W1 is less than CL2 at gross weight W2, you can see that a greater angle of attack will be required.
Most of you can probably stop reading here, because you know intuitively that more thrust will be required to maintain level flight at the same airspeed at a higher angle of attack, and we haven’t even talked about thrust yet, but I can try and show that with a graph too.
That same wing will also have a CD vs alpha curve, which may look something like one of these. (Note these are parabolas but if you squint at them they become straight lines which is the educated guess @ppragman made.
View attachment 68829
And there’s a drag equation that is identical to the lift equation but you replace CL with CD:
D = q*CD*S
T=D so T = q*CD*S
CD = T/(q*S), so you could draw the same CD1 vs alpha1 and CD2 vs alpha2 picture only this time you would input the known alpha1 and alpha2 and see whether the drag coefficient at the higher weight is greater than the drag coefficient empty, and it is, therefore you’re going to need more thrust.
Finally, some of you will notice that my drag coefficient graph is Cd not CD, this means it’s for a 2D airfoil and not a 3D wing on a 3D airplane. When you translate from the 2D wind tunnel world into the 3D wing world a bunch of complicated crap happens (that I’d like more practice with myself) due to this equation:
View attachment 68830
So basically this means the engineers have to do a “drag buildup” where the parasitic (and other) drag coefficients for all the different parts of the airplane are all calculated and then added together to get CD0, and then needing to account for induced drag due to the wingtip vortices - which itself is related back to CL from earlier because it’s dependent on how much lift the wing is producing.
I think the moral of all this is whether you count induced drag or not (which depends on how slow you’re flying) you can see from the second graph relating CD vs CL that induced drag just adds more drag, but it doesn’t change the fact that even back at the 2D airfoil level and the lift equation we could intuitively see what was going to happen…
TL;DR if you add more weight, you’ll need more lift to balance it out. More lift means a higher CL. In order to get a higher CL at the same airspeed and atmospheric conditions (q) and same airplane (wing area, S) you have to increase your angle of attack (alpha). Increasing that angle of attack results in a higher drag coefficient, which requires more thrust to remain balanced.
Way back to the origin of all this, I don’t fly jets but I’ve also heard of the using the fuel flow trick to set thrust just because the precision is just better, even though it’s a secondary indication so there may be a little more lag(?) But other than it being a non-standard indicator, it sounds like you guys just memorized a look-up table (or developed an algorithm) to relate airspeed to power setting. And knowing an initial setting to try and then fine tune as required is going to be a good bet especially when the spread between weight/CG combinations is smaller, as discussed above.
Hope that helped or was at least mildly entertaining.
This is an awesome write up. I bailed on aerospace engineering after three years of school (probably should have stuck it out until the end but oh well) and this sort of problem solving was what mostly kept me interested those three years. I think I was still able to follow along on about 50% of it which I'll take as a huge win.
ppragman
FLIPY FLAPS!
We need a mathematical rule of thumb thread where we go through and evaluate rules of thumb, then maybe we collectively start generating new rules of thumb…
I love this sort of thing!
bLizZuE
Calling for engine starts en français
Hey champ, thanks for giving everyone else a whole day before writing the post of the friggin year.@BobDDuck is correct as usual, because he’s a super smart dude. @ppragman is also a super smart dude and pulled a mathematician move
When I first thought about this problem I skipped ahead to drag counts and parasitic vs induced drag and the relationship between CL vs CD at different airspeeds…
View attachment 68826
…and then I realized I think @BobDDuck ’s point can be made by looking at some graphs and just using the lift equation to relate the Lift force (L) back to the non-dimensional lift coefficient (CL) and then using the CL vs alpha curve to back out the required angle of attack (alpha), so let’s do that.
Recall in level un accelerated flight Lift must be equal to Weight and Thrust must be equal to Drag. L=W, T=D.
Let’s think about @mikecweb ‘s MD-11on an empty repo flight vs at max gross weight. Empty is W1, max gross is W2.
On both flights the lift force produced by the wing has to be equal to the current weight, so empty lift is L1 and max gross lift is L2 because L=W.
Onward to the lift equation:
View attachment 68827
We can call everything in red dynamic pressure (q) and say q = 1/2*rho*v^2. S is the wing area (usually in ft^2). The cool thing about this equation is that since CL is what’s related to angle of attack we don’t actually have to care about dynamic pressure or wing area and we can divide both those things out to get the lift coefficient by itself:
L = 1/2*rho*v^2*CL*S
But L = W, so
W =1/2*rho*v^2*CL*S
W = q*CL*S
CL = W/(q*S)
So for our two weights:
CL1 = W1/(q*S) (empty) and
CL2 = W2/(q*S) (max gross)
The MD-11 wing is going to have a relationship between angle of attack and CL that looks something like this (and you could take a section of it and throw it in a wind tunnel and verify experimentally and your plot would look similar to this):
View attachment 68828
So in other words as you increase angle of attack your CL increases linearly up to the hump, which is CL max, after which lift starts to drop off and your wing stalls.
You can use the sketch above to translate CL back to angle of attack, so knowing that CL1 at empty weight W1 is less than CL2 at gross weight W2, you can see that a greater angle of attack will be required.
Most of you can probably stop reading here, because you know intuitively that more thrust will be required to maintain level flight at the same airspeed at a higher angle of attack, and we haven’t even talked about thrust yet, but I can try and show that with a graph too.
That same wing will also have a CD vs alpha curve, which may look something like one of these. (Note these are parabolas but if you squint at them they become straight lines which is the educated guess @ppragman made.
View attachment 68829
And there’s a drag equation that is identical to the lift equation but you replace CL with CD:
D = q*CD*S
T=D so T = q*CD*S
CD = T/(q*S), so you could draw the same CD1 vs alpha1 and CD2 vs alpha2 picture only this time you would input the known alpha1 and alpha2 and see whether the drag coefficient at the higher weight is greater than the drag coefficient empty, and it is, therefore you’re going to need more thrust.
Finally, some of you will notice that my drag coefficient graph is Cd not CD, this means it’s for a 2D airfoil and not a 3D wing on a 3D airplane. When you translate from the 2D wind tunnel world into the 3D wing world a bunch of complicated crap happens (that I’d like more practice with myself) due to this equation:
View attachment 68830
So basically this means the engineers have to do a “drag buildup” where the parasitic (and other) drag coefficients for all the different parts of the airplane are all calculated and then added together to get CD0, and then needing to account for induced drag due to the wingtip vortices - which itself is related back to CL from earlier because it’s dependent on how much lift the wing is producing.
I think the moral of all this is whether you count induced drag or not (which depends on how slow you’re flying) you can see from the second graph relating CD vs CL that induced drag just adds more drag, but it doesn’t change the fact that even back at the 2D airfoil level and the lift equation we could intuitively see what was going to happen…
TL;DR if you add more weight, you’ll need more lift to balance it out. More lift means a higher CL. In order to get a higher CL at the same airspeed and atmospheric conditions (q) and same airplane (wing area, S) you have to increase your angle of attack (alpha). Increasing that angle of attack results in a higher drag coefficient, which requires more thrust to remain balanced.
Way back to the origin of all this, I don’t fly jets but I’ve also heard of the using the fuel flow trick to set thrust just because the precision is just better, even though it’s a secondary indication so there may be a little more lag(?) But other than it being a non-standard indicator, it sounds like you guys just memorized a look-up table (or developed an algorithm) to relate airspeed to power setting. And knowing an initial setting to try and then fine tune as required is going to be a good bet especially when the spread between weight/CG combinations is smaller, as discussed above.
Hope that helped or was at least mildly entertaining.
ERfly
Well-Known Member
Yes, it does. Either in the descent or when the main tanks get below a certain level, the fuel starts moving forward out of the tail tank so that the tail tank is empty by landing.
You have to have a total fuel of 60,000 lbs or great and the Fuel System Controller has to be in Auto for Tail Fuel Management to work.
It's a neat system and allows us to actually make fuel on longer flights. It also depends on how we're loaded as to how much Tail Fuel Management we get. Even when it's working (or we have a high enough fuel load), they always seem to want to load us with an aft CG to start with.
It seems that she's constantly moving fuel to different tanks all the time. I tend to brief jumpseaters sitting up front that aren't familiar with the 11 that they're going to see lights going on and off on the fuel panel all the time. That's in addition to seeing all the Pack and Bleed lights come on as soon as we push the throttles up for takeoff.
ASpilot2be
Qbicle seat warmer
Also need to pass it along to Captains.
L-16B
Well-Known Member
Pretty sure on the airbus that’s the fire missle button