Thrust
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Takeoff Rolls And Climb Rate Questions

Thu Jul 24, 2008 8:28 pm

What's up. Listen, I've got some questions about the takeoff roll lengths of certain airplanes. Why do certain airplanes take longer to get off the ground than others? Like surely the heavier airplanes must be able to compensate to complete a flight in the same amount of time as a much smaller airplane on the same route? Like does a 747 typically accelerate faster than a smaller plane like the 757 once it is in the air, or what? I guess this is a question I've been pondering for some time. I would think that engineers would realize possible disadvantages of a heavy airliner and would probably try and design the plane in such a way that these disadvantages could be compensated for. If my question needs further clarification, please don't hesitate to let me know.
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Starlionblue
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RE: Takeoff Rolls And Climb Rate Questions

Thu Jul 24, 2008 10:41 pm



Quoting Thrust (Thread starter):
Why do certain airplanes take longer to get off the ground than others?

- Thrust to weight ratio factoring in any derate.
- Take off weight for that flight.
- Decision to delay rotation in order to achieve higher climb speed.

Quoting Thrust (Thread starter):
Like surely the heavier airplanes must be able to compensate to complete a flight in the same amount of time as a much smaller airplane on the same route? Like does a 747 typically accelerate faster than a smaller plane like the 757 once it is in the air, or what?

Most of any flight except very short ones will be at cruise speeds. Acceleration is nowhere near as important as, say, a car, with starts and stops, slowdowns, etc... So the acceleration time to cruise speed is not a huge factor. Time to climb to cruise altitude is probably more critical.

Quoting Thrust (Thread starter):
I would think that engineers would realize possible disadvantages of a heavy airliner and would probably try and design the plane in such a way that these disadvantages could be compensated for.

Well, heavier airliners will have proportionally more engine power so there's the compensation.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
vikkyvik
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RE: Takeoff Rolls And Climb Rate Questions

Thu Jul 24, 2008 11:26 pm



Quoting Starlionblue (Reply 1):
Acceleration is nowhere near as important as, say, a car, with starts and stops, slowdowns, etc...

A good explanation I read paraphrased it thusly:

"Airlines aren't buying a sports car. They're buying a truck."

If I could only remember who wrote that.................

Quoting Thrust (Thread starter):
If my question needs further clarification, please don't hesitate to let me know.

Could you clarify? I'm not sure I understand what exactly you're asking.
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CosmicCruiser
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RE: Takeoff Rolls And Climb Rate Questions

Thu Jul 24, 2008 11:45 pm



Quoting Starlionblue (Reply 1):
Thrust to weight ratio factoring in any derate.
- Take off weight for that flight.
- Decision to delay rotation in order to achieve higher climb speed.

Except "decision to delay rotation to achieve a higher climb speed". That would NEVER happen with the exception of a WINDSHEAR alert on t/o roll then you have a predetermined added factor you use. A higher rotation speed has really nothing to do with climb speed. On rotation the FD command bars direct you to a pitch that is, with all engs running, V2+10. After flap/slat retraction you begin your acceleration to your CLIMB speed. That can vary from airline to airline or particular SID.
 
tdscanuck
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RE: Takeoff Rolls And Climb Rate Questions

Fri Jul 25, 2008 4:16 am



Quoting Thrust (Thread starter):
Why do certain airplanes take longer to get off the ground than others?

Because TOW varies, wings vary, and the thrust/weight ratio is different for different airplanes (and for the same airplane on different takeoffs).

Quoting Thrust (Thread starter):
Like surely the heavier airplanes must be able to compensate to complete a flight in the same amount of time as a much smaller airplane on the same route?

Cruise speed has no direct connection to weight. Cruise speed is primarily a function of aerodynamics with some adjustment for the value of fuel vs. the value of time.

Quoting Thrust (Thread starter):
Like does a 747 typically accelerate faster than a smaller plane like the 757 once it is in the air, or what?

Typically, no. A 757 has considerably more excess proportional thrust than a 747.

Tom.
 
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Starlionblue
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RE: Takeoff Rolls And Climb Rate Questions

Fri Jul 25, 2008 4:29 am



Quoting Vikkyvik (Reply 2):

A good explanation I read paraphrased it thusly:

"Airlines aren't buying a sports car. They're buying a truck."

If I could only remember who wrote that.................

 Smile

Quoting CosmicCruiser (Reply 3):
Except "decision to delay rotation to achieve a higher climb speed". That would NEVER happen with the exception of a WINDSHEAR alert on t/o roll then you have a predetermined added factor you use. A higher rotation speed has really nothing to do with climb speed.

Perhaps I'm not phrasing it clearly but I am pretty sure I am not hallucinating about this.

With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.

I assume this is only with certain types and if concomitant with company procedures.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
vikkyvik
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RE: Takeoff Rolls And Climb Rate Questions

Fri Jul 25, 2008 5:34 am



Quoting Starlionblue (Reply 5):
With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.

I assume this is only with certain types and if concomitant with company procedures.

It sounds like what Cosmic is saying is that this isn't just a spur-of-the-moment decision. I assume it's planned for ahead of time. Correct me if I'm wrong.
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Starlionblue
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RE: Takeoff Rolls And Climb Rate Questions

Fri Jul 25, 2008 6:22 am



Quoting Vikkyvik (Reply 6):

It sounds like what Cosmic is saying is that this isn't just a spur-of-the-moment decision. I assume it's planned for ahead of time. Correct me if I'm wrong.

Yeah well I agree with that.  Wink Certainly not spur of the moment.
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zeke
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RE: Takeoff Rolls And Climb Rate Questions

Fri Jul 25, 2008 10:08 am



Quoting CosmicCruiser (Reply 3):

Except "decision to delay rotation to achieve a higher climb speed". That would NEVER happen with the exception of a WINDSHEAR alert on t/o roll then you have a predetermined added factor you use. A higher rotation speed has really nothing to do with climb speed. On rotation the FD command bars direct you to a pitch that is, with all engs running, V2+10. After flap/slat retraction you begin your acceleration to your CLIMB speed. That can vary from airline to airline or particular SID.

It is called V2 overspeed (or improved climb procedure), and is used by airlines the world over.

V1/VR/V2 are increased (but still withing runway and screen height limits), such that the new V2 is closer to the optimum climb speed giving a better takeoff gradient or the same gradient with a higher takeoff weight.

Requires a runway/stopway in excess of the minimum length, good surface and the aircraft in good condition.
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CosmicCruiser
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 12:40 am



Quoting Zeke (Reply 8):
V1/VR/V2 are increased (but still withing runway and screen height limits), such that the new V2 is closer to the optimum climb speed giving a better takeoff gradient or the same gradient with a higher takeoff weight.

The more I looked into it it appears it was initially an Airbus thing but Boeing & MDD also published some data on it. Interestingly it looks like V2+20 or in some cases +30 would be the best climb gradient. Here we never change Vr or V2 but an all eng climb is always V2+10. Lose an eng and it's back to V2. Vr, as I said can change with a windshear alert for the arpt. Then it changes for brute energy at rotation.
 
lowrider
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 2:53 am



Quoting Starlionblue (Reply 5):
With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.
I assume this is only with certain types and if concomitant with company procedures.

We use a performance program that does this for us. It looks at current conditions and the various structural and performance limitations. If it can improve our payload by enhancing the energy for the second segment, it will automatically do so. This can make for some startlingly late rotations, but its not like the airport gives us a discount for using less runway. If we can get a few more pounds of revenue on board, it is worth it.
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2H4
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 3:26 am



Quoting Lowrider (Reply 10):
its not like the airport gives us a discount for using less runway.

How interesting things would be if that were indeed the case...  scratchchin 

2H4
Intentionally Left Blank
 
lowrider
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 3:30 am



Quoting 2H4 (Reply 11):
How interesting things would be

It would make it harder to cater to photographers who like those end of runway overflight shots. That is, of course, the real reason we do it.
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2H4
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 3:34 am



Quoting Lowrider (Reply 12):
That is, of course, the real reason we do it.

And here I thought it was just to spite NIMBYs...

2H4
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lowrider
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 3:37 am



Quoting 2H4 (Reply 13):
And here I thought it was just to spite NIMBYs...

Just an added benefit. One of the many services we provide.
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tdscanuck
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 4:24 am



Quoting Starlionblue (Reply 5):

With a long enough runway, pilots can decide to rotate later (perhaps with a lower flap setting) and thus get a higher take-off speed. This will make segment two (I think it is) climb rate better.

It will get you to optimum climb rate faster, but it won't change your climb rate. The steady state climb rate is fixed by aircraft geometry, weight, attitude, and engine power.

Tom.
 
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Starlionblue
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 6:51 am



Quoting Tdscanuck (Reply 15):

It will get you to optimum climb rate faster, but it won't change your climb rate. The steady state climb rate is fixed by aircraft geometry, weight, attitude, and engine power.

So much to learn! Thx!
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Pihero
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RE: Takeoff Rolls And Climb Rate Questions

Sat Jul 26, 2008 10:23 am



Quoting Tdscanuck (Reply 15):
It will get you to optimum climb rate faster, but it won't change your climb rate. The steady state climb rate is fixed by aircraft geometry, weight, attitude, and engine power.

We are here talking *Takeoff performance*, and thus we are talking about *one engine out* climb (in this case).
So, a V2, whether "raw" or *improved* is still a V2 for piloting aspects, and the *improved V2* gives a better second segment climb gradient and over-obstacle performance with all engines running.
The acceleration comes later.
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draigonair
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RE: Takeoff Rolls And Climb Rate Questions

Sun Jul 27, 2008 9:19 am

When you are taking off and use a lower flap setting you will need to go faster to generate enough lift to lift off. However, due to the lower flap setting, your climb performance will be better (an increase in VY (climb rate) because you use less flaps and hence less drag). This could be used if there is a limited obstacle take off clearance.
However, using less flaps will also decrease the maximum takeoff weight. So, the heavier the plane, the more flaps are needed to generate enough lift permitting there is enough runway.

Derated take off is also used to safe engine life: less thrust is used and hence it will take longer to accelerate along the runway.

Also depending on the weather, the warmer it is outside the longer it will take for the a/c to accelerate, this also is the same for airports that are located at a large altitude. (warm, air is less dense hence need more airflow over the wing is needed to generate the needed lift, same for engines)
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Pihero
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RE: Takeoff Rolls And Climb Rate Questions

Sun Jul 27, 2008 9:42 pm

Draigonair,

Quoting DRAIGONAIR (Reply 18):
due to the lower flap setting, your climb performance will be better (an increase in VY (climb rate) because you use less flaps and hence less drag).

We are not really interested in rate of climb, but in climb gradient

Quoting DRAIGONAIR (Reply 18):
However, using less flaps will also decrease the maximum takeoff weight.

You'll have to qualify that as there are a good number of runways that are long enough so that they do not impact on the TOD or the Accelerate-stop distance.

Quoting DRAIGONAIR (Reply 18):
So, the heavier the plane, the more flaps are needed to generate enough lift permitting there is enough runway.

See above

As a general rule, on an international runway, the second segment limitation comes before the TOD, ASD limits (See the example of a hot-and-high runway) so a lesser flap configuration is generally a good idea to consider.

Look up somewhere on the net a publication called "Getting to grips with aircraft Performance".
Worth a read.
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AAH732UAL
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RE: Takeoff Rolls And Climb Rate Questions

Sun Jul 27, 2008 10:06 pm

The standard rate of climb is 200 feet per NM. That starts 35 feet above the DER(Departure End Runway). The OCS(Obsatcle Clearence Surface) is the 40:1 slope..... or 152 feet per NM. The 200 feet per NM is the standard used which allows the ROC(Required Obstacle Clearence) to be meet. This is changed when something is in the OCS and then a note is made in the ODP(Obstacle Depature Procedure) or SID that plane has to meet in its climb gradient to fly it. They use these numbers all the way to the MEA, MORA, MOCA, filed cruise etc. as well as positive course guidence (PCG). New TERPS has an initial climb area (ICA) set to allow at least 400 feet above the DER as well as PCG.

There is also a ton of other stuff that goes into this but the above is the BASIC(rest is a ton of math crap & stuff  Smile ) stuff used to design the SID/ODP, rate of climb, etc. This is according to FAA TERPS and pilots really don't need to know all this stuff, just how to fly them  

I go into this stuff, because the designers have already taken into account the climb performace and has designed the stuff off their numbers, in turn the planes must meet these set fourth to fly the procedures!

That is why there will be different ways that airlines have to help planes meet these. I doubt the 757 would have problems most of the time BUT a fully loaded 744 has problems a lot of times. That of course depending on the TORA, TODA, etc etc.....

Sorry if I got to tech but at least this provides some light into how they figure this stuff out.
     

[Edited 2008-07-27 15:29:12]
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SlamClick
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RE: Takeoff Rolls And Climb Rate Questions

Sun Jul 27, 2008 10:22 pm



Quoting Thrust (Thread starter):
would think that engineers would realize possible disadvantages of a heavy airliner

You are right. The engineers would have us fly around empty. Fortunately the accountants who understand the advantages of heavy airplanes, prevail and we sell as many tickets and as much belly freight as we possibly can.

Quoting Starlionblue (Reply 1):
- Take off weight for that flight.

You meant to say weight AND configuration, right?

Quoting CosmicCruiser (Reply 3):
"decision to delay rotation to achieve a higher climb speed". That would NEVER happen

Actually yes, not that uncommon "hot and high" where second-segment climb performance is a factor. Involves selecting a higher V1 and VR for a takeoff. V2 would not be changed and the selected V1 speed ultimately will be limited by accelerate-stop distance. In practical terms, however, a given hotrod two-holer might be able to accelerate to tire limit speed and still reject the takeoff on a very long runway. The reason for the V1 and VR overspeed is twofold. (1) The plane will accelerate faster from original charted V1 to the new VR faster with the weight on the wheels than wallowing through the air, and (2) the new liftoff speed is nearer L/DMAX and that improves the initial climb performance.
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AAH732UAL
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RE: Takeoff Rolls And Climb Rate Questions

Sun Jul 27, 2008 10:33 pm



Quoting CosmicCruiser (Reply 3):
That would NEVER happen

Yes it sorta does..... I have been part of a sim session when they put DEN at crazy hot and stuff. It was V1, Vr, V2, and then called "rotate"...... like 10-15 knots higher then V2...... We were really moving but still climbed like crap  Smile
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Pihero
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RE: Takeoff Rolls And Climb Rate Questions

Sun Jul 27, 2008 11:45 pm



Quoting SlamClick (Reply 21):
Quoting CosmicCruiser (Reply 3):
"...decision to delay rotation to achieve a higher climb speed". That would NEVER happen

Actually yes, not that uncommon "hot and high" where second-segment climb performance is a factor. Involves selecting a higher V1 and VR for a takeoff. V2 would not be changed...

Actually, it could be changed. There is now the possibility of a *V2 range* where :
1.1 Vs < V2 <1.35 Vs (VS here is the Vs 1g and my signs read "at most equal to...)
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SlamClick
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 12:25 am



Quoting Pihero (Reply 23):

Yep. Fair enough.
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zeke
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 12:30 am



Quoting SlamClick (Reply 21):
Actually yes, not that uncommon "hot and high" where second-segment climb performance is a factor. Involves selecting a higher V1 and VR for a takeoff. V2 would not be changed and the selected V1 speed ultimately will be limited by accelerate-stop distance. In practical terms, however, a given hotrod two-holer might be able to accelerate to tire limit speed and still reject the takeoff on a very long runway. The reason for the V1 and VR overspeed is twofold. (1) The plane will accelerate faster from original charted V1 to the new VR faster with the weight on the wheels than wallowing through the air, and (2) the new liftoff speed is nearer L/DMAX and that improves the initial climb performance.

Increasing V1 does not directly improve climb gradient, improving the V1/VR will improve MTOW, but will not improve the gradient. The best climb gradient is achieved when V2//Vs1g is at a maximum.

Gradient improvement is done by a higher V2, hopefully (V2//Vs1g) max. We also use a lower flap setting, that may change Vr & V2, as Vr must be ≥ Vmca, and V2 must be V2 ≥ 1.1 VMCA V2 must be ≥ 1.13 Vs1g on the FBW aircraft (V2 must be ≥ 1.2 Vs non FBW).
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SlamClick
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 12:57 am



Quoting Zeke (Reply 25):
improving the V1/VR will improve MTOW

Isn't that the entire point? I mean what company cares about the climb gradient, so long as it is equal to, or greater than the legally required gradient? It is about how much payload can we lift while carrying required fuel.
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zeke
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 2:33 am



Quoting SlamClick (Reply 26):
I mean what company cares about the climb gradient, so long as it is equal to, or greater than the legally required gradient?

If you are limited by obstacles, changing V1 will not change the MTOW. Increasing V1 beyond optimum can actually decrease your MTOW as you are moving the rotation closer to the obstacle.

I would hope every "company cares about the climb gradient" before MTOW.
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lowrider
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 2:50 am



Quoting Zeke (Reply 27):
I would hope every "company cares about the climb gradient" before MTOW.

Only to the extent that not meeting climb gradients can get expensive. Particularly if terrain is the determining factor.
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SlamClick
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 2:41 pm

Quoting Zeke (Reply 27):
If you are limited by obstacles, changing V1 will not change the MTOW. Increasing V1 beyond optimum can actually decrease your MTOW as you are moving the rotation closer to the obstacle.

I would hope every "company cares about the climb gradient" before MTOW.

I know all that, I was only discussing a case where initial, or second-segment climb was the limiting factor in maximum allowable takeoff gross weight.

Since the o/p is from the US and I am too, I was speaking from the US point of view. Every takeoff by every US air carrier considers all factors, whether the pilots or even the load planners are aware of this or not:

structural weight limits
accelerate stop
VRMAX due to tire speed limit
VRMAX due to brake energy
accelerate-go with all engines operating
accelerate-go with an engine failure
initial climb gradient also considering actual obstacles
second segment climb gradient also considering actual obstacles
final climb gradient also considering actual obstacles
return-to-field landing climb performance
enroute climb gradient
climb gradient 1500 feet above a presumed driftdown alternate
And the following considering burnoff to destination:
landing structural limit (the most common limiter on short segments)
landing climb gradient all engines
landing climb gradient with an engine inoperative
landing field length limit

All of the above may be further reduced by MELs or other conditions. The lowest number encountered will limit TOGW but often it can be improved by a runway or configuration change or, in the case of a takeoff climb, by "improved climb" procedure which was the only case we were talking about.

Which makes this the long-winded post I was trying to avoid.

BTW most of these are buried within software or charts and procedures and are invisible to the casual user of the system.

edit: Wow, the font looks really different for some reason.

[Edited 2008-07-28 07:43:07]
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PGNCS
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 2:56 pm



Quoting DRAIGONAIR (Reply 18):
When you are taking off and use a lower flap setting you will need to go faster to generate enough lift to lift off. However, due to the lower flap setting, your climb performance will be better (an increase in VY (climb rate) because you use less flaps and hence less drag). This could be used if there is a limited obstacle take off clearance.
However, using less flaps will also decrease the maximum takeoff weight. So, the heavier the plane, the more flaps are needed to generate enough lift permitting there is enough runway.

This is not necessarily correct. Heavier aircraft very frequently require lower flap settings to deal with climb limit requirements. The corollary is that the runway requirements will be longer; that's why so many hot and high airports (DEN, for example) have outlandishly long runways.

Quoting SlamClick (Reply 21):
Quoting CosmicCruiser (Reply 3):
"decision to delay rotation to achieve a higher climb speed". That would NEVER happen

Actually yes, not that uncommon "hot and high" where second-segment climb performance is a factor.

Yes it does happen; the last two carriers I have worked for used improved performance takeoff speeds in situations where doing so can increase MTOW.
 
SlamClick
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RE: Takeoff Rolls And Climb Rate Questions

Mon Jul 28, 2008 3:55 pm



Quoting PGNCS (Reply 30):
Yes it does happen; the last two carriers I have worked for used improved performance takeoff speeds in situations where doing so can increase MTOW.

You copied my post then appear to think we disagree. Read mine: "not that uncommon"
- means it is common
- means it does happen
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EssentialPowr
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RE: Takeoff Rolls And Climb Rate Questions

Tue Jul 29, 2008 4:56 am



Quoting SlamClick (Reply 21):
Quoting Thrust (Thread starter):
would think that engineers would realize possible disadvantages of a heavy airliner

You are right. The engineers would have us fly around empty.

Baloney. Engineers design an a/c to haul payload. If an a/c cannot be profitable, then the design is poor or short lived...

Weight control of a new a/c is one of the most fundamental elements of a/c design, and a singular descriptor of a certain design is how far over or underweight it is at key points in its development.
 
David L
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RE: Takeoff Rolls And Climb Rate Questions

Tue Jul 29, 2008 9:57 am



Quoting SlamClick (Reply 31):

I think he was backing you up.  Smile
 
411A
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RE: Takeoff Rolls And Climb Rate Questions

Wed Jul 30, 2008 3:26 am



Quoting CosmicCruiser (Reply 9):
The more I looked into it it appears it was initially an Airbus thing but Boeing & MDD also published some data on it.

Clearly you haven't been around long enough.
The overspeed (improved climb) procedure was used on the first swept wing civil jet transport...the B707, long ago.

Very useful on runways with obstructions...runway 16 at ZRH is a perfect example.
Personally used the improved climb procedure there many times with the B707 and L1011, to increase allowable payload.
 
PGNCS
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RE: Takeoff Rolls And Climb Rate Questions

Thu Jul 31, 2008 9:44 pm



Quoting PGNCS (Reply 30):
Quoting SlamClick (Reply 21):
Quoting CosmicCruiser (Reply 3):
"decision to delay rotation to achieve a higher climb speed". That would NEVER happen

Actually yes, not that uncommon "hot and high" where second-segment climb performance is a factor.

Yes it does happen; the last two carriers I have worked for used improved performance takeoff speeds in situations where doing so can increase MTOW.



Quoting SlamClick (Reply 31):
Quoting PGNCS (Reply 30):
Yes it does happen; the last two carriers I have worked for used improved performance takeoff speeds in situations where doing so can increase MTOW.

You copied my post then appear to think we disagree. Read mine: "not that uncommon"
- means it is common
- means it does happen

SlamClick: I was actually trying to support what you said as correct. If I copied it in a confusing manner, I apologize. I agree with you.
 
SlamClick
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RE: Takeoff Rolls And Climb Rate Questions

Thu Jul 31, 2008 11:26 pm

Well I agree with us both, I guess.  Smile
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