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Take off segments net and gross gradients

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Take off segments net and gross gradients

Old 16th May 2018, 20:50
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Take off segments net and gross gradients

The second segment of Net Take Off Flight Path requires a net gradient of 2.4% and there is a 0.8% safety margin to add to get the gross gradient. 2.4% equals to 144 fpm/nm but I am unsure on how to calculate the gross gradient.
If I add 8% to 144 fpm I get 155 fpm. If I add 0.8 to 2.4% I get 192 fpm/nm. (3.2x60=192).

I know the numbers but I have never had to make any calculation during my groundschool period nor exams and now I would like to get this lack of 'knowledge' fixed
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Old 16th May 2018, 21:07
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Originally Posted by Banana Joe View Post
If I add 0.8 to 2.4% I get 192 fpm/nm. (3.2x60=192)
Yes, that's the calculation you need to make, although the units of the result are ft/nm, not fpm/nm.

Last edited by DaveReidUK; 16th May 2018 at 22:33. Reason: Edited to clarify which is the correct calculation
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Old 16th May 2018, 21:43
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Yes,correct, ft and not fpm. So which one? The second method?
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Old 16th May 2018, 21:48
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Quickest way to approximate required rate of climb is gradient times ground speed.
ie just say V2+5 is 160 kts IAS, 2.4% gradient is 384 fpm required rate of climb.
Simples
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Old 16th May 2018, 21:55
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@CaptainSouth, I wrote my initial message poorly. What I am after is how to find the required gain of altitude/height per nautical mile according to both the net and gross gradient.
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Old 16th May 2018, 22:13
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The second calc is correct. If you want to make the adjustment to the 144 ft/nm answer, you need to increase it by 0.8/2.4 = 1/3rd increase, not 8%.
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Old 16th May 2018, 22:19
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The minimum second segment climb gradient two engines is 2.4% gross
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Old 16th May 2018, 22:27
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Thanks. Multiplying by 60 is just a rule of thumb that I came across from my ATPL days, but I can't seem to find any precise formula in my notes, but I guess that 2.4% of 6076 ft would give me the 'mathematically' correct figure.
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Old 16th May 2018, 22:27
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Originally Posted by Alex Whittingham View Post
The minimum second segment climb gradient two engines is 2.4% gross
Thanks Alex.
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Old 17th May 2018, 00:44
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Originally Posted by Alex Whittingham View Post
The minimum second segment climb gradient two engines is 2.4% gross
every thread you write all posts following yours including yours are cursive/italic.

i would suspect it's some html code in the bristol.gs under your username but that's just a guess.
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Old 17th May 2018, 23:46
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i would suspect it's some html code in the bristol.gs under your username but that's just a guess.

Pretty close. I referred the problem to the higher-up-the-totem-pole-folk and it turned out to be an HTML problem of our making but, apparently, associated with Alex's profile details. Fixed now so, hopefully, we won't see it again.

Re the thread, I'll run up a post in the next day or two to explain some stuff about the gradient thing which might help sort out some problems evident in a couple of posts.
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Old 18th May 2018, 04:35
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Originally Posted by JT
I'll run up a post
I hope you've got an RTOW chart for that. I'd hate you to be half way up and have an engine failure and be overweight....
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Old 18th May 2018, 08:42
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touché ...
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Old 18th May 2018, 21:52
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Another question regarding gradients, though not in the take-off and departure phase: what distance from the runway, according to certification standards, does the gradient requirement change from approach climb (2.1% for a twin jet with OEI in approach configuration) to landing climb (3.2% AEO in landing configuration)?
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Old 18th May 2018, 22:44
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Originally Posted by Banana Joe View Post
Another question regarding gradients, though not in the take-off and departure phase: what distance from the runway, according to certification standards, does the gradient requirement change from approach climb (2.1% for a twin jet with OEI in approach configuration) to landing climb (3.2% AEO in landing configuration)?
Seems like not a matter of distance but configuration.
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Old 23rd May 2018, 23:33
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Some considerations ..

(a) as part of the certification process, an aircraft will have weight limits which address a set of required WAT gradient limits. These vary with the number of engines and look both at a reasonably to be expected gross capability and a terrain clearance lesser climb gradient to use when running obstacle clearance sums for the OEI escape.

The important thing about WAT limits is that they represent a line in the sand set of performance criteria so that the aircraft is likely to be able to keep going up when that is required. If your weight is less than the relevant WAT limit weight, then your performance, pretty obviously, will be better than that which might be achieved at the WAT limited weight.

(b) for the more common two-engine aircraft, for the second segment as an example, the minimum gross gradient required (which will apply if your takeoff were at the WAT-limiting TOW) is 2.4%. With the two-engine margin factor of 0.8% applied, the net gradient required is 1.6%. Both of these can be converted to an expected ROC using the usual trigonometric sums while keeping an eye on whether the air is non-ISA for any instrument considerations so that we can reasonably match the numbers to the rocky bits.

(c) Multiplying by 60 is just a rule of thumb Generally, if you are running sums involving angles, the 60 factor comes from radian angular measure conversions where the appropriate number is around 57 (if my memory isn't failing too much .. =180/pi). That's pretty close to 60 so we use 60 and accept any small errors associated with the calculation.

(d) what distance from the runway, according to certification standards, does the gradient requirement change from approach climb (2.1% for a twin jet with OEI in approach configuration) to landing climb (3.2% AEO in landing configuration)? As another poster observed, the figures you quote are certification WAT gradient requirements and, as such, provide a line in the sand for maximum weights on the day. They have nothing specifically to do with managing the approach and landing but will be incorporated in configurations for the approach and landing and, more particularly, the missed approach.

(e) we need to keep in mind the consideration that the WAT limits provide for the maximum allowable weight in whatever situation you are looking at. Calculating numbers based on WAT limits will be relevant if you are at the relevant limiting weight. If you are lighter, then you will perform better and the calculated numbers will be conservative to whatever extent. More important to remember that the AFM performance data incorporates all the various WAT limits .. the important thing for the pilot is to fly the aircraft as the AFM expects so that there is a reasonable expectation of achieving or exceeding the climb capabilities incorporated in the AFM data.
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