as long as you are within WAT limits and there are no obstacles to consider you are not climb limited
Got to disagree with you, if you cannot achieve the required certification climb gradients you are climb limited, if you cannot clear obstacles in the takeoff flight path then you are obstacle limited.
Regardless of the nomenclature on your takeoff charts, these are the limitations. To test it, pick an airport with no obstacles and a long runway, then increase the airport elevation and temperatures until you start to see restricted takeoff weights less than the structural, what do you call these weights? Mr Boeing calls them CLIMB LIMIT weights.
Yes mutt I mixed up climb with obstacle, i was thinking of it as 2nd segment limited and thought of it as a climb limit which is solely the case for the minimum regulatory climb gradient. When there are obstacles in the 2nd segment requiring a higher gradient it is an obstacle limit.
So I mixed the 2 up.
So what I meant to say is that within the WAT limits the aircraft will always achieve 2,4%.
However I'm not the one stating that the 2,4% regulatory gradient offers a 1,6% Net flight path. In my view both are completely seperate. One is certifying minimum the other based on the actual performance and weight on the day that considers local obstacles.
I presume you realise that this is an instant value and not a constant value, so you will achieve 2.4% at the start of the climb but it can be less at the end of the 2nd segment.... Surprised? So was I when Mr Boeing stated it
Look at far 25.111 and 115 and see if you still think the same thing about the climb gradient.
Apologies .. not the point of my comment. The main concern is to avoid the new chums getting confused along the way.
In my view both are completely seperate
Not quite the case. The 2.4% gross data should be viewed as an actual minimum climb capability or even marginally less than that obtained. The 1.6% is the expected gross data reduced by 0.8% to provide a reasonable, real life, margin for obstacle etc., clearance calculations.
Thus, gross approximates what you expect to see on the gauges while climbing out while net is the calculated data upon which one bases the obstacle clear flight profile for preflight calculations.
The two are inextricably linked.
but it can be less at the end of the 2nd segment
Used to cause heartache in Australia where the local rule book required that the performance be maintained throughout the second segment.
far 25.121 b) Takeoff; landing gear retracted. In the takeoff configuration existing at the point of the flight path at which the landing gear is fully retracted, and in the configuration used in §25.111 but without ground effect, the steady gradient of climb may not be less than 2.4 percent for two-engine airplanes, 2.7 percent for three-engine airplanes, and 3.0 percent for four-engine airplanes, at V2 and with --
(1) The critical engine inoperative, the remaining engines at the takeoff power or thrust available at the time the landing gear is fully retracted, determined under §25.111, unless there is a more critical power operating condition existing later along the flight path but before the point where the airplane reaches a height of 400 feet above the takeoff surface; and
(2) The weight equal to the weight existing when the airplane's landing gear is fully retracted, determined under §25.111.
far 25.117 Compliance with the requirements of §§25.119 and 25.121 must be shown at each weight, altitude, and ambient temperature within the operational limits established for the airplane and with the most unfavorable center of gravity for each configuration.
So according to FAR 25 within the WAT limits a 2 engine jet is certified to achieve a steady 2,4% climb gradient.
Can you point me too a reference that says it only momentary?
I'm not sure what you are getting at with Far25.111 and 115. First of all 115 simply defines the takeoff flightpath and nett takeoff flightpath. 111 determines another regulatory climb gradient 1,2%.
(a) No person operating a turbine engine powered airplane may take off that airplane at a weight greater than that listed in the Airplane Flight Manual for the elevation of the airport and for the ambient temperature existing at takeoff.
(d) No person operating a turbine engine powered airplane may take off that airplane at a weight greater than that listed in the Airplane Flight Manual --
(2) In the case of an airplane certificated after September 30, 1958 (SR 422A, 422B), that allows a net takeoff flight path that clears all obstacles either by a height of at least 35 feet vertically, or by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries.
(e) In determining maximum weights, minimum distances, and flight paths under paragraphs (a) through (d) of this section, correction must be made for the runway to be used, the elevation of the airport, the effective runway gradient, the ambient temperature and wind component at the time of takeoff, and, if operating limitations exist for the minimum distances required for takeoff from wet runways, the runway surface condition (dry or wet). Wet runway distances associated with grooved or porous friction course runways, if provided in the Airplane Flight Manual, may be used only for runways that are grooved or treated with a porous friction course (PFC) overlay, and that the operator determines are designed, constructed, and maintained in a manner acceptable to the Administrator.
(f) For the purposes of this section, it is assumed that the airplane is not banked before reaching a height of 50 feet, as shown by the takeoff path or net takeoff flight path data (as appropriate) in the Airplane Flight Manual, and thereafter that the maximum bank is not more than 15 degrees.
(g) For the purposes of this section the terms, takeoff distance, takeoff run, net takeoff flight path and takeoff path have the same meanings as set forth in the rules under which the airplane was certificated.
So 189 (a) determines amongst others the certified climb limit weights determind during certification. And 189 (d) determines obstacle limited weights/climb gradients.
The 0,8% penalty is just for defining the net takeoff flightpath and is used during obstacle climb limit determination. Not performance climb limit determination.
I agree that any obstacles penetrating the 35ft obstacle clearance on a net takeoff path of 1,6% will lead to a higher required net takeoff path and thus gross takeoff path.
But this is a far 121.189 Operator requirement not a far25 certifying requirement like the 2,4% gradient.
In that sense the 1,6% figure is irrelevant, because as operators we need to take the actual obstacles into account and assure that the net path clears them by 35ft.( 121.189 d) We don't do this by starting of at 2,4%. It is done by taking all relevant obstacles into account, add 35 feet, determine the net path. Then add the penalty and thus determine the gross flightpath required. This could be 2%, 3% 5% etc... For all these percentages you could make a specific WAT table that determines the limiting weight.
If there are no obstacles we are still limited by certifying limits (121.189a) like the 2,4% 2nd segment WAT limit (or required groos obstacle climb gradient is less than the certifying limit)
But this is a far 121.189 Operator requirement not a far25 certifying requirement like the 2,4% gradient.
One needs to keep in mind that second segment WAT is 2.4% and not related to nett. However, whatever gross is required for obstacle or other operational need is then linked to a net capability 0.8% less. Put it down to minor emphasis in meaning.
WAT limits ... a steady 2,4% climb gradient. Can you point me too a reference that says it only momentary?
The physics of the matter is the reference.
As soon at the ambient conditions (Hp/OAT) vary, the steady climb performance will vary ergo the gradient may reduce through the climb. To satisfy yourself on this one, have a look at any WAT chart with different entry arguments.
The reference to steady is to preclude enterprising things, such as a bit of zooming, etc.
Necessary, also, to keep in mind that the regs can't be read in isolation. One needs to review ACs, policy letters and the like to get a story which, itself, varies over the years .. hence one Type will have different rules applied compared to another.
I edited my post above. I simply make a clear distinction between what we as operators should do far121.189 and what the certifying manufacturer should do far25.
Off course they are linked, but as long as we stay within AFM limits the aircraft will perform to its certified limits ie. 2nd segment 2,4% WAT. As you said it is not related to net and thus not to obstacles.
For us as pilots the important things are to not bust AFM LIMITS like 2,4% WAT in 2nd segment. And ensure the net takeoff path clears all obstacles by 35ft by ensuring the aircraft is not too heavy to achieve the required gross path.
The mentioned 1,6% over complicates this and is simply not relevant. In fact it is only relevant where obstacles determine that exactly a 1,6% net path is required. But this is determined by obstacles not certifying limits and would be a pure coincidence.
In other words we check what climb gradient is required based on obstacles and AFM limits and take the highest to determine our limited weight.
Not really the case .. and the AFM leads one by the hand in any case.
Easy for you to say, jt...
Having detailed and accurate obstacle data is the problem .. which is why pilot calculations generally are based on simplified data appropriate to obstacle clear surface data.
Oh, I'd agree that most pilots could find the WAT limits if there were no obstacles in the regulatory departure corridor.
I'm only suggesting that if one gave a reasonably complex departure problem with both close-in and distant obstacles -- one that would introduce thrust time limits and selections of clean-up heights -- to 100 line pilots, it would be surprising if not impossible to find more than five correct answers to the question of "what is the maximum allowable TOW?"
a reasonably complex departure problem with both close-in and distant obstacles
Generally, the pilot doesn't have the time to run a detailed calculation during line operations.
The traditional method used in Oz works OK -
(a) the aerodrome owner publishes runway distance and obstacle clear splay data
(b) the operator publishes a simplified set of performance data which is suited to runway distance and obstacle clear gradient calculations. This can be in a variety of presentations but none is too difficult for Government work.
We occasionally used this sort of stuff on the line, generally when a NOTAM or similar temporarily invalidated the normal company RTOW data for a runway. The general caveat is that the technique is restricted to simple straight ahead departures.
Yes it does. The computer is updated every 2 weeks /every month or so. It even gets modified for Notams, and has specified engine out procedures for each runway.
In addition you can add obstacles yourself. Reduce runway length at begin or end, adjust for 90 degree or 180 degree lineup etc etc etc And do multiple runway calculations at the same time. And off course it has all usable intersections for departure listed as well.
Oh and off course you can add all kinds of performance affecting MEL and CDL items.
The laptop is called LPC for airbus (my username is a bit outdated and the software FOVE.
It also does W&B landing perf dispatch and inflight and offcourse contaminated runway perf. Again all with MEL/CDL integration.