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Old 19th Sep 2011, 17:19   #1 (permalink)
 
Join Date: Jul 2005
Location: heaven
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Net/Gross Takeoff?

Hey folks,

Can someone explain the difference between "net" and "gross" takeoff flight path? I guess the "net" takeoff flight path has to clear obstacles by 35ft, but I dont understand the definition of "net" and "gross."

Thanks for any feedback.
Macgyver is offline   Reply
Old 19th Sep 2011, 17:48   #2 (permalink)
 
Join Date: May 2005
Location: France
Age: 59
Posts: 294
'Gross' performance is what has been observed and measured during flight testing/certification.

'Net' performance is that 'gross' performance degraded by a small percentage, to take into account 'real-world' variables (handling skill, wear and tear of engine and airframe, accuracy of data, e.g. meteorolgical, runway).

The percentage depends on number of engines and phase of flight.

Take-off performance is the most complex, involving several 'segments', all with their own percentage.

Cruise performance is also degraded from 'gross' to 'net', especially in the case of engine-out 'driftdown'

Landing performance is degraded in the case of go-around climb gradients.

I'm sure that more detailed info is available, but that is a quick overview.

Hope it helps.

Eckhard

P.S. Just found this:

Sec. 25.115 Takeoff flight path.

(a) The takeoff flight path begins 35 feet above the takeoff surface at the
end of the takeoff distance determined in accordance with Sec. 25.113(a).
(b) The net takeoff flight path data must be determined so that they
represent the actual takeoff flight paths (determined in accordance with Sec.
25.111 and with paragraph (a) of this section) reduced at each point by a
gradient of climb equal to--
(1) 0.8 percent for two-engine airplanes;
(2) 0.9 percent for three-engine airplanes; and
(3) 1.0 percent for four-engine airplanes.
(c) The prescribed reduction in climb gradient may be applied as an
equivalent reduction in acceleration along that part of the takeoff flight
path at which the airplane is accelerated in level flight.



Sec. 25.117 Climb: general.

Compliance with the requirements of Secs. 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.



Sec. 25.119 Landing climb: All-engine-operating.

In the landing configuration, the steady gradient of climb may not be less
than 3.2 percent, with--
(a) The engines at the power or thrust that is available eight seconds
after initiation of movement of the power or thrust controls from the minimum
flight idle to the takeoff position; and
(b) A climb speed of not more than 1.3 VS.



Sec. 25.121 Climb: One-engine-inoperative.

(a) Takeoff; landing gear extended. In the critical takeoff configuration
existing along the flight path (between the points at which the airplane
reaches VLOF and at which the landing gear is fully retracted) and in the
configuration used in Sec. 25.111 but without ground effect, the steady
gradient of climb must be positive for two-engine airplanes, and not less
than 0.3 percent for three-engine airplanes or 0.5 percent for four-engine
airplanes, at VLOF and with--
(1) The critical engine inoperative and the remaining engines at the power
or thrust available when retraction of the landing gear is begun in
accordance with Sec. 25.111 unless there is a more critical power operating
condition existing later along the flight path but before the point at which
the landing gear is fully retracted; and
(2) The weight equal to the weight existing when retraction of the landing
gear is begun, determined under Sec. 25.111.
(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 Sec. 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 Sec. 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 Sec. 25.111.
(c) Final takeoff. In the en route configuration at the end of the takeoff
path determined in accordance with Sec. 25.111, the steady gradient of climb
may not be less than 1.2 percent for two-engine airplanes, 1.5 percent for
three-engine airplanes, and 1.7 percent for four-engine airplanes, at not
less than 1.25 VS and with--
(1) The critical engine inoperative and the remaining engines at the
available maximum continuous power or thrust; and
(2) The weight equal to the weight existing at the end of the takeoff path,
determined under Sec. 25.111.
(d) Approach. In the approach configuration corresponding to the normal
all-engines-operating procedure in which VS for this configuration does not
exceed 110 percent of the VS for the related landing configuration, the
steady gradient of climb may not be less than 2.1 percent for two-engine
airplanes, 2.4 percent for three-engine airplanes, and 2.7 percent for four-
engine airplanes, with--
(1) The critical engine inoperative, the remaining engines at the available
takeoff power or thrust;
(2) The maximum landing weight; and
(3) A climb speed established in connection with normal landing procedures,
but not exceeding 1.5 VS.



Sec. 25.123 En route flight paths.

(a) For the en route configuration, the flight paths prescribed in
paragraphs (b) and (c) of this section must be determined at each weight,
altitude, and ambient temperature, within the operating limits established
for the airplane. The variation of weight along the flight path, accounting
for the progressive consumption of fuel and oil by the operating engines, may
be included in the computation. The flight paths must be determined at any
selected speed, with--
(1) The most unfavorable center of gravity;
(2) The critical engines inoperative;
(3) The remaining engines at the available maximum continuous power or
thrust; and
(4) The means for controlling the engine-cooling air supply in the position
that provides adequate cooling in the hot-day condition.
(b) The one-engine-inoperative net flight path data must represent the
actual climb performance diminished by a gradient of climb of 1.1 percent for
two-engine airplanes, 1.4 percent for three-engine airplanes, and 1.6 percent
for four-engine airplanes.
(c) For three- or four-engine airplanes, the two-engine-inoperative net
flight path data must represent the actual climb performance diminished by a
gradient of climb of 0.3 percent for three-engine airplanes and 0.5 percent
for four-engine airplanes.
eckhard is offline   Reply
Old 19th Sep 2011, 17:51   #3 (permalink)
 
Join Date: Mar 2011
Location: Mumbai
Age: 25
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Gross Performance is what is required for certification purposes.It is the average performance expected from a group/fleet of aircraft.
Net Performance is the performance obtained by reducing the gross performance
in specified manner to allow for degradation that cannot be accounted for,due to factors such as difference in operating techniques,due to age-ing of the acft and different climatic conditions at different places.

Net T/O flight path is obtained by reducing the gross climb gradient by 0.8%.
It is used for obstacle clearance purposes.
A much better explanation is here
Gross vs. Net takeoff performance [Archive] - PPRuNe Forums

AF.
attitudeFlying is offline   Reply
Old 19th Sep 2011, 17:54   #4 (permalink)
 
Join Date: Mar 2011
Location: Mumbai
Age: 25
Posts: 37
@eckhard
I just Ctr C Crtl V'ed that
Thank you
AF.
attitudeFlying is offline   Reply
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