Gulfstream has made much of the climb performance question easy for us by certifying the airplane to meet second segment climb numbers — 2.4% — all the way up to 1,500'. Of course the Feds have complicated things a bit with "gross" and "net" climb numbers, and a Hodge Podge of obstacle clearance criteria. Fear not, it actually makes sense if you break down the regulatory requirements to their basics, and consider what information the airplane gives you.
Everything here is from the references shown below, with a few comments in blue.
[14 CFR 25 §25.111]
(a) The takeoff path extends from a standing start to a point in the takeoff at which the airplane is 1,500 feet above the takeoff surface, or at which the transition from the takeoff to the en route configuration is completed and VFTO is reached, whichever point is higher. In addition—
(2) The airplane must be accelerated on the ground to VEF, at which point the critical engine must be made inoperative and remain inoperative for the rest of the takeoff; and
(3) After reaching VEF, the airplane must be accelerated to V2.
(b) During the acceleration to speed V2, the nose gear may be raised off the ground at a speed not less than VR. However, landing gear retraction may not be begun until the airplane is airborne.
(c) During the takeoff path determination in accordance with paragraphs (a) and (b) of this section—
(1) The slope of the airborne part of the takeoff path must be positive at each point;
(2) The airplane must reach V2 before it is 35 feet above the takeoff surface and must continue at a speed as close as practical to, but not less than V2, until it is 400 feet above the takeoff surface;
(3) At each point along the takeoff path, starting at the point at which the airplane reaches 400 feet above the takeoff surface, the available gradient of climb may not be less than—
(i) 1.2 percent for two-engine airplanes;
(ii) 1.5 percent for three-engine airplanes; and
(iii) 1.7 percent for four-engine airplanes.
Part 25 does not mandate the first, second, third, and final segment nomenclature but does mandate minimum climb performances for these segments at the noted altitudes: (2-engine aircraft numbers)
Gulfstream III, IV, V, 450, and 550 meet 14 CFR 25 requirements by maintaining second segment performance beyond 1500 feet, eliminating the third and final segments.
[14 CFR 25 §25.121]
(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 § 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 § 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 § 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 § 25.111 but without ground effect:
(1) 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 with:
(i) 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
(ii) The weight equal to the weight existing when the airplane's landing gear is fully retracted, determined under § 25.111.
(d) Approach. In a configuration corresponding to the normal all-engines-operating procedure in which VSR for this configuration does not exceed 110 percent of the VSR for the related all-engines-operating landing configuration:
(1) 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—
(i) The critical engine inoperative, the remaining engines at the go-around power or thrust setting;
(ii) The maximum landing weight;
(iii) A climb speed established in connection with normal landing procedures, but not exceeding 1.4 VSR; and
(iv) Landing gear retracted.
Gulfstream computes the takeoff configuration with 10° or 20° flaps, allowing you to at 2.4% with the gear retracted. Gulfstream's approach configuration is always with 20° flaps, gear retracted. More on this below.
[14 CFR 25 §25.115]
Figure: Obstacle Analysis Vertical Performance, from Departure Obstacle Avoidance / Aircraft Performance.
(a) The takeoff flight path shall be considered to begin 35 feet above the takeoff surface at the end of the takeoff distance determined in accordance with § 25.113(a) or (b), as appropriate for the runway surface condition.
(b) The net takeoff flight path data must be determined so that they represent the actual takeoff flight paths (determined in accordance with § 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.
When the aircraft is certified, manufacturers are required to reduce their actual climb performance numbers by this safety factor, 0.8% in the case of the two-engine aircraft. What you see in the AFM, PH, and FMS are these "net climb gradient" numbers. Of critical importance to you as a pilot is this knowledge: the airplane will do 0.8% better than the book is telling you.
[14 CFR 135 §135.379]
(d) No person operating a turbine engine powered large transport category airplane may take off that airplane at a weight greater than that listed in the Airplane Flight Manual—
(2) For an airplane certificated after September 30, 1958 (SR422A, 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.
Part 91 doesn't address obstacles at all, you just need to clear them. Part 135 says you need to have at least 35 feet vertically over any obstacles within 200 feet horizontally within the airport boundary, and 300 feet horizontally outside the airport boundary. The 14 CFR 121 requirement is identical.
(a) An operator shall ensure that the net takeoff flight path clears all obstacles by a vertical distance of at least 35 ft or by a horizontal distance of at least 90 m plus 0·125 x D, where D is the horizontal distance the aeroplane has travelled from the end of the take-off distance available or the end of the take-off distance if a turn is scheduled before the end of the take-off distance available. For aeroplanes with a wingspan of less than 60 m a horizontal obstacle clearance of half the aeroplane wingspan plus 60 m, plus 0·125 x D may be used. (See IEM OPS 1.495(a).)
(b) When showing compliance with subparagraph (a) above, an operator must take account of the following:
(1) The mass of the aeroplane at the commencement of the take-off run;
(2) The pressure altitude at the aerodrome;
(3) The ambient temperature at the aerodrome; and
(4) Not more than 50% of the reported head-wind component or not less than 150% of the reported tailwind component.
(c) When showing compliance with subparagraph (a) above:
(1) Track changes shall not be allowed up to the point at which the net take-off flight path has achieved a height equal to one half the wingspan but not less than 50 ft above the elevation of the end of the take-off run available. Thereafter, up to a height of 400 ft it is assumed that the aeroplane is banked by no more than 15°. Above 400 ft height bank angles greater than 15°, but not more than 25° may be scheduled;
(2) Any part of the net take-off flight path in which the aeroplane is banked by more than than 15° must clear all obstacles within the horizontal distances specified in sub-paragraphs (a), (d) and (e) of this paragraph by a vertical distance of at least 50 ft; and
(3) An operator must use special procedures, subject to the approval of the Authority, to apply increased bank angles of not more than 20º between 200 ft and 400 ft, or not more than 30º above 400 ft (See Appendix 1 to JAR-OPS 1.495(c)(3)).
(4) Adequate allowance must be made for the effect of bank angle on operating speeds and flight path including the distance increments resulting from increased operating speeds. (See AMC OPS 1.495(c)(4)).
(d) When showing compliance with subparagraph (a) above for those cases where the intended flight path does not require track changes of more than 15°, an operator need not consider those obstacles which have a lateral distance greater than:
(1) 300 m, if the pilot is able to maintain the required navigational accuracy through the obstacle accountability area (See AMC OPS 1.495(d)(1) & (e)(1); or
(2) 600 m, for flights under all other conditions.
(e) When showing compliance with subparagraph (a) above for those cases where the intended flight path does require track changes of more than 15°, an operator need not consider those obstacles which have a lateral distance greater than:
(1) 600 m, if the pilot is able to maintain the required navigational accuracy through the obstacle accountability area (See AMC OPS 1.495 (d)(1) & (e)(1)); or
(2) 900 m for flights under all other conditions.
Advisory Circular 120-91 expands the engine out horizontal distance to 300' either side of center line once past the airport boundary gradually increasing to 2,000 feet either side of center line once 5 NM from the end of the runway. If a turn is involved, the distance continues the gradual increase until about 8 NM from the end of the runway where it is 3,000 feet either side of center line. See Departure Obstacle Avoidance for more about this.
TERPS Volume 4, Chapter 6 is different from the other requirements in that TERPS procedures "are predicated on normal aircraft operations for considering obstacle clearance requirements." If you lose an engine, TERPS does not apply.
Figure: Obstacle Analysis Horizontal Performance, from Departure Obstacle Avoidance / Aircraft Performance.
Most pilots apply TERPS criteria, the most restrictive, to engine-out planning. While this is a conservative approach, it needlessly reduces payload on aircraft that can easily navigate to tighter tolerances than given in TERPS. The obstacle clearance requirements in 14 CFR 135 and JAROPS err on the opposite side, allowing crews to ignore obstacles very close to probably flight paths. The answer is found in AC 120-91. See Departure Obstacle Avoidance for a primer on how to do this safely.
[G450 Airplane Flight Manual §5.3-1]
Figure: Maximum Allowable Takeoff Grossweight, from G450 Airplane Flight Manual pg. 5.3-2.
Takeoff gross weight is limited when available climb gradient is less than the minimum allowable climb gradient. For a takeoff flap setting of 20°, the minimum climb requirement of 2.4% gradient below 400 feet establishes the maximum allowable takeoff gross weight limit based on climb capability. For a takeoff flap setting of 10°, however, the minimum approach climb gradient of 2.1% (computed for the approach configuration of 20° and an approach weight 1,000 pounds less than the associated takeoff weight) establishes the maximum climb limited takeoff gross weight. The takeoff minimum final segment requirement of 1.2% climb gradient above 400 feet is a less restrictive criteria at all conditions.
If you plan a 20° flap takeoff, your numbers will always assure you can also make the 2.1% approach climb numbers. If you plan a 10° takeoff, you will need to also consider the 2.1% approach climb requirements.
[G450 Aircraft Operating Manual §2B-26-10] Performance information in the FMS is based on data entered by the pilot and calculated by the FMS. Mission planning data has not been evaluated by the Federal Aviation Administration (FAA) for accuracy and is not approved by the FAA.
The data, however, is based on the AFM and will meet the 2.4% climb requirement.
Obstacle clearance requirements vary depending on if you are flying under 14 CFR 91, 135, 121, JAA, or AC 120-91. You can enter up to three obstacle elevation/distance pairs into the performance computer and come up with gradients and heights. But will these be good enough? You can simplify your airport obstacle analysis chores if you understand the regulatory requirements and how horizontal clearance is as important as vertical clearance. More on that here: Departure Obstacle Avoidance.
The Gulfstream III, IV, V, 450, and 550 meet the 14 CFR 25 requirements by maintaining second segment performance beyond 1500 feet, eliminating the third and final segments. V2 is flown from 35 feet to at least 1500 feet, which will achieve the 2.4% second segment climb requirement and exceed the required third and final segment minimums. The recommended level off height is a function of the obstacle height and the horizontal distance from reference zero. It is further limited by the operating time on the operating engine at takeoff thrust. This limit is five minutes on the G-III and ten minutes on the G-IV, V, 450, and 550. [AFMs Section 5.6]
While VFS is defined for the G-III, G-IV, V, 450, and 550 as 1.25 times VSO, it is not used procedurally in the G-IV, V, 450, or 550. In those aircraft, takeoff with an engine failed passed V1 calls for flying from V2 to VSE. The entry for VFS will be blank on the G450 and G-550 unless set through aircraft personality module options. [G450 and G-550 AFM 2B-05-30, 1.D] The entry will be blank on GV aircraft with JAA certification. [GV AFM 2B-05-30, 1.H] The entry will appear in the G-IV and non-JAA GVs, but it is shown for information purposes only and is not used procedurally.
Figure: LFPB ATREX Departure, from Jeppesen FlightDeck LPPB pg. 30-3A.
The G450 does not have the GV OIS which provides two-engine performance data. It does, however, have some two-engine climb data in the OM, Section 11. An example problem at Le Bourget can serve to illustrate its use:
Figure: Twin Engine En Route Climb ISA + 10°, from G450 Aircraft Operating Manual §11-04-20, figure 13.
Can the G450 climb 608 ft/NM up to 15,000? The climb data in the AFM is for the right engine failed. But the Operating Manual provides this chart:
Based on this chart, at 66,000 lbs the aircraft can climb at a rate of 15,000 / 24 = 625 ft/NM — so the answer is yes at this weight and temperature deviation.
Note: For an airport with a higher elevation, it may be wise to compute a more conservative gradient using the difference between the elevation and the obstacle clearance altitude. For example, let's say our airport was at 8,000 feet elevation. We could subtract 5,000 feet data from 15,000 for a more conservative climb rate: (15,000 - 5,000) / (24 - 6) = 556 ft/NM.
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
14 CFR 135, Title 14: Aeronautics and Space, Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft, Federal Aviation Administration, Department of Transportation
Advisory Circular 120-91, Airport Obstacle Analysis, 5/5/06, U.S. Department of Transportation
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
Joint Aviation Authorities JAR-OPS 1, Commercial Air Transportation (Aeroplanes), 10 May 2007
United States Standard for Terminal Instrument Procedures (TERPS), Federal Aviation Administration 8260.3B CHG 19, 5/15/02