Enroute Performance

- James Albright (a former G450 driver)

Updated: 2020-10-01

En route performance, a piece of cake, eh?

Well, there are a few things you need to consider:

• What is the minimum altitude needed to clear obstacles, if you lose an engine?

• What is your maximum possible altitude after you've climbed and then lose an engine?

• What will your cruise fuel be?

• How does the FMS consider winds and temperature?

Everything here is from the references shown below, with a few comments in an alternate color.

 Minimum Safe Altitudes: General

[14 CFR 91 §91.119] Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:

(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.

(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.

(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

Minimum Altitudes for IFR Operations

[14 CFR 91 §91.177]

(a) Operation of aircraft at minimum altitudes. Except when necessary for takeoff or landing, no person may operate an aircraft under IFR below—

(1) The applicable minimum altitudes prescribed in parts 95 and 97 of this chapter. However, if both a MEA and a MOCA are prescribed for a particular route or route segment, a person may operate an aircraft below the MEA down to, but not below, the MOCA, provided the applicable navigation signals are available. For aircraft using VOR for navigation, this applies only when the aircraft is within 22 nautical miles of that VOR (based on the reasonable estimate by the pilot operating the aircraft of that distance); or

(2) If no applicable minimum altitude is prescribed in parts 95 and 97 of this chapter, then—

(i) In the case of operations over an area designated as a mountainous area in part 95 of this chapter, an altitude of 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown; or

(ii) In any other case, an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown.

(b) Climb. Climb to a higher minimum IFR altitude shall begin immediately after passing the point beyond which that minimum altitude applies, except that when ground obstructions intervene, the point beyond which that higher minimum altitude applies shall be crossed at or above the applicable MCA.

En route limitations: One engine inoperative

[14 CFR 135 §135.381

(a) No person operating a turbine engine powered large transport category airplane may take off that airplane at a weight, allowing for normal consumption of fuel and oil, that is greater than that which (under the approved, one engine inoperative, en route net flight path data in the Airplane Flight Manual for that airplane) will allow compliance with paragraph (a) (1) or (2) of this section, based on the ambient temperatures expected en route.

(1) There is a positive slope at an altitude of at least 1,000 feet above all terrain and obstructions within five statute miles on each side of the intended track, and, in addition, if that airplane was certificated after August 29, 1958 (SR422B), there is a positive slope at 1,500 feet above the airport where the airplane is assumed to land after an engine fails.

(2) The net flight path allows the airplane to continue flight from the cruising altitude to an airport where a landing can be made under § 135.387 clearing all terrain and obstructions within five statute miles of the intended track by at least 2,000 feet vertically and with a positive slope at 1,000 feet above the airport where the airplane lands after an engine fails, or, if that airplane was certificated after September 30, 1958 (SR422A, 422B), with a positive slope at 1,500 feet above the airport where the airplane lands after an engine fails.

(b) For the purpose of paragraph (a)(2) of this section, it is assumed that—

(1) The engine fails at the most critical point en route;

(2) The airplane passes over the critical obstruction, after engine failure at a point that is no closer to the obstruction than the approved navigation fix, unless the Administrator authorizes a different procedure based on adequate operational safeguards;

(3) An approved method is used to allow for adverse winds;

(4) Fuel jettisoning will be allowed if the certificate holder shows that the crew is properly instructed, that the training program is adequate, and that all other precautions are taken to ensure a safe procedure;

(5) The alternate airport is selected and meets the prescribed weather minimums; and

(6) The consumption of fuel and oil after engine failure is the same as the consumption that is allowed for in the approved net flight path data in the Airplane Flight Manual.

You have everything you need to do this, but it isn't exactly straight forward. We'll use a sample problem to help illustrate:

• Departure airport: 4,000' elevation, 20°C

• Obstacle: 8,000' MSL, 60,000' from Departure End of Runway

• Need to clear obstacle by: 1,000' to satisfy 14 135.381 (a)(1)

Determine climb gradient required

You could do some math: (9,000 - 1,000 - 4,000) / 60,000 = 0.0833 or 8.33%

Or you could look at a chart (AFM 5.6, Distant Obstacle Clearance Required):

This also shows you that you will be well within your ten-minute engine-out rated thrust limit, since you are to the left of the "Max Level Off Height" line.

Determine maximum weight to clear obstacle

You'll need AFM 5.6 Available Net Gradient, page 2 first:

Transfer the number to the first page:

This tells you the most weight you can depart from this airport and still clear the obstacle following an engine failure at V1 is 54,000 lbs. Doing so, you know you can clear the obstacle by 1,000 ft.

 Maximum Possible Altitude Following Engine Loss

It seems obvious that if you can make it to the specified altitude on one engine, you should be able to get there with two engines and stay airborne after losing one, but here goes. Pull out the QRH, turn to EB-16, and see that the lowest drift down altitude at a much higher weight is higher than our planned cruise altitude. The data is available in the FMS:

Unlike the GV, the G450 is probably cruising very close to drift down speed and will be coming down at twice the rate, about 600 fpm.

Want more on this, look at this: G450 Engine Fail Drift Down.

 Cruise Fuel

Back in the GIII we would say the first hour of flight takes 5,000 lbs of fuel, the next two hours take 4,000 lbs of fuel each, and then it is 3,000 lbs from then on. In other words, 5/4/4/3...

I've heard 4/3/3/2... in the G450 but I heard the same thing in the GV, which uses less fuel.

Using the 0.80MT/ISA chart in section 11 of the AOM, you see the fuel flow starts at 4,626 PPH. Assuming you use that hour of fuel to climb to FL410 and end up 4,000 lbs less, your next hour will use 3,225 PPH. Burning that fuel takes you to 3,078 PPH. It will take seven hours to get the fuel flow down to below 2,500 PPH. Maybe we can devise a . . .

Rule of Thumb:

Cruise fuel flows in the G450 are generally 4,000 / 3,500 / 3,000 PPH during hours one, two, and three. From that point on you can budget 3,000 PPH.

 Wind/Temperature Modeling

[G450 Aircraft Operating Manual §2B-26-50] The FMS wind and temperature model blends wind and temperature entries with the current position sensed wind and temperature. The sensed wind and temperature are blended in proportion to the distance away from the aircraft. For example, at present position, sensed wind and temperature are blended at 100%. At 200 NM, sensed is blended 50% and entered at 50%. At 400 NM, the blend is 20% sensed and 80% entered. When viewing the WIND/TEMP page, the blended wind and temperature are displayed.

If wind or temperature are not entered on any page, a wind of zero and ISA temperature is assumed for each waypoint at every altitude. Performance planning is based on zero wind and ISA temperature plus the blended sensed wind and temperature.

If an average wind and/or temperature deviation (ISA DEV) is entered on the PERF INIT 4/5 page, it applies to every waypoint in the flight plan. The wind is ramped down from the entered altitude to produce a lower wind at lower altitudes. At altitudes above the tropopause, the wind is assumed to be constant.

Wind and temperature can also be entered at each waypoint on the WIND/TEMP page. When an entry is made at an individual waypoint, it erases any previous entry. The entry is applied to each waypoint forward in the flight plan until a waypoint with another entry is encountered.

If the wind and temperature are forecast to be fairly constant over the route of flight, an average wind and temperature (ISA DEV) entered on the PERFORMANCE INIT 4/5 page is sufficient. If the wind and temperature are predicted to be significantly different at various flight plan waypoints, waypoint entries should be made. This can be done after an average entry is made or in place of average entries.

Being an old 89th guy from back in the days when block times were measured in seconds, I like having an FMS that is smarter than I am. The problem is sometimes we make it stupider. If you have a fairly straight route home with twenty waypoints and center offers you direct the last one, you are probably going to say yes. Think about that next time. Your ETAs with twenty some odd waypoints will be based on predicted winds along the way. If you delete every waypoint for the next 500 miles, your winds are based on your present position winds blended down the road to where they are mostly based on that last waypoint. I mentioned that to a few pilots a few jobs ago and the answer was universal, "Do I look like I care?" Well, if you care about the ETA shown on your Airshow, think twice about the change in winds before accepting direct.

 References:

14 CFR 91, Title 14: Aeronautics and Space, General Operating and Flight Rules, 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

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013

Gulfstream G450 Quick Reference Handbook, GAC-AC-G450-OPS-0003, Revision 34, 18 April 2013