Photo: Haskel's shot of G-IV FMS typo
The topic of climb speed was pretty ugly in the G-IV, mostly muddled by an FMS typographic error which led to the "Drift Up" nonsense taught at SimuFlight and, to a lesser extent, at FlightSafety. (For that story, see Flight Lessons / Drift Up.) Thankfully all that is behind us in the G450, but we are still left with the question of how slow can we fly if asked to expedite our climb.
From fast to slow, I think of climb speeds in these terms:
Many novice jet pilots are tempted to wax poetic about VX and VY but that misses the point in a high performance, high altitude jet. What works at sea level doesn't always work at 45,000 feet and certainly not when the temperature can go from ISA to ISA+15 in just a few thousand feet. I always know what happened when I hear a Challenger pilot request a lower altitude through the North Atlantic Track System. They planned a climb to what seemed a good altitude flying over New Foundland and were alarmed to see the airplane mush into a buffet speed once over the water.
Flying a Gulfstream we don't have that problem but the issue of climb speed still deserves our respect. I climb at 300 knots until it becomes 0.75 Mach. If asked to expedite, I'll go as low as 250 KCAS and 0.70 Mach. My bottom line numbers are 0.60 and 230 KCAS, if I see that I get worried. But I temper all of this with the Angle of Attack.
One of the lessons from flying very heavy receiver aircraft behind air refueling tankers is that you learn it takes a very long time to recover lost speed when you are slow at altitude. The lesson? Don't get slow.
I know this isn't definitive, so maybe some source material is in order. What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
As you climb above 27,500 feet your true airspeed tends to suffer and your ground speed may actually decrease with altitude. As you climb above the troposphere and the temperature lapse rate nears zero, your fuel advantages with climb become smaller. You may want to minimize your climb rate if forward groundspeed is a priority and winds do not improve at higher altitudes. What is your minimum rate of climb to keep ATC happy? 500 fpm.
[Aeronautical Information Manual §4-4-10.d.] When ATC has not used the term “AT PILOT’S DISCRETION” nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC.
Photo: from Haskel's cockpit.
In the Air Force our flight manuals were published in "technical orders." A "tech order climb" is a climb at the speed schedule mandated by the manual. There is no such animal in the G450. The best we can do is the PlaneView defaults:
But how do you do that? A reader asks the question...
Good Afternoon Mr. Haskel,
I wanted to thank you for the tremendous amount of time and effort you have put into this website. It has offered an invaluable amount of information to myself and our flight department. I wanted to ask about your climb to altitude technique. I have most of my time in the GIII, and with the pilot we have on the G450, they all have a different way of climbing. Below are some of the techniques and I am trying to figure out which one is most appropriate.
I respectfully ask for what you have found works best for the G450. Thank you very much for your time.
Call me Eddie.
I use the third option you listed, setting the target speed to a higher value than the one I intend to fly in order to keep the engines at climb thrust. I don’t like VS mode during climb because we pilots tend to get distracted and I’ve seen too many pilots forget about it and end up running out of airspeed. But I don’t think the airplane always figures out the IAS to Mach transition and from that point on the porpoising begins and I have no choice but to go to VS. So at that point I set M0.80 and use VS to bracket between M0.75 and M0.77. I keep my focus on airspeed and if something comes up in the cockpit I put my hand on the VS knob or hand control of the airplane to the other pilot.
Hope that helps.
When asked to expedite climb, I automatically head for 250 KCAS or 0.70 Mach. There is no science behind this other than those number are higher than those that follow. . .
Figure: En route climb speed, from G450 Airplane Flight Manual, pg. 5.7 - 2.
The only en route climb chart in our books gives climb speeds given an engine failure. You can reason that if it can do it on one engine, it can do it on two. If that is true, you can fly this airplane all the way to its ceiling at maximum weight at 230 knots and still be above VSE.
G450 Airplane Flight Manual §5.7.] Charts are presented for choosing the en route climb speed (VSE) and for determining the en route climb gradient. The en route climb speed is based on using the minimum FAR/JAR criteria of 1.3 times the stall speed. The gradient data shown are Net Gradient values (actual reduced by 1.1% gradient).
Photo: Display Controller With VREF, from Eddie's aircraft.
The display controller gives you an accurate VREF based on the flap handle position all the way to 15,000' and from there all the way to 45,000' it appears to be very close. See: G450 Normal Procedures & Techniques / VREF / Display Controller for more about this.
Gulfstream's recommended rough air procedure is to maintain at least 1.5 times VS to maintain an adequate stall margin. Since VREF is 1.3 VS, keeping at least 20% above the VREF shown in the display controller will serve you well. More about this: G450 Abnormal Procedures / Turbulence Penetration Speed.
Figure: Buffet Boundary, from G450 Airplane Flight Manual pg. 5.8 - 2.
The G450 Airplane Flight Manual §5.8 gives us a buffet boundary chart as a function, it says, "of speed, weight, and load factor." But the chart includes altitude as a factor and becomes a useful tool for deciding where we will and will not operate the aircraft. The buffet boundaries are defined by the stick shaker on the low end and the airplane's natural high speed buffet on the high end.
You could take this chart and tape it to your yoke and vow to keep an eye on it every time your speed gets away from you, or you can run the chart for a worst case scenario. Say you are climbing one day after a maximum weight takeoff and find yourself with the autopilot engaged keeping a watchful eye on the autopilot's performance. Using the 30° of bank line gives you a pad for maneuvering and a bit of turbulence.
At 20,000 feet you will get the stick shaker at Mach 0.40, the green line. At 30,000 feet you will get the stick shaker at Mach 0.50, the blue line. At 40,000 feet you will get the stick shaker at Mach 0.60. See a pattern here?
During a maximum weight climb to altitude you will get the stick shaker at a Mach twenty higher than your altitude in thousands. At 20,000 feet it occurs at Mach .40, at 30,000 feet at Mach 0.50, and at 40,000 feet at Mach 0.60.
More about this here: G450 Flight Envelope.
What of VX and VY? What of it? I don't believe in them above traffic pattern altitude. . .
[Dole, Page 82] The maximum climb angle will occur where the maximum excess thrust exists. For a turbojet aircraft, with thrust assumed to be constant with velocity, this will be where Tr (drag) is a minimum. This, of course, is at (L/D)max.
Of course we are not flying a pure turbojet, but with that understanding we know the Angle of Attack that produces (L/D)max is 0.30, and our best climb angle will be at 0.30 AOA.
[14 CFR 1]
VX — means speed for best angle of climb.
VY — means speed for best rate of climb.
[14 CFR 25 §25.123]
(a) For the en route configuration, the flight paths prescribed in paragraph (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 a speed not less than VFTO, 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—
(1) In non-icing conditions; and
(2) In icing conditions with the en route ice accretion defined in appendix C, if:
(i) A speed of 1.18 VSR0 with the en route ice accretion exceeds the en route speed selected for non-icing conditions by more than the greater of 3 knots CAS or 3 percent of VSR; or
(ii) The degradation of the gradient of climb is greater than one-half of the applicable actual-to-net flight path reduction defined in paragraph (b) of this section.
(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.
The concept of best angle of climb and best rate of climb are not flushed out fully in the regulations, but it is clear they do not consider altitude. You are much better off flying 0.30 AOA than VX or VY. See Basic Aerodynamics / Angle of Attack for more about this.
14 CFR 1, Title 14: Aeronautics and Space, Definitions and Abbreviations, Federal Aviation Administration, Department of Transportation
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013