Eddie Sez:

We sometimes think of VREF as the most critical number in our day-to-day flight operations and maybe it is. I think we should temper our reverence to reference speed with frequent checks of AOA, but that is just me.

The FAA doesn't dictate the method used to determine VREF, only that is must be at least 1.23 times the stall speed of the aircraft in a landing configuration. Most of us think the multiplier has to be 1.3 but it isn't always the case. (The Challenger 604, for example, uses the 1.23 minimum.) As it turns out, the G450 does indeed use 1.3.

For more about the technical and regulatory side of VREF, see Technical / VREF.

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.


Definitions

[G450 Airplane Flight Manual §5.1-3] VREF, LANDING REFERENCE SPEED - the landing threshold speed for the selected landing configuration and atmospheric conditions.

[14 CFR 1 §1.1.] Reference landing speed means the speed of the airplane, in a specified landing configuration, at the point where it descends through the 50 foot height in the determination of the landing distance.

[14 CFR 1 §1.2] VREF means reference landing speed.

[14 CFR 25 §25.125] Landing.

(a) The horizontal distance necessary to land and to come to a complete stop (or to a speed of approximately 3 knots for water landings) from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane):

(1) In non-icing conditions; and

(2) In icing conditions with the landing ice accretion defined in appendix C if VREF for icing conditions exceeds VREF for non-icing conditions by more than 5 knots CAS at the maximum landing weight.

(b) In determining the distance in paragraph (a) of this section:

(1) The airplane must be in the landing configuration.

(2) A stabilized approach, with a calibrated airspeed of not less than VREF, must be maintained down to the 50-foot height.

(i) In non-icing conditions, VREF may not be less than:

(A) 1.23 VSR0;

(B) VMCL established under §25.149(f); and

(C) A speed that provides the maneuvering capability specified in §25.143(h).

(ii) In icing conditions, VREF may not be less than:

(A) The speed determined in paragraph (b)(2)(i) of this section;

(B) 1.23 VSR0 with the landing ice accretion defined in appendix C if that speed exceeds VREF for non-icing conditions by more than 5 knots CAS; and

(C) A speed that provides the maneuvering capability specified in §25.143(h) with the landing ice accretion defined in appendix C.

(3) Changes in configuration, power or thrust, and speed, must be made in accordance with the established procedures for service operation.

(4) The landing must be made without excessive vertical acceleration, tendency to bounce, nose over, ground loop, porpoise, or water loop.

(5) The landings may not require exceptional piloting skill or alertness.


AFM Charted VREF

Figure: Threshold Speed, from G450 Airplane Flight Manual §5.1-3.

Section 5 of the G450 Airplane Flight Manual contains threshold speed charts for a variety of configurations up to 15,000 feet.


Performance Handbook VREF

Figure: G450 Landing Speed Schedule, from G450 Performance Handbook pg. PC-2.

The G450 Performance Handbook also gives you VREF for a variety of conditions. The table here is for 50,000 lbs.


AFM Charted VSR0 (Stall Speed)

Figure: G450 Stall Speed, from G450 Airplane Flight Manual §5.1.

Section 5 of the G450 Airplane Flight Manual provides VSR0 stall speeds in the landing configuration. Comparing these to the charted reference speeds reveals that in the G450, VREF = 1.3 VSR0.

For more about this: Technical / VSR - Reference Stall Speed.


Display Controller VREF

Photo: Display Controller With VREF, from Eddie's aircraft.

Figure: G450 Stall Speed, from G450 Airplane Flight Manual §5.1.

We are told from day one that the display controller provides VREF for any altitude, any gross weight, for the configuration dictated by the flap handle. But this isn't written anywhere. Is it true?

It appears to be accurate where you need it most, on approach at typical approach altitudes. I've looked at it a few times at altitude and the display controller appears to give a number lower than the charts would suggest, but not much lower. For example, one day flying at FL 300 at a gross weight of 55,621 lbs the display controller showed a VREF of 177 knots. The charted stall speed is 138 KCAS which would mean a VREF of 179 KCAS. Close, I agree. But I wouldn't fly that slow at that altitude. See G450 Flight Envelope for more about this.


References

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

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

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

Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011