Gulfstream G450 Performance
Nothing is as it seems when it comes to G450 landing performance, but generally what you see is conservative so that's a good thing.
But sometimes we are required to delve into "demonstrated" versus "actual," or "factored" versus "unfactored." Fear not, you can make better decisions about landing performance if you understand the smoke and mirrors involved.
Approach and Landing Speeds
[G450 Airplane Flight Manual §5.11-1] and [G450 Aircraft Operating Manual §13-03-10] Threshold speeds, VREF, for landing distance are shown for all flap settings, with final approach speeds computed as the landing threshold speed plus appropriate speed margins. The final approach speed margin must not be greater than that which can be bled off before the point of crossing the runway threshold. For the G450, a final approach speed margin of 5 KCAS is recommended but not mandatory
[G450 Aircraft Operating Manual §06-05-10] Cross runway threshold at VREF. Retard power levers as necessary for airspeed control, leaving power levers at idle after touchdown. Glide path should allow for a smooth flare into the touchdown zone. At touchdown, the PF will gently lower the nosewheels to the runway and, as necessary, deploy the thrust reversers and apply brakes to slow the aircraft.
You are planning on crossing the threshold at VREF and holding this speed until touchdown unless the runway is contaminated. If the runway is contaminated, you plan on losing 8 to 9 knots prior to touchdown. (See G450 Normal Procedures / Landing for more on this.
[G450 Airplane Flight Manual §5.11] If gusty wind conditions are present, add ½ of the steady state wind plus the full gust value to a maximum additive of 20 knots (VREF + 20). VREF will still be the target speed at the threshold.
The G450 uses a 30% factor above stall speed for VREF. (This is explained here: G450 VREF.) So, for example, if your VREF is 130 knots, you have all the way down to 100 knots, which is below VMCA. Your Angle of Attack will be around 0.50 which is half that of a stall. No problem right? That excess speed represents energy and it takes energy to flare. Besides, a gust can rob you of that margin in an instance. Don't mess with VREF.
[G450 Airplane Flight Manual §5.1] The maximum demonstrated 90° crosswind component for takeoff and landing is 24 knots which was demonstrated with tiller steering operative and rudder pedal steering off.
This value was "demonstrated" and represents the minimum number required by certification based on VREF. Most of us Teterboro veterans know we've exceeded this because KTEB tower has been notorious for lying about the winds. Still, we treat 24 knots as a limit.
For more about the fact and fiction behind the G450 crosswind number, see G450 Crosswinds.
Wet/Contaminated Runway Crosswind Limits
[G450-OIS-02, page 26] Although Gulfstream has not conducted dedicated flight testing to determine crosswind capabilities for wet and contaminated runways, Transport Canada has published a generic chart that can be used to determine crosswind capabilities as a function of the runway surface condition.
Figure: Runway Surface Condition Equivalent, from G450-OIS-02, Table 6a.
It sounds like a lawyer's limit which makes it our limit.
- You first convert the Runway Surface Condition (RSC) to the Canadian Runway Friction Index (CRFI) using Table 6a:
- Remember that anything over 0.125 inches is contaminated.
Figure: Runway Surface Condition Equivalent, from G450-OIS-02, Table 1a.
- You then use Table 1a to find your limit.
A wet runway with just a thin coat of water can have a CRFI as high a 0.60 and the resulting limit actually exceeds the G450 demonstrated value.
A wet runway with just a tenth of an inch of water, however, brings the CRFI down to 0.30 and your crosswind limit to just 10 knots.
Landing Field Length
Figure: Landing Field Length, from G450 Airplane Flight Manual §5.11, pg. 5.11-9.
[G450 Airplane Flight Manual §5.1] Landing Field Length - the actual distance required to land from a 50 foot height and come to a complete stop on a hard surfaced runway, with the distance increase by appropriate conservatism factors in accordance with applicable operating rules.
[G450 Airplane Flight Manual §5.11] The landing distance shall be the horizontal distance required to land and come to a complete stop from a point at a height of 50 feet above the landing surface. Landing distances are shown as a function of flap position, anti-skid functions (operative or inoperative), landing gross weight, airport pressure altitude, wind condition, spoiler deployment (automatic or manual), and possible speed increase (ΔV) to the threshold speed (VREF). The threshold speed increase (ΔV) correction grid provides landing distance increments which would occur on occasions such as Category II approaches or when the pilot elects to use autothrottle all the way to touchdown. Note that landing distances are presented here for both dry and wet runway conditions.
[G450 Airplane Flight Manual §5.11] NORMAL FLAP LANDINGS: The following conditions apply for normal flap landings:
- All engines operating at high idle before touchdown (high idle automatically established with gear down, flaps 39°) and reduced automatically to ground idle about 5 seconds after touchdown.
- Flaps 39° and gear down.
- Landing distances based on 3.0° glide path at 50 feet and 6 FPS sink rate at touchdown.
- Distances are shown for both anti-skid operative and inoperative. Either ground spoilers armed and automatically deployed on touchdown or manual speed brakes selected after touchdown.
[G450 Aircraft Operating Manual §13-03-20] Landing distances based on 3.0° glide path at 50 feet and 6 FPS sink rate at touchdown.
[14 CFR 25 §25.125(b)(5)] The landings may not require exceptional piloting skill or alertness.
The landing field length charts in the G450 AFM produce your most reliable numbers but the results on the bottom can be confusing.
- The bottom line labeled "Landing Distance" is just that, the amount of runway it takes to stop the airplane using the assumptions listed above. (More on that below.) In the example problem, about 3,400 feet.
- Intended Dest Dry, Alternate Dest Dry & Wet is just the landing distance divided by 0.60, allowing for the 14 CFR 135.385(b) safety factor which applies to your intended destination, unless it is wet, and your alternate destination which does not require the wet correction. What does "intended" mean? More on that below. In the example problem, our distance is just under 5,700 feet. That makes sense, figuring 3,400 / 0.60 = 5,667.
- Intended Dest Wet is the Intended Dest Dry number multiplied by 115%, as required by 14 CFR 135.385(d). In our example the distance is just over 6,500 feet. That again makes sense, figuring 5,667 x 1.15 = 6516 feet.
If you cross the fence at 50 feet, flaps 39, off a 3° glide path, touchdown at 6 FPS, and use optimal braking, you will stop the airplane in the chart's "Landing Distance" line. Is this hard to do? Not really, the only challenge is 6 FPS is a pretty firm landing: 360 fpm. That means you are only cutting your descent rate in half. We can often beat the number without the hard landing only because we usually have a headwind. The reversers? This kind of stop is so quick they barely get a chance to spool up before you have to stow them.
What about the "intended" numbers? The chart author meant "intended" to satisfy the 14 CFR 135.385 proviso that required you to apply these factors to your destination before you actually depart. If you are operating under 14 CFR 91 and your company doesn't further restrict you, these factors do not apply to you. Even if you are operating under 14 CFR 135 they only apply prior to departure. Say when you depart the destination is forecast dry but starts raining once you are airborne. The factors do not apply.
The G450 manual assumes you are not a Part 91 only operator and throws in various factors for you. This can be a problem because you will end up outperforming those numbers more times than not and you end up with a poor understanding about your airplane's landing distance.
Commercial Operations Factored Distances (The "60% Rule")
Figure: G450 Landing Field Length Table, from G450 Performance Handbook pg. PC-1.
[14 CFR 135 §135.385(b)] Except as provided in paragraph (c), (d), (e), or (f) of this section, no person operating a turbine engine powered large transport category airplane may take off that airplane unless its weight on arrival, allowing for normal consumption of fuel and oil in flight (in accordance with the landing distance in the Airplane Flight Manual for the elevation of the destination airport and the wind conditions expected there at the time of landing), would allow a full stop landing at the intended destination airport within 60 percent of the effective length of each runway described below from a point 50 feet above the intersection of the obstruction clearance plane and the runway.
[G450 Airplane Flight Manual §5.11]
- Intended Destination Dry and Alternate, Dry and Wet: No airplane shall takeoff at weight in excess of that which, in accordance with landing distances shown in the Operations Manual for the airport elevation and for wind conditions anticipated there at the time of landing, would allow full stop landing within 60% of the effective length of the runway from a point 50 feet directly above the intersection of obstruction clearance plane and the runway.
- Intended Destination Wet: No airplane shall takeoff when appropriate weather reports and forecasts, or a combination thereof, indicate that runways may be wet or slippery at the estimated time of arrival unless effective wet runway length is at least 115% of dry runway length. Note that wet runway distances are valid only for .125 in (3 mm) or less of standing water.
The G450 Performance Handbook offers an easy source of landing field length data, but note the table says "Intended Destination." So these numbers have the 60% factor even if you don't want them.
14 CFR 91K Factored Distances (The "80% Rule")
[14 CFR 91 §91.1037(c)(2) The airplane's weight on arrival, allowing for normal consumption of fuel and oil in flight (in accordance with the landing distance in the Airplane Flight Manual for the elevation of the destination airport and the wind conditions expected there at the time of landing), would allow a full stop landing at the intended destination airport within 80 percent of the effective length of each runway described below from a point 50 feet above the intersection of the obstruction clearance plane and the runway.
With the advent of 14 CFR 91K, those "sorta charter" operators, we now have the "80% rule." While this factor is mentioned in the G450-OIS_02 on contaminated runway operations, it isn't actually reflected in the charts.
The "80%" rule only applies to 14 CFR 91K operators but offers a good compromise for those wanting a safety pad above the basic 14 CFR 91 numbers but not as extreme as the 14 CFR 135 criteria.
SAFO 06012 Factored Distances (The "15% Rule")
[SAFO 06012] Once the actual landing distance is determined an additional safety margin of at least 15% should be added to that distance.
This factor came out of the Southwest 1248 runway excursion incident in Chicago Midway but hasn't caught up with the AFM yet, but most of the numbers in the contaminated runway OIS do. (More on that below.)
The G450 flight manual offers only the number with the 60% factor. You will either have to add 15% to the basic number or multiple the factored number by 0.60 to get the real wet landing distance.
[14 CFR 135 §135.385(d)] Unless, based on a showing of actual operating landing techniques on wet runways, a shorter landing distance (but never less than that required by paragraph (b) of this section) has been approved for a specific type and model airplane and included in the Airplane Flight Manual, no person may take off a turbojet airplane when the appropriate weather reports or forecasts, or any combination of them, indicate that the runways at the destination airport may be wet or slippery at the estimated time of arrival unless the effective runway length at the destination airport is at least 115 percent of the runway length required under paragraph (b) of this section.
[14 CFR 135 §135.385(e)] A turbojet airplane that would be prohibited from being taken off because it could not meet paragraph (b)(2) of this section may be taken off if an alternate airport is selected that meets all of paragraph (b) of this section.
[G450 Airplane Flight Manual §5.1] A runway is considered wet when it is well soaked (there is sufficient moisture on the runway surface to cause it to appear reflective) but without significant areas of standing water.
To compute contaminated landing distance, you will need G450-OIS-02 where you are presented with charts, tables, and a one page solution. The charts give you the most accurate numbers but are time consuming and error prone. The tables offer an easy solution with just a few options of contamination depth. The one page solution is a thing of beauty:
[G450 Airplane Flight Manual §5.1] The runway is considered contaminated when more than 25% of the runway surface area (whether in isolated areas or not), within the required length and width being used, is covered by surface water more than 0.125 inch (3 mm) deep, or by slush or loose snow equivalent to more than 0.125 inch (3 mm) of water.
The title says "En route Landing Distances" so this is one of the few charts that offers basic landing distances. You enter the first column with the basic, unfactored number and end up with data that has the 15% factor added.
FMS Performance Computer.
The FMS performance computer produces good numbers, but knowing what each line is telling you can be a challenge. The AOM doesn't explain every line and some of the explanations are misleading.
Photo: From Haskel's aircraft.
[G450 Aircraft Operating Manual §2B-26-80, ¶1.B]
- LAND WT: Projected landing weight (1L) (From the landing/ga init page.)
- MAX LAND WT: Maximum landing weight (2R), based on initialization conditions
- WIND: Headwind and crosswind components (3L) (From the wind input on init page.)
- RWY USE: Usable runway length (length minus displaced threshold) (1R)
- RWY REQ: (This appears to be the AFM landing distance with no additives.)
- DRY RWY: (This appears to be the required runway length with US FAA 135 dry calculation criteria. Landing distance divided by 0.60.) *
- WET RWY: (This appears to be the required runway length with US FAA 135 wet calculation criteria. [G450 OM 2B-26-80§1.B.] Landing distance divided by 0.60 and then multiplied by 1.15.) *
- 39° VREF: Based on landing init.
- APP CLB SPD: Climb speed with critical engine inoperative, 20° flaps, landing gear retracted.
- LND CLB SPD: Climb speed with all engines operating, 39° flaps, landing gear extended.
I like to brief both numbers: RWY REQ and DRY RWY. I know I can easily stop the airplane between those two figures and the closer I want to get to RWY REQ the more aggressive I will have to be. If the runway is wet, I add 15% to the RWY REQ number and brief that along with the WET RWY number.
* About the "divided by 0.60" requirement. If you are a commercial operator you know that you cannot dispatch unless you can land on 60% of the planned runway at your destination. Most of us multiple our landing distance by 1.67 to make sure we have enough runway. Let's say your RWY REQ is 3,000 feet. If you multiply that by 1.67 you get 5,010 feet and if your runway is longer than that, you are good to go. If the runway is 5,000 feet, your landing distance can be no longer than (5000) x (60%) = 3,000 feet. But wait, the multiply by 1.67 procedure said we needed 5,010 feet. It is a rounding error. Dividing by 0.60 gives you a more accurate answer. In this example: (3,000) / (0.60) = 5,000 feet. Is 10 feet worth worrying about? Not really. But the engineer in me prefers the divide by 0.60 routine because it is more mathematically "pure."
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
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
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
Gulfstream G450 Operational Information Supplement, G450-OIS-02, Contaminated Runway Performance, Revision 1, August 3, 2011
Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011
Safety Alert for Operators, SAFO 06012, Landing Performance Assessments at Time of Arrival (Turbojets), 8/31/06, U.S. Department of Transportation