Figure: Brake Synoptics, extracted, from FSI G450 PTM, Figure 16-13.
The wheel brakes on the G450 are very good and, with few exceptions, the only time you will have to worry about their capability is following a series of short hops where you had just a few minutes between turns. Of course we do that in this business, don't we? Fortunately the brake temperature is automatically measured and the Brake Temperature Monitoring System (BTMS) reports to you in the brake synoptic page (shown above). More about the BTMS: G450 Systems / Landing Gear, Wheels, and Brakes / BTMS.
Our first concern is: will we be able to stop if we have to abort the takeoff at V1? With some airplanes this could be a problem on your very first takeoff of the day if you were heavy and had a long taxi to the runway. With the G450 this is rarely a problem. In fact, even if you just landed, picked up passengers, and are ready to go in just a few minutes, chances are you will be okay. But the airplane helps you out in this instance too. The Brake Temperature Monitoring System (BTMS) tracks your brake usage and logs away the time of the peak temperature. The airplane's performance computer then figures out how much energy it will take to abort the next takeoff and warns you if you are in danger of exceeding the capability of the brakes.
So you may not have to worry about this at all. But the BTMS doesn't always report for duty when it should and there are times when you would like to know well in advance of showing up at the end of a runway to find out you need to cool your brakes before you takeoff. You really should understand the magic inside the black boxes:
Everything here is from the references shown below, with a few comments in blue.
Figure: Brake Energy, from G450 PH, page PH-143.
[G450 AFM, Appendix C, Page C-1] The maximum BKE capacity of the G450 carbon brake system was demonstrated to be 115 million foot-pounds (MFP) during rejected takeoff flight testing. The brakes must be inspected per aircraft maintenance manuals if energy level of 91 million foot-pounds (MFP) is exceeded during aircraft braking operations.
The limit on the brakes is measured in million foot-pounds of energy, which doesn't do you, the pilot, much good. The energy level, however, does correlate to a temperature and if you add that to the AFM notes, you have a guide.
[G450 AFM, Appendix C, Page C-6] Fuseplug release not likely; BKE less than 75 MFP.
If you see less than 625°C you are in the "normal zone," but remember the temperature keeps rising for about 30 minutes, especially if you continue to taxi or have to set the brakes.
[G450 AFM, Appendix C, Page C-6] Fuseplug release possible; BKE between 75 - 91 MFP.
If you are between 625°C and 750°C, you are in the caution zone.
[G450 AFM, Appendix C, Page C-6] Fuseplug release probable; BKE greater than 91 MFP.
If you are above 750°C, you are in the danger zone.
[G450 AFM, Appendix C, Page C-1] The energy the brakes can absorb during a stop is determined by the previous energy absorbed, the airplane gross weight, the speed at brake application, the taxi operations and the cooling conditions. The BKE/Cool Down chart may be used to determine the expected operational brake energy levels and cooling times. The Brake Kinetic Energy (BKE) presented in the BKE/Cool Down chart is based on the aircraft total kinetic energy with no credit for aerodynamic drag or reverse thrust, as these components are most often offset by unequal energy distribution throughout the four (4) brake assemblies.
Figure: Brake Energy, from FSI G450 PTM, figure 16-13.
In order to use the BKE/Cool Down chart to determine the BKE absorbed and possible cool down time required, the operator must note the initial brake application speed, or use the Brake Temperature Monitoring System (BTMS) maximum indicated peak temperature for correlation with the BKE absorbed. If the brake application speed is noted from the airspeed indicator (calibrated airspeed), it must first be adjusted by the runway component of wind velocity (add for tailwind, subtract for headwind). Then this wind adjusted airspeed must be converted to a true ground speed using corrections for pressure altitude and temperature. If the FMS or IRS ground speed is used to determine the BKE, these wind corrections are not required. When the BTMS peak temperature is used, the operator can simply compare the BTMS and the BKE scales on the chart to read the BKE that corresponds to the recorded BTMS peak temperature.
[G450 AFM, Appendix C, Page C-1] The BTMS peak temperature will occur within approximately 5 minutes after brake application. The brakes synoptic page will provide displays of the brake temperatures for each brake, the highest peak temperature, which wheels recorded the highest brake temperature, and the time of occurrence of the highest peak temperature. The peak BTMS temperature is the highest brake temperature recorded since the last landing, and is the only temperature that can be used as a measure of the expended or residual BKE. A Brake Overheat CAS message will illuminate at 625° C and indicates the possibility of fuseplug release. The BTMS displays will appear in an amber color when the peak temperature is equal to or greater than 625°C.
Your BTMS should record the peak temperature experienced by your brakes following landing or a rejected takeoff. Contrary to the AFM statement, it is my experience that the brake temperature keeps increasing for 30 minutes after brake application.
You will not get a CAS message or an FMS scratchpad message if the BTMS thinks your next takeoff will place you in the caution or danger zones, it will simply refuse to let you initialize your takeoff data.
For less than those marginal conditions, the BTMS can be relied upon to keep you out of trouble, if it got a good reading on the last brake application. If that happened, your life will be a lot easier in the steps that follow. But it doesn't always happen and you should get into the practice of noting the speed at which you applied the brakes on every landing and whenever you reject a takeoff. More about that follows.
When you start your day your brakes should be at ambient temperature, almost never above 50°C. You will rarely see over 300°C after landing. Since the caution zone on the brake energy chart begins at around 620°C, you should get the idea that brake energy is something you will rarely have to worry about. There is a slight complication to that:
[G450 AFM, Appendix C, Page C-1] Energy levels absorbed by the brakes during taxi operations are typically low. But, during long taxi operations, the energy absorbed from such sources as engine idle thrust, stops, and turns can be a factor. To account for these considerations, an estimate of brake energy requirements of 3.4 MFP per statute mile of taxi is used.
This 3.4 MFP BKE is based upon a 20 knot taxi speed and one 20 knot stop per statute mile. In addition, the presence of taxiway slope (or runway slope when used to taxi) can add significantly to the brake energy absorbed during taxi. That is, for each statute mile of taxi on a downhill slope, an additional BKE contribution of 4 MFP should be assumed to accumulate in the brakes for each 1 % of downhill slope.
Reverse idle thrust can be used on an unlimited basis to control taxi speed and to stop the airplane during taxi operations. In these instances accountability for taxi BKE's is not required.
It generally takes about 4 miles of taxi under benign conditions to raise the temperature of the brakes 100°C. You can expect about a 4 mile taxi at some airports so it would be a good idea to add 100°C to what you see in the chocks when considering a subsequent takeoff. As a technique, if I see 300°C or higher before takeoff, I will plan on keeping the landing gear extended after takeoff until the temperature goes below 200°C. I am told this is overkill; I call it "operating with an abundance of caution." I do warn my passengers it will be a little noisy after takeoff.
Figure: Takeoff Init Page 5/5, from Eddie's aircraft.
[G450 AOM, §2B-26-00, pg. 39.] 1L and 1R-- The BTMS configuration is displayed at this line. When ENABLED, brake temperature computations are used in computing the TAKEOFF DATA. When DISABLED brake temperature computations are not used in computing the TAKEOFF DATA. The default is ENABLED. Should the FMS not be able to perform brake temperature monitoring computations, ENABLED is displayed in reverse video.
Though there isn't much written about this, experience tells us that the performance computer uses this information to ensure the brakes will remain below 90 million ft lbs if the takeoff planned in the PERF INIT and TAKEOFF INIT pages were to be aborted at V1. It looks like the only way you will find this out is if the TAKEOFF INIT fails to initialize. The only messages you could get are from two other problems:
[G450 AOM, §2B-26-00, pg. 51.]
This is simply a problem of the system getting invalid temperature or time data. (These are computers after all; I've seen this a handful of times.) If you know the brakes are cold you could simply disable the BTMS in order to complete the takeoff initialization; the BTMS will reset itself at 60 knots. Here's how to do that: G450 Normal Procedures / FMS: Takeoff Data Initialization.
The BTMS uses the chart which is pretty much unusable below 50,000 lbs, hence the warning. Section 2B of the operating manual is written for both the G450 and G550. I think the 50,000 lbs. is for the G550 because I have taken off several times below 50,000 lbs without getting this warning. G450 AFM, Appendix C, Page C-5 hints that the real number for the G450 is 45,000 lbs.
[G450 AFM, Appendix C, Page C-6] A constant 3-mile taxi distance is always assumed as part of the next planned takeoff. Taxi distances in excess of this distance can result in higher taxi BKE requirements. Computed BKEs do not consider the effects of prolonged downhill taxi distances or downhill rejected takeoff distances.
As shown above, Normal (Low Stress) Operations, a normal taxi adds to the energy absorbed by your brakes. As a technique, you can plan on the brakes getting 100°C hotter after a taxi of 4 miles. If you are near (but below) the limit, the BTMS will allow you to initialize your takeoff data without considering the taxi distance required to get to the runway. I don't know what would happen at that point, will the data "unbox?" If you know you will be close, you should figure your brake energy manually, as shown below.
The AFM uses 115 million foot-pounds as the brake energy limit which is deep into the "danger zone." If you need to reject the subsequent takeoff at V1, the fuse plugs will release, the airplane will need to be evacuated, and the airplane will be unflyable until the wheels are torn down for inspection. You might be able to forgo all that if you simply wait an hour. You can compute your brake energy in the chocks and make an educated decision of wait time versus risk. More about that next . . .
[G450 AFM, Appendix C, Page C-3] When planning quick turn-around operations (less than 30 minutes ground time), pilots must take into account the brake energy that will accumulate and the limited benefit of a short cooling period. A typical scenario would be a deadhead flight to pick up passengers for a subsequent intercontinental trip. The landing at the pickup point is at maximum landing weight with the subsequent takeoff at or near maximum takeoff weight. The resulting brake takeoff temperature becomes critical if a rejected takeoff is required.
For the above scenario, the pilots must track the cumulative energy inputs into the brakes which include braking on landing, braking during taxi-in and taxi-out operations, and brake requirements in the event of a rejected takeoff. If the total airplane cumulative BKE for these operations exceeds the maximum brake capacity of 115 million foot-pounds, a cool down period is required between the landing and the subsequent takeoff. The required cool down time can be determined from the Brake Kinetic Energy Chart using the cumulative BKE's from previous operations and the estimated BKE for the next operation, including taxi.
The AFM gives a quick turn example which works as well as any. The chart, however, can be confusing because of the two steps (landing and subsequent takeoff) can obscure the process. So we'll take their example in steps. . .
[G450 AFM, Appendix C, Page C-8 and C-9]
Figure: Brake Energy Example, from G450 PH, page PH-143.
First determine the landing BKE using either the noted brakes on speed or the recorded BTMS temperature.
Figure: Brake Energy Example, from G450 PH, page PH-143.
Next, for the takeoff gross weight of 69,000 pounds, a maximum takeoff decision speed of 141 KCAS is read from the applicable G450 AFM chart. Note that a lower V1 can be employed if the field length is adequate; but, for this case the V1 was selected to simplify illustration of the example (i.e., V1 = VR = 141 KIAS). The corresponding brakes-on speed for a rejected takeoff is 142 KIAS (141 + 1).
Note that times seen for the "cooling time on ground" scale are those required to achieve a recovery of full braking capability (i.e., 115 MFP) for the applicable BKE levels. Hence, if the takeoff BKE requirement is 115 MFP, a full energy recovery after the landing in this example would require a 1.55 hours of brake cooling for the landing BKE of 54 MFP.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013
Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011