Aircraft Performance Planning

[hoover]

This may seem an odd question, but do military pilots plan their flights to account for engine out performance on takeoff? For example, does a B-1 or F-15 pilot limit his takeoff weight so that in the event of an engine failure at V1 he can still takeoff? Or do they just load up with ordanance and hope that an engine doesn't quit.

 

[bearcat]

A-10 specific...we typically fly local sorties so we aren't that concerned with single engine performance where I am at. We have an oh sh!t button that will jettison all of our external stores (with some limits) with a push of a button. It would be interesting to jettinson a whole bunch of live munitions immediately after takeoff.

 

[pilotyip]

Depends upon the mission, flying in a combat Zone where the airplane must fly such things, as SE climb performance is not taken into account. No balanced field lenght either. I remember takeoffs in the P-3A with the -10W engines over max gross where if we lost water injection before we reaching 110 Kts coming out of Sangley Point, we could neither fly nor stop on the remaining runway. Talking to the KC-135 guys in the club at Utapao, they had very close to the same conditions on hot days on the 11,000+ foot runways

 

[VVJM265]

Multi-engine guys get in the charts for sure.

 

[MarineGrunt]

We could go on for hours with TOLD, but a quick answer your question:

V-CEFS is the speed you can accelerate to, lose an engine and take off or stop in the same distance. This distance (critical field length), in almost every case, must be equal to or shorter than usable runway. This is calculated for every flight.

V1 (the military calls S1 for some strange reason) is based off of a whole slew of speeds but will never be less than V-CEFS. So an engine fails at V1 (S1), you will be able to continue the takeoff.

For normal mission planning, takeoff and climbs are figured one engine inop. There are certain exceptions where all engines operating TOLD can be used as noted in an earlier post, but thats getting more into what I feel like typing!

 

[CobraKai]

Talking to the KC-135 guys in the club at Utapao, they had very close to the same conditions on hot days on the 11,000+ foot runways

The new engines make that a little better now. Although I have seen the 1 or 2 boards in the desert.

As for what MarineGrunt said Ditto in the -135. All our data is for one engine inop.

 

[Herc130AV8R]

The Herc is a whole different beast!! We cannot take off below Vmca which accounts for our ability to laterally control the A/C in the air with 1 engine inop. (Having props makes things much more interesting). With our ground run and roll being so short (<2000 ft!!!!) it usually is not a factor until we go to the 3,000 ft assault zone!! We still must make Vmca before Vt/o on the assault zone without a waiver. It CAN be done which is why the Herc is such an amazing A/C for the tactical mission.

 

[TankerDriver]

In the 135, we use a TOLD program before each mission that calculates our takeoff performance. We just punch in the parameters: runway length, grade, heading, condition-dry/wet/slush, temps, winds, pressure alt, tell it we want to do a MAX mode takeoff or an ACCEL climb, 20 or 30 degrees flaps, TRT or specific climb gradient. Hit the calculate button and it spits out numbers. We use this as a planning baseline, but always run realtime numbers in the jet before takeoff with the most current weather. We punch it all into our FMS, hit run and about 15 minutes later, it spits out numbers. Well, maybe not 15 minutes, but the darn thing seems to take that long. It depends on how fast the hampster is turning the wheel in there. Ah, the wonderful world of avionics upgrades in the AF. After it's done with its calculations, it gives us all our speeds that we need.

In the desert, it's a little different in that we don't normally run TOLD calculations for planning purposes before we go out to the jet. We probably "should", but unless there are some abnormal variables, like strong crosswinds or a wet runway (which doesn't happen very often where we fly out of), we takeoff from the same runway, at the same temperature, at the same weights, with pretty much the same winds every day. We just get to the jet and punch the numbers in while sitting in the chocks. The numbers are pretty much identical every time we fly. The "box" gives us a 3 engine climb speed and rate of climb also, so we can see what our performance would be if we lost one on takeoff. At the heavy weights we were at, using TRT, we were seeing around 7-800 fpm climb for a 3 engine climb if I recall.

 

[AvroGuy]

The plan is to take off with what you need to do the mission. I would start out with a rolling Reduced Thrusst. If that didn't work I would go to a rolling TRT, Max power for the day. IF that didn't work the last option I had was a static TRT. IF the static didn't work, I would let the AC know and tell him/her how much weight we had to loose in order to get off the runway with 3 engines operating. I did all of this in the charts because I was the FE on the E-3. As long as the runway was dry we could useally get off the ground with full fuel. When it was wet, the problems started..

Data was always done no matter where we were. The only thing that changed was in a deployed location we would use canned data that would work within the TOLD tollerances. Most of us Engineers would verify the data when we arrived at the deployed location.

 

[prozac]

To further complicate things, how many of you guys have charts that give you good climb gradient info? In the 135 performance manual, there is a rediculous discussion on climb gradient. It basically leaves you with no good information for departure planning. For instance, if a departure requires a minimum climb gradient of 450ft/nm to 15,000 ft, we do not have a chart that will tell us if we can make it. Some of us try to interpolate the data from other charts (obstacle clearance, initial climb rate, etc), but most just put their trust into the big CFM fans and hope for the best. Anybody in a similar situation?