In the last chapter we looked at how the weight and balance of the plane is critical to being able to fly in a controlled manner.
Today we'll look at another critical set of numbers - takeoff data. In a heavy jet we don't just "kick the tires and light the fires". Before we go fly we make sure we can safely get off the ground with the runway we have to work with.
This is critical stuff because we're trying to accelerate a couple hundred thousand pounds (or more) up to race-car speeds and we've only got mile and half or so to do it in.
Today we have computer programs that do most of the work for us, but we still have to know what the numbers are telling us.
Back in my prehistoric B-52 days, we had to know how to work takeoff data using a huge binder full of charts and a pencil. You'd trace a line on a graph, get a number, use that number on the next chart and so on. Compounding your errors all the time. The thickness of your pencil might make a 5 knot difference in takeoff speed.
Today we have a laptop computer in the cockpit which will calculate this for us, and probably more accurately than I could do it. During our pre-flight checks we calculate our takeoff data and then double-check it.
First off we need to know a few things. Like what runway we're going to takeoff from. To get that information we get the ATIS (Automated Terminal Information Service) either over the radio or over our datalink. This will give us the current weather, runways in use and other nice to know information like a taxiway being closed.
Then we start plugging numbers into the computer:
RUNWAY - The computer has a database of the runways we use so it knows how long they are, how high up they are (elevation), what direction they face and if they slope up or down. Taking off uphill will take more runway than downhill. If I'm not sure which runway they may send us off I'll calculate data for more than one.
WIND - Taking off into the wind is always better. It might sound counter intuitive, but a headwind is free airspeed, and we know that airspeed is what makes the wings fly.
Take this to its extreme and it will make more sense - with a 150 knot headwind we could hover (except I don't want to fly in any weather that can produce 150 knot headwinds!). We can takeoff with a tailwind, but no more than 10 knots is allowed and we'll use more runway.
TEMPERATURE - Colder is better. Cold air is denser than warm air. The engines produce more thrust with denser air. Denser air also gives the wings more to work with.
PRESSURE - This is the current barometric pressure. What we use is the local altimeter setting in inches of mercury (US) or Hectopascals (Europe). "Standard" pressure is 29.92 inches of mercury in case you want to amaze your friends the next time the Weather Channel is on.
Using those numbers the computer will calculate the DENSITY ALTITUDE, which is an important number (except the computer doesn't show me what it is, go figure). A high density altitude means that the air is very thin. That means less air molecules going through the engine to make thrust. Since those air molecules are further apart (less density), we'll need a much higher speed across the ground (ground speed) to get enough of those little guys across the wings to produce lift.
This gets really important at "hot and high" airports. Taking off from Bogota (8,300 ft elevation) in the summer is an experience.
WEIGHT - A heavier plane will accelerate more slowly and will need a higher airspeed to fly, as we've learned in earlier chapters (see how this all comes together). It will also be more difficult to stop if we need to reject the takeoff (more on that later).
CG - Center of Gravity. This determines what our stabilizer trim will be set at for takeoff. This determines how much pitch control we'll have at rotation. Too far forward and we might not be able to pull the nose off the runway. Too far back and it might try to fly before we're ready. We need that Goldilocks "just right" setting.
Runway Condition - A wet or icy runway would definitely affect our stopping distance if we need to reject. A slush or snow covered runway might also affect our acceleration.
Climb profile - We have two standard climb profiles we can do. The one we normally use has us climb to 1000 feet with takeoff flaps and then clean up the airplane, accelerate to 250 knots and climb at 250 knots to 10,000'. This is our most fuel efficient climb because we get the flaps up fairly early. It also gets us up to speed and away from the runway quicker so other planes can take off behind us.
OK, hit "Enter" and the computer spits a bunch of numbers at me:
Takeoff EPR - This will be our thrust setting. If you remember from when we talked about the engines this is called Exhaust Pressure Ratio. We don't always use maximum power for takeoff. If we can get away with using less thrust and still have adequate performance, it's easier on the engines. A reduced thrust setting also makes the plane easier to control if we lose an engine because there isn't as much asymmetric thrust.
Nominal N1 - Another engine related number. Since we're using EPR to set thrust, the N1 (actual RPMs of the fan) is a "common sense" check to make sure the EPR probes are working properly. Back in 1982 an Air Florida 737 ended up in the Potomac because their EPR probes were iced up and gave them false readings.
Flap setting - More flaps gets us off the runway sooner but hurts our climb performance once we're airborne. In the 757 we normally use flaps 5 (degrees) for takeoff but we can go up to flaps 20.
Trim - Stabilizer trim setting. We set this before takeoff and validate it as part of our before takeoff checklist.
V1 - This is an important number. This is our "go no-go" speed. If we lose an engine prior to V1 we need to stop, because we won't be able to accelerate to takeoff speed on the remaining engine in the runway available. After V1 we need to continue the takeoff, because we wouldn't be able to stop in the remaining runway. We set a "bug" on our airspeed indicator to V1.
Would you ever reject a takeoff after V1? Only if something really catastrophic happened. Taking the plane into the air would have to be worse than running it into a ditch somewhere past the end of the runway. Jets are made to GO, and for most things I'd take it into the air.
VR - Rotation speed. This is the speed at which we bring the nose up for takeoff. Too soon and we won't accelerate as quickly (the plane is acting as an air brake) too late and we use more runway. VR is always greater than or equal to V1. Stopping isn't a factor once we rotate because we'll be flying.
There's a little technique involved here. You don't want to yank the plane off the ground or you might drag the tail, especially with a long plane like a 757. The flight manual says to rotate at 2.5 to 3 degrees per second up to 15 degrees of pitch. So rotation should take about a 5 or 6 count.
We set our second "bug" at VR.
V2 - Single engine climb speed. If we lose an engine on takeoff, we climb at this speed (or faster if we've got it) until we're high enough to speed up and retract the flaps. V2 is always higher than VR. If both engines are working we'll climb at V2+15 (in the 757).
We set the third bug at V2.
This kills a lot of pilots of light twins. They lose an engine, get below VMCA, and the plane flips over on its back. There's a saying about light twins - the second engine just carries you to the crash site.
Engine Out Acceleration Altitude - Usually 1000 feet above the airport. If we lose an engine this is where we'll level off and accelerate.
Climb gradients - Some departures require extra climb performance because there may be obstacles. Places like Reno that have mountains around them tend to have required climb gradients. These are sometimes given in feet/nautical mile or as a percentage. The 757 can usually double or triple the required climb gradient (I love this plane!) but we check just to make sure.
Flap speed(s) - Each flap setting has a minimum maneuvering speed. On a normal 757 takeoff we're going to set airspeed bugs for flaps 5, flaps 1 and flaps up (clean). After we get airborne and accelerate we'll bring the flaps up "on schedule".
Vref - Reference speed. This is actually an approach speed. If we have to come back around and land in a big hurry we may not have time to look this up so calculate it before takeoff.
Now what if the computer tells us we can't take off? We have a few options:
1. Find a longer runway. Maybe we're not using the longest runway available at this airport.
2. Use Max takeoff power instead of reduced thrust. We don't like to use it but it's there if we need it.
3. Turn the air conditioning off. Yes, really. The air conditioning works off bleed air from the engines' compressors. We only have so much bleed air and we can use some of it to run the air conditioning or we can send it all out the tailpipe as thrust. Once airborne we can turn the A/C back on. We had to do this in the 727 sometimes because it was a little underpowered.
4. Lighten the load. We really don't want to do this, but we could offload some freight or some fuel (if we have extra) to make the plane lighter.
So what happens if we lose an engine on takeoff? This is a very critical event so we practice it a couple times a year in the simulator.
If our speed is below V1 we'll reject the takeoff. We'll use maximum braking, spoilers and the thrust reverser on the remaining engine to stop.
If our speed is above V1 we'll accelerate to VR, rotate a little bit less than a normal takeoff and then climb at V2 to V2+15 (if we have extra speed we keep it).
We'd prefer not to turn, but if we have to we can only use 15 degrees of bank with our speed at V2.
The old saying about "don't turn into a dead engine" doesn't really apply to jet airliners. If we have to turn into a dead engine to avoid a mountain - guess what.
Once we get to 1000' we level off and accelerate. We're probably coming back to this airport and landing, so we may leave the flaps at 5 while we come around the pattern. If we need to go to another airport for some reason we'll bring the flaps up.
The main difference from a normal takeoff is the plane obviously won't climb as well and we need to hold rudder into the good engine to keep us going straight. Other than that it's not a big deal.
Now if we're someplace like Reno or Bogota that whole "not climb as well" thing can really bite you. That's why we have a special procedure to fly if we lose an engine.
It looks complicated but it's really just "turn left and fly up the valley".
If we can see outside we'll just avoid the big rocks by looking out the window, but we might have to fly this in the weather. One important point is that this procedure is specific to our aircraft and our airline so Reno approach control has no idea of what this looks like. If we lose an engine and I tell Reno "We're flying the engine out departure" they're just going to say "Huh?"
If we lose an engine we'll tell them something like "We've lost an engine and we're turning left up the valley". That lets them know what we're doing and also that we're kind of busy right now and we'll get back to them when we have things sorted out.