This emulator will allow you to learn or practice the art of programming the Multipurpose Control & Display Unit (MCDU).
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by Eric Bradley, 19 December 2015
Of all the things we receive questions about, it seems takeoff speeds are the source of the most confusion, and/or misunderstandings.
It's possible I didn't give due thought to this matter in the tutorials because it was assumed that most users would be real-world Airbus pilots going through the proper training syllabus, and the emulator (and tutorials) were intended as a quick top-up / refresher, or an opportunity for free and easy practice.
As things appear to have turned out, while we do have thousands of regular visitors who are indeed Airbus pilots, or captains making the transition from Boeing, we also have many more visitors who are private pilots and simulator enthusiasts, and for those people there's a big chunk of information missing.
So for the benefit of everyone, I'm going to use this article to try and fill in the blanks. Please do contact me for information if you have any questions or if you notice any errors in the information provided here. We hope to encourage better learning and especially safety for everyone who is using our resources here.
Takeoff speeds are one of the most important things you need to understand as a pilot. Of course landing speed is even more important, but that tends to be relatively uniform, as you need to maintain enough knots to stay airborne until just before you touch down. So landing speed is a lot simpler than take off speed.
For most airline pilots, your company will provide reference charts for each airport that you work out of (at the very least), and these will have the appropriate settings for V1, V2 and VR. Simulator pilots, you have a bit more work to do, because you won't have ready access to those references (we'll try to make a database available in the future, for simulator users to have the same references, however your simulator software may not have real-world accuracy so the settings may still need some adjustment).
Minimum Speed for Control on the Ground
The first speed you have to be ready for is not even one of the 3 reference speeds. This is the speed required to maintain minimum control on the ground, and is predictably enough written as "VMCG". The actual velocity does vary slightly with altitude above MSL.
Now what you need to know about VMCG is that it's main purpose is to ensure you have the ability to control the aircraft while it's still on the ground and one of your engines cuts out while you're rolling. You can probably predict what will happen in that situation, because you'll have asymmetric thrust, and quite possibly also quite strong thrust as this is more likely to happen during the actual takeoff.
When you have asymmetric thrust, the aircraft will try to move laterally, and the only way you can steer against it and keep the aircraft straight on the runway is to apply full rudder in the opposite direction. Obviously your goal initially will be to slow the aircraft from takeoff thrust (but only if you're below V1). PNF should also notify ATC that you have an emergency on the ground, so they will be ready to mobilize emergency services and delay other waiting aircraft, unless you are super-confident that you have the situation completely under control.
The yaw can be expected to happen very suddenly in this situation and you need to act very quickly. In many cases, you will go off the runway before you can react quickly enough. The worst thing that could happen is to exceed V1 while moving laterally on the ground.
Engine Failure Speed
Between VMCG and V1, we also have VEF, which is "engine failure speed". This one is a tiny window of time, always one second or less, during which you actually notice that you're past VMCG and below V1 and you have an engine failure. That's your one second to react before reaching V1. Neither VMCG or VEF need to be called out during the takeoff roll.
Why you need to know the VEF for your aircraft is because you must ensure that V1 is higher than VEF, and the amount it must be higher by is different depending on whether the runway is wet or dry (due to decreased braking in wet conditions). Fortunately your airline's reference charts take all this into account for you.
Now we come the V1. This is your "decision speed". Once you are past this speed, you no longer really have a realistic chance to abort the takeoff, because you won't be able to stop the aircraft on the ground without crashing, and crashing on the ground is still very dangerous. V1 is the first speed PNF should call, this alerts the PF that you've reached V1.
If you have an engine failure before or exactly at V1, you can try to abort the takeoff. As the PF you will have to make a very quick decision at V1 whether you will continue the takeoff or not. As you have a single engine failure on a 2 engine aircraft, the yaw will be significant. We'll talk more about single engine takeoff in another article.
When you add V1 in the MCDU PERF page, the PFD will be updated with a "1" symbol in cyan, as in the illustration below, marking the position of V1 on the speed tape.
Drawing: Eric Bradley [© 2015]
There's another not-well-known speed in the mix, which is the "maximum unstick speed", written as VMU. This is technically a speed where you can takeoff even if you create a minor tailstrike when pitching up. While it's not a speed you'll be particularly interested in, here's a video that does a sweet job of demonstrating VMU, and if you'd like to see how engineers determine VMU for the Airbus, here's another video. Watching pilots intentionally create a tailstrike may be an interesting experience for you.
The speed which actually matters is "rotation speed", written as VR, and this another called speed. The task for the PF is to pitch up smoothly on reaching this speed (note, some fighter aircraft and light high-powered aircraft don't require any physical input past V1 to become airborne).
VR is a speed where there is at least enough velocity already gained that V2 will still be attainable 35 feet from the ground, even if an engine fails during lift-off. The actual speed at 35 feet will therefore be greater than V2 because normally both engines are working when you reach that altitude.
Minimum Speed for Control in the Air
Next we come to VMCA, which as you can probably guess is the minimum speed for control in the air. The same basic idea applies as for VMCG, except in this case it is that you must be able to maintain a particular heading through the air using the rudder to counter asymmetric thrust. The VMCA is not a called speed.
Takeoff Safety Speed
Then we have "takeoff safety speed", or V2. This is the minimum speed that needs to be maintained until you reach acceleration altitude (so that means you can go faster than V2 but not slower). Of course in the real world, you can only do what the plane allows, so if it doesn't have the power to maintain V2 while climbing, you'll have to improvise.
When you add V2 in the MCDU PERF page, the PFD will be updated with a magenta triangle, as in the illustration below, marking the position of V2 on the speed tape.
Drawing: Eric Bradley [© 2015]
With both engines operating normally, the usual recommendation is V2+10 or even V2+15 (this means 10 or 15 knots above V2). When there is not much power available from the engines due to failure or malfunction, you may be able to maintain V2 by reducing pitch below the optimal rate of climb. Where it gets tricky is when there are obstacles such as buildings, hills, and trees that need to be cleared. Under normal conditions, it's not difficult to do that, but with under-powered engines, a lot of weight, and bad weather conditions, you could be in for a real fight maintaining V2 with a shallow pitch and those obstacles to worry about. Not that I'm trying to scare you or anything!
Something else you should know is that although V2 is considered an "engine out" speed, if you've already reached a speed higher than V2 when an engine failure occurs, you don't need to slow down to V2. Just keep going at the higher speed as long as you can, so you don't interrupt your climb. At this point, altitude will give you time to go around or (maybe) get the failed engine started.
Engine Out vs Low Power (and what you can do)
Now, I would just like to say that in just about every reference and document dealing with matters affecting takeoff performance you will hear this talk about single engine operation and engine failure, but really that's not the only kind of problem you could encounter.
For the moment we'll not get into matters dealing with damage to flight controls or the aircraft body, but even in the case of engine thrust, it can be the case that both engines are running, but one or more of them may not be operating correctly.
If both engines are running but one has a problem, you'll notice a tendency for the aircraft to yaw due to asymmetric thrust. You can fix this by using the rudder or by reducing power on the engine that is performing better. Of the two options, during takeoff it is smarter to use rudder to correct the problem, because if the under-powered engine cuts out, you may stall, and there's no guarantee that you'll be able to get the other engine back up to full power again after reducing thrust.
If both engines are under-powered, then you'll have to wing it. In some cases it will be smarter to climb in a series of shallow steps in order to maintain speed while climbing, but that may not always be possible, especially if there's a lot of terrain. You could also do a circuit while climbing, so you could request to hold at some area and circle around it (obviously you want all traffic clear of the area that you'll be doing this) doing shallow climbs until you have enough altitude to do a proper go-around. This is not normal procedure, but I am talking about an extreme situation where you are barely able to maintain V2.
A related problem to the engine-out problem—possibly we could consider it the opposite of that problem—is where the rudder is stuck in one direction. You can obviously fix this by reducing power on one engine to create asymmetric thrust in the direction opposite the undesired yaw.