RFT 485: Takeoff Climb Gradients
Image source: FAA-H-8083
From AOPA:
In the airline world, there are a number of new rules, limits, and terms a pilot needs to learn. One area in which a new understanding needs to be had is in the takeoff.
Gone are the days when, as a general aviation pilot, you can just eyeball the runway, the load, the airplane, measure the wind with your thumb, and go for it. When you are flying passengers and cargo for hire, you need to be able to comply with the segmented climb. Specifically—-and this is key—-you need to be able to meet the climb requirements on a single engine (assuming you are flying a twin-engine jet) as a result of an engine failure at V1 [takeoff decision speed, but a beyond the scope of this post]. It is assumed that you will meet all the requirements if every engine is running.
The first segment is short—it ends when the airplane is airborne and the gear is retracted. Not partially retracted, but fully up-and-locked retracted. The airspeed must be up to V2, commonly known as “takeoff safety speed,” but in technical terms, the speed for best climb gradient.
The second segment requirement is often the most difficult one to meet. Segment two begins when the gear is up and locked and the speed is V2. This segment has the steepest climb gradient: 2.4 percent. This equates to a ballpark figure of around 300 feet per minute, and for a heavy airplane on a hot day with a failed engine, this can be a challenge. Often, when the airlines announce that a flight is weight-limited on hot summer days, this is the reason (the gate agent doesn’t know this kind of detail, and nor does she care; she just knows some people aren’t going).
The magic computers we use for computing performance data figure all this out, saving us the trouble of using charts and graphs. All we know is that we can either carry the planned load or we can’t.
Second segment climb ends at 400 feet, so it could take up to a minute or more to fly this segment. Think of all the obstacles that might be in the departure path in the course of 60 seconds or more.
Third segment climb begins at 400 feet, and here the rules can vary. The climb gradient is now half of what it was before: 1.2 percent. However, we are also required to accelerate to a speed called VFS (final segment climb speed). In graphs and publications, the third segment of the climb is often depicted as being a flat line for the acceleration. In many turboprops, that’s exactly the way it’s flown. The airplane is leveled off (and the pilot is using a very tired leg to overcome the increasing yaw tendency via the rudder) and accelerated before the final climb begins.
In jets, however, there is generally enough power in the remaining engine to avoid a level-off. If the airplane can continue to accelerate during the third segment, it may continue to climb, so long as it can do so without a decrease in speed or performance. In fact, during the climb it must continue to meet the climb gradient while accelerating to VFS.
Third segment climb ends upon reaching VFS.
The fourth and “final segment” begins upon reaching VFS and completing the climb configuration process. It is now permissible (and maybe necessary) to reduce thrust to a Maximum Continuous setting. The climb gradient is again 1.2 percent, and VFS must be maintained to 1,500 feet above field elevation.
V1 cuts and single-engine climbs are a staple of turboprop and jet training. It is critical that a pilot of such equipment understand what the objective is when it comes to performing the maneuver, and why the requirements are what they are. This material is taught in much greater detail in ground school than I presented here. In fact, there may be a few deviations and exceptions to the above, as this is a general introduction (there are, like many things in aviation, always caveats, so bear that in mind).
Some pilots dread V1 cuts, but the best way to approach them is to take them as a challenge and constantly push yourself to master them and excel in your performance.