My previous post had a 10-second video of a takeoff, and asked what happens right at the end of the video. A bit of a loaded question it was…
Note I will be talking about transport-category aircraft in this series. Those that are certified under FAR Part 25, or EASA CS-25. I’ll use the US regulations, but EASA’s are very similar.
The Answer
Here’s my answer. And another question, further down.
The gear having fully retracted and the doors closed, the aircraft reached the end of what is called the first segment in Aircraft Flight Manuals.
Some details about the first segment. Not hugely important.
Is this information about the first segment important? Well, in the grand scheme of all things Obstacle, not that much. But what is important is that you be able to answer the question down at the bottom of this post. And it might help with that.
You’ll have heard the term. Pilots tend to also remember that the first segment starts at lift-off, and that for two-engine aircraft, the climb gradient must only be positive *. And then try to forget what seems like unclear and worrisome knowledge.
In practical terms, that blue line in the diagram above has to be going up after lift-off and until the wheels are in the well. Though it doesn’t have to go up much.
Fictitious example: that twinjet of yours can do a 0.5% first-segment climb gradient. You’re taking off from a 5000-foot runway, lift off at the 2000-foot mark. Well, you’re gonna cross the departure end of the runway at 15 feet.
The folks who certified the airplane were happy with this. Are you? Probably not. There’s a different bunch of folks who design departure procedures for airports, and they have a completely different set of hurdles they want airplanes to overcome when taking off. And we have a vital interest in overcoming these.
The first requirement is usually that you’ve got to cross the departure end of the runway at a minimum of 35 feet. There’s more hurdles, but let’s do one thing at a time. Here’s the diagram again, with the designer’s first hurdle drawn in red.
The next question
So here’s the next question I’ll leave you to mull over for a while.
What did you do, before your most recent takeoff, to ensure that you could indeed make that first hurdle at 35 feet?
Allow me to highly doubt you pulled up your first-segment climb gradient charts, regular pilots don’t regularly do that.
Yet I’m sure you did check. How’d you do it?
My answer tomorrow.
- For three-engined aircraft the requirement is a bit better, the gradient must be a still-paltry 0.3%, and 0.5% if you have four engines. Technically, the first segment really starts when the aircraft reaches VLOF. Read up on it here, § 25.121 specifically.