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Plane Answers: Sudden acceleration on landing and lining up on final approach
Welcome to Gadling's feature, Plane Answers, where our resident airline pilot, Kent Wien, answers your questions about everything from takeoff to touchdown and beyond. Have a question of your own? Ask away!Darren from Engadget asks:
Hey Kent!
Just got back from a LIR -> ATL -> RDU trip, and I thought of a question. When the plane (we were on a 737) is just seconds from touching down on the runway, it feels & sounds like the plane suddenly accelerates. For a few minutes leading to landing, it feels like we're slowing down, and right before the rubber hits the road, there's like a small burst in speed, followed by touch down and then massive wind as I assume the engines are thrown in reverse to stop us.
So, what's that acceleration just before touch down for? Or am I dreaming? Thanks!
Hi Darren,
Nope, I'm sure you experienced this.
Occasionally if a pilot is a bit slow (say 2 or 3 knots below your 'target speed') they can add a small boost of power in the flare to cushion the inevitable thump of a landing. But it's really not a very good technique to use regularly.
And some pilots don't just use it when they're a tad slow, but they use it as a substitute for a finessed flare on every landing.
It can lead to a very 'flat' and fast landing. Touching down like this eats up a lot of runway unnecessarily and puts more wear on the brakes and tires.
You'll hear in this takeoff and landing video the instructor repeat "flare and squeeze" to the captain as he's about 30 feet over the runway. He's telling him to start his flare, or round out the glide path angle to allow for a smoother touchdown and to "squeeze" or pull the power back to idle before touching down.
And you're right, the noise you're hearing after landing comes from the reverse thrust mechanism which is simply a set of 'blocking doors' that divert the thrust out the sides of the cowl and forward, angled away from the engines.
Ainsley asks:
How important is a reference point in lining up for landing?
Hi Ainsley,
If the weather is clear, we are often able to make up our own final approach to landing in, for example, the Caribbean. At densely populated areas and in the weather (flying on instruments) we fly an approach that usually has a straight-in segment of about 10 miles. We're almost never turned in any closer than 3 miles out.
As you can see from the video linked in Darren's question above, while on the final approach it's easy to tell if you're lined up correctly with the runway.
In San Juan, Puerto Rico, we line up for one runway (10) and in the last 300 feet make a turn to another runway (8) for landing. It's a 'charted' visual approach procedure that's rather fun.
The approach is similar to the ending of this video I took while flying in a small Diamond DA-20 airplane:
Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he'll use it for next Monday's Plane Answers. Check out his other blog, Cockpit Chronicles and travel along with him at work.
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Reader Comments (Page 1 of 1)
Allan Jun 29th 2009 5:35PM
Approximately How many mph. does an average commercial jet roll on the runway before lifting off.
Arrey Jun 30th 2009 11:27AM
Allan it really depends on a lot of factors; what kind of aircraft it is, how much it ways, what the weather condition is like, etc. I found this link I think you might find interesting. http://www.aerospaceweb.org/question/performance/q0088.shtml
Sak (SJU) Jun 29th 2009 6:39PM
Kent, I was reading with interest your second answer because years ago I remembered pilots using the Levittown water tank as a reference and I wondered if they still use it. To my surprise you used the SJU approach and answered my question! I live very close to that water tank.
Many years ago, a 757 (I think) got confused and landed on runway 9 at the Ribas Dominicci airport which handles only small planes (5,000 ft runway vs. 10,000 in SJU). They had to unload the airplane for take off because the runway was really short. Someone said it was his first time.
Anyway, thanks for you answers and blog.
Kent Wien Jun 29th 2009 7:00PM
What a coincidence SAK.
They will often line us up even farther out, so that might be why you don't see us in a pylon turn over the Levittown water tank. We'll flash the lights at you next time!
Denny Dubay Jun 29th 2009 11:31PM
Another explanation for the feeling of last minute acceleration upon landing would be the reduced drag of the aircraft flying in ground effect.
BTW Ken, I love reading your blog and managed to read through your many entries in only two days! Keep up the great work!
Gordon Jun 30th 2009 3:19AM
Thanks for the link to the DA-20 video. I did all my training on Katanas -- beautiful airplanes! I'm not surprised you had so much fun flying them. :)
Nathan Jun 30th 2009 9:01AM
I recently flew into PGUM at night, and we lined up no more than a mile before touchdown. It was pitch dark outside, although the sky was also clear...is this usual?
zondaracer Jul 24th 2009 11:26PM
I believe that it is standard practice on the MD-80 series to increase throttle in the flare to cushion the landing. Since the aircraft is so low to the ground, increasing throttle instead of flaring on the yoke reduces the chance of a tailstrike.