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I was recently on a flight between LGB and PHX. I've been flying since I was 5, am now 31. After takeoff, as the plane approached cruising altitude, the plane slowed down. A lot. Then accelerated again, followed by another slowing down. This happened at least 4-5 times during the short flight, and before landing we made a complete circle around the airport, I'm assuming to comply with which runway to land on. My question is: why, or what cause, does a plane slow down in mid flight where you move forward in your seat, like hitting the brakes in your car too hard multiple times. I understand training and all that, I want to know the cause. To put it in perspective, I assumed the engines stopped. And to increase the nausea, we were delayed by a mechanical issue for an hour.

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    $\begingroup$ Are you sure that the plane slowed down? Your inner ear and your body can only feel acceleration, they cannot tell where that acceleration comes from, it will always be the sum of all accelerating forces acting on your body. For example: if the plane accelerates with 1g, you will be pressed into the back of the seat with your entire body weight. If the plane stands on its tail without moving, you will actually be accelerated downwards by gravity with 1g, i.e. you will be pressed into the back of the seat with your entire body weight. The sensation will be exactly identical, you cannot … $\endgroup$ – Jörg W Mittag Apr 1 '18 at 9:22
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    $\begingroup$ … tell the difference without having a reference to the attitude of the plane with respect to the gravitational field. Likewise, the sensation of decelerating is the same as the sensation of pitching down: you fall forward in your seat. Unless you can see the horizon, you will not know the difference. Note that even trained pilots typically lose orientation after less than 15 seconds after losing visibility, unless they are trained to not trust their body and ignore every cue and just look at the instruments. They can be upside down plummetting to the ground and swearing they are just in a … $\endgroup$ – Jörg W Mittag Apr 1 '18 at 9:22
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    $\begingroup$ An aircraft can't decelerate that rapidly in flight. Spoilers aren't very effective as the disk brakes on the wheels, and reverse thrust, which are both used on the ground. Someone should be able to quantify the typical max rate of deceleration in cruise, but I can't see it being very high. So, it was probably more a sensation of decelerating, which maybe caused by pointing the nose down, than a real deceleration. If you have ever watched an aircraft simulator, it tips forward and backwards, but inside, you think you are accelerating and decelerating. $\endgroup$ – Penguin Apr 1 '18 at 9:46
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    $\begingroup$ "I assumed the engines stopped" If the (all) engines ever do stop in mid-flight on an airliner you're on, then trust me, you'll know. $\endgroup$ – a CVn Apr 1 '18 at 10:28
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Without knowing the specifics of the flight, there could be a lot of reasons for this.

One guess for the sensation of accelerating and decelerating We’re intermittent steps at fixes on the departure procedure the pilot flew out of KLGB at certain altitudes and airspeed either as part of the DP or as assigned by ATC in order to facilitate departure from SoCal airspace to enroute. That and a little sloppy airmanship on the part of the flight crew.

As for the “circling” at KPHX, most likely that was the pilot flying a STAR with ATC final vectoring to a visual approach or SIAP for runways 25L/25R/26 at Sky Harbor Intl.

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