Flight Question

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networkn

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#225415 17-Nov-2017 17:00

I keep meaning to ask someone. Almost always when I fly, which I have done a lot over the years and happened on my flight from CHCH last night, the plane feel like it surges forward for a few seconds, and the engines make a lot of noise. Mid flight or sometimes right before they are landing, on final approach. Doesn't seem long enough to make a material difference to not being fast enough for approach etc.

Was wondering if anyone else noticed and knows why?

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#1903353 17-Nov-2017 17:00

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Dingbatt

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#1903364 17-Nov-2017 17:25

An A320 by any chance?
If the requested speed and the actual speed are too far apart or are diverging, the auto-thrust system aggressively applies power to recover. Effectively 'revving up' and then slowing again. This may happen when the change is as little as 10km/h so as soon as the increase is felt, it is over.
In cruising flight it may be due to the areoplane being asked to speed up by air traffic control. On approach, it may be due to the aircraft reaching its approach speed for its particular configuration of flaps and landing gear. The engines 'rev up' to maintain that speed. In a Boeing the throttles move under the control of the auto throttle system and can be overpowered by the pilots, so they can reduce the movement if it isn't appropriate. In the Airbus the levers do not move with auto thrust changes and so it is more difficult for the pilots to prevent the surge.

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Oblivian

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#1903370 17-Nov-2017 17:41

Also to compensate for the airspeed decrease and flaps being on high attack

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#1903617 18-Nov-2017 11:28

Your sense of motion is not to be trusted when in an aircraft. For example, you shouldn't feel any lateral movement when you're turning. The floor of the aircraft will still feel "down" but if you look out the window the horizon will move up or down. Similarly, slowing down may actually feel like you're dropping, even though the aircraft is flying straight & level. So you may not actually be surging forward. More likely, the nose of the aircraft is pitching up slightly, so that gravity becomes slightly aft of what you believe to be "down". You perceive this as a forward acceleration because the seatback presses against your back a little more.

The objective is to be descending at a specific rate whilst maintaining a specific airspeed *and* being on a line that leads to the right touchdown point on the runway. It's a complex juggling act where airspeed, aircraft nose up/down pitch, thrust, lift, and drag all interact, so changing one setting affects multiple things. In addition there's wind speed, which varies with altitude and gusts. So the pilot (human or robot) is continually adjusting thrust and pitch to maintain the desired profile.

Probably what's happening is that a little extra gust of headwind or a downdraft is pushing the aircraft below/behind the glideslope line. The pilot (perhaps a robot, perhaps human) responds by advancing the throttles and pulling back a little on the stick. Increasing power alone would move back to the glideslope, but increase airspeed. Pulling back on the stick alone would move the aircraft towards the glideslope, but at the cost of a decrease in airspeed. So, if airspeed is right, you need to do both.

You feel the stick-pull/pitch-up as a "surge in speed", and hear the engine noise increase.

Batman

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#1903653 18-Nov-2017 12:54

It's to adjust the plane's speed and altitude. You don't want to touch the ground a hundred metres too short or too long. It's not a helicopter that can come down vertically if you overshoot or go up vertically if you undershoot, it's a diagonal line that cannot be too steep (impact too hard) or too shallow (too fast).

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#1903658 18-Nov-2017 13:21

Batman: It's to adjust the plane's speed and altitude. You don't want to touch the ground a hundred metres too short or too long. It's not a helicopter that can come down vertically if you overshoot or go up vertically if you undershoot, it's a diagonal line that cannot be too steep (impact too hard) or too shallow (too fast).

Really?

Geektastic

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#1904012 19-Nov-2017 13:14

Batman: It's to adjust the plane's speed and altitude. You don't want to touch the ground a hundred metres too short or too long. It's not a helicopter that can come down vertically if you overshoot or go up vertically if you undershoot, it's a diagonal line that cannot be too steep (impact too hard) or too shallow (too fast).

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#1904016 19-Nov-2017 13:27

Speed brakes. This are applied to slow the aircraft, when they are retracted, it feels like a surge forward.All to attain the correct air speed on approach. Not used that often but if the wind conditions are variable and the aiircraft doesnt bleed knots as expected, a quick way to adjust the speed