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How to not speed up whilst descending?

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How to not speed up whilst descending?

Postby Flight876 » Fri May 05, 2017 11:08 am

Hey guys,
When I tried to descend in the MD-11, I kept on speeding up. I have no idea how other pilots do it, but how can I descend without speeding up? I can't quite figure that out.
Any help would be greatly appreciated.
Thanks :)
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Re: How to not speed up whilst descending?

Postby Bomber » Fri May 05, 2017 11:52 am

Put your engines to idle.
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Re: How to not speed up whilst descending?

Postby sp-lcz » Fri May 05, 2017 12:01 pm

It depends on your vertical speed. Typically I descend with 1800 ft/min. You can use your speed breaks but if I good remember it is not necessary with this speed limit...

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Re: How to not speed up whilst descending?

Postby Johan G » Sun May 07, 2017 11:35 am

Bomber is right.

This basically boils down to energy management. In essence, if you keep the power setting (and configuration) you will always trade kinetic energy (airspeed) for potential energy (altitude) and vice versa.

If you keep the same power setting (and configuration) and descend by pointing your nose down you will invariably speed up. I you keep the power setting and point the nose up you will loose speed.

If you on the other hand change the power setting you will change the total energy without changing the airspeed.

If you play around with this you will see that this only hold true in the longer term (several minutes), but that the short term reaction (a few minutes) from the aircraft is the opposite and more intuitive (pitch up and gain altitude, add power and increase airspeed). Often this is called the primary effects (long term) and secondary effects (short term) of the rudder and power controls.

This can be a real killer on the final approach where the total energy is rather small (both low airspeed and altitude) as pitching up to gain altitude can quickly drop off enough airspeed to stall or on the final turn stall and spin.
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