Let's say you need greater descent rates than you can get with the full use of the speed brake in an airliner. Does it help to use the above flap positions in getting greater descent rates?
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asked Feb 3, 2018 at 14:27
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2$\begingroup$ General operational rule, as my training colleagues used to say: „Flaps are not speed brakes“. Still, more drag means steeper descend gradient capability. What scenario(s) are you after? $\endgroup$– Cpt ReynoldsCommented Feb 3, 2018 at 20:04
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$\begingroup$ The scenario I had in mind was you need a greater descent rate to meet an altitude restriction for STAR when you were belatedly instructed by ATC to continue descent after a level-off. $\endgroup$– lemoninciderCommented Feb 4, 2018 at 0:03
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2$\begingroup$ @lemonincider While one should make every reasonable effort to comply with controller instructions, doing show should not entice you to exceed limitations or, for that matter, your own comfort level. That said, if you're comfortable with it and below max gear extension speed, you could consider doing that. You can always tell ATC you're unable, and you can always suggest an alternative, like maybe a 360. Also, you can be pro-active. If you think the controller is behind the curve, you can request lower. Don't be afraid to negotiate. $\endgroup$– TerryCommented Feb 4, 2018 at 1:07
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1$\begingroup$ In your scenario, deploying flaps to steepen descent might be technically possible, but is operationally unfeasible for most types, as on the STAR most often the speed is kept high enough to fly clean for most aircraft types. This means that for most types, at best, slats can be used (they come out before the flaps and withstand higher speeds), and they won’t make much difference. Notifying/negotiating with ATC to vary speed and/or course are the methods of choice in your scenario, if speed brakes don’t cut it. $\endgroup$– Cpt ReynoldsCommented Feb 4, 2018 at 22:05
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2 Answers
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It depends what you intend to do with the speed. If all you are looking for is absolute descent rate: a high-speed, spoilers out and high speed dive will get you the best rate.
What you are usually interested in a normal approach however is not descent rate (vertical speed) as much as descent angle. In this respect, all/any flaps will decrease the L/D ratio, which is going to increase you descent angle. Each stage of flaps will have increasingly higher descent angles, but mind you at a lower speed.
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$\begingroup$ Flaps improve lift and pitch angle, but they are not used to control descent angle on approach. $\endgroup$ Commented Feb 3, 2018 at 19:57
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1$\begingroup$ uhm... Granted that’s not their main purpose, but actually they can be used exactelly for that.(I do it almost daily). Be carefull when saying ‘flaps improve lift’, that can trick you into the wrong conclusions. All flaps will add more drag for the lift they provide. Since glide ratio is basically L/D , you glide angle will always be steeper with flaps. So when high on approach, slow down and lower some flaps... you will be surprised ! $\endgroup$– Radu094Commented Feb 4, 2018 at 9:01
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As @Radu094 said, a spoilers out, high speed dive will generally get you the highest rate of descent. For that max rate of descent you would put your airspeed right up against the limit.
Using the 747-100/200 as an example, that max airspeed is 350 knots as I remember, and you don't have the option of also using flaps because operation of the speed brakes with any degree of flaps is prohibited.
Even if the speed brakes prohibition wasn't there, the max speed on a -200 for flaps 1, 5, 10 is 275, 250, 238 respectively, considerably lower than the 350 knots for the max rate of descent with a clean wing.
You could extend the gear. Speed brake usage with the gear extended is allowed, and the max gear-extended speed is 320 kn. Though 30 kn slower than than the max speed without the gear, all that drag allows you to put the nose down further. I can't remember how the rate of descent in this configuration compares with gear-up 350 kn, but you'll have a much steeper angle (and a lot more noise), and you really come out of the sky.
There's a trade-off though. While the max gear-extended speed is 320 kn, the max gear-operation speed is 270, so you have to first slow down before using the 320 kn speed.
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$\begingroup$ That and all you really risk in extending the gear past 320kts is damage to the fairing doors. I guess that’s a risk which may be appropriate, depending on how dire the emergency is. $\endgroup$ Commented Feb 3, 2018 at 19:28
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$\begingroup$ I've also heard using flaps and speed brakes together is not recommended but couldn't come up with a reasonable explanation. If you don't mind, could you explain why the prohibition is there? Is it a manufacturer's limitation or just a common op specs rule? $\endgroup$ Commented Feb 3, 2018 at 23:50
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$\begingroup$ @lemonincider It's a design limitation determined by the manufacturer. You could design for higher flap extension speeds, but that would mean increasing the weight of the structure to be able to handle the higher loads, and the size of the flap extension motors to move the flaps against the higher airspeeds. The design goal for flaps should include getting the weight as low as possible without incurring normal operational difficulties. $\endgroup$– TerryCommented Feb 4, 2018 at 0:55
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1$\begingroup$ @lemonincider I'd suggest asking that as a separate question in those words. I'm not an aerodynamicist, but I suspect the answer lays in the fact that their use together produces undesirable flight characteristics. I once saw a captain go to approach flaps while having forgotten he had the speed brakes out. He was capturing a glideslope from above. Bringing the power up did not stop our sink, and adding much more power and bringing the nose up more got us the stall warning clacker at a much higher speed than one would expect. Stowing the speed brake stopped the sink and the clacker. $\endgroup$– TerryCommented Feb 4, 2018 at 6:43
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1$\begingroup$ @lemonincider - see: Why is deploying speed brakes not recommended with the flaps extended? $\endgroup$– user14897Commented Feb 4, 2018 at 6:59