Do airline pilots use this high-speed, high-drag technique for losing altitude?

Question

This question and another more recent one got me thinking about this.

Years ago on my MEII checkride the DPE showed me a technique for losing altitude on final when the aircraft was high. Typically, I would reduce power, increase drag (flaps, landing gear, etc), hold nose up to lose speed, and sink like a rock (if necessary slip). However, what he showed me kind of blew my mind. I rarely do it, except when just having fun.

He said, "Do you remember what happens when airspeed doubles?" I said, "Yes, drag quadruples." More precisely the parasite drag quadruples. Drag $= pv^2$, where $p$ is some constant representing parasite drag and $v$ is velocity.

So, we set up high on final, and he dropped the flaps and descended at $v_{FE}$. I was surprised to find that the maneuver used up a lot of the aircraft's energy, and the aircraft was brought down to the glide path and slowed down with plenty of time for a normal landing.

My DPE was a Captain for Southwest back when Southwest was a small airline. My question is whether this is a technique used by airline captains nowadays?

I don't have access to a B737 to try this out, but in a small aircraft like a Cessna 152 you have a $v_{s_{0}}$ of about 35 and $v_{FE}$ of 85.

The idea is you can descend with full flaps at 42.5 knots and power off and it will sink slowly but in a somewhat nose up attitude. Or, you can put the nose down and speed up to 85 knots and quadruple the drag and get on your glide path sooner so that the last bit of the landing is at a proper glide path, attitude, and speed.

Answer 1

Airline pilots certainly do use gear/flaps/spoilers to descend more quickly if needed. However, SOP typically requires an approach to be stable upon reaching a certain altitude on the approach. This means having speed and airplane configuration set for landing, and being lined up with the runway and on the glideslope. Unstable approaches are more likely to result in landing mishaps.

Some situations such as a runway change may require late changes to the approach. It is up to the crew whether they can safely make this change or if a go-around is the better option. S-turns are another way to lose further altitude.

When Air Canada Flight 143 ran out of fuel, the pilot used his glider experience to perform a forward slip to lose altitude. These sorts of maneuvers would be reserved for an emergency on an airliner.

Answer 2

I actually got asked a similar question on a checkride two weeks ago. Right after I was asked to demonstrate an emergency descent through a hole. I heard the talk about the advantages of the substantially increased drag of the Fowler flap, and was quizzed about impact of drag with higher speeds. Meanwhile I extended full flaps, and slowed the plane. Midway through the hole, I was "corrected" that the proper way would be to fly closer to Vfe. I disagreed and pointed out that at a slower speed, in a 60 degree bank, our turn radius was smaller, and we would remain VFR within the hole.

But to answer your question, the procedure depends upon what airline, and what country. Two US carrier captains say that if the procedure isn't in their books, then they can't do it, except in an emergency. My take is that configuration changes take time, have risk, and flying slow or fast have risks. So for one US carrier, the answer from two of their senior pilots is, no we do not do that.

As a final note, pilots who tow gliders and fly in drop zones are motivated to get down quickly. Most will dirty the plane, close the cowl flaps, pull back on the power, and roll in lotsa bank. I do the 60 degree turns with enough airspeed to keep me away from an accelerated stall.

Congrats on your checkride. ps: The two guys I was talking with about this do not fly for Southwest.

Answer 3

It's hard to get above Vfe in a Cessna with full flaps once you've slowed down. At least in the lightweight 152s and 172s I used to fly. You could go to 45 deg nose down easily. But as one reply said, that's hard on the engine.

I guess I never tried it at full power, but I got used used to making pretty much all "major" descents with full flaps so I could keep partial power to prevent shock cooling the engine.

Answer 4

• G protection on a clean jet ( no flaps/ slats) is HIGHER
• typically there’s no negative G protection with flaps/slats extended
• Also typical tear and wear of the flaps/slats mechanism is higher when operated near their operating limits ( speed)
• F/S usage has limitations ( Max alt, VFe)

Keeping the above in mind, the "normal " procedure would be to use speedbrakes to increase the rate of descent rather than lowering the flaps / slats.

This being said, some line pilot elect to lower the F/S instead of using speedbrakes, when the conditions allow ( smooth air, speed and altitude ) They do this mostly: - for comfort (to prevent the typical buffeting created by the speedbrakes) - in anticipation of a further speed reduction imposed by ATC

NOTE : Another way to achieve a higher rate of descent, without compromising the G protection would be to lower the gear. But this is very noisy, and limited ( typical max speed for lowering the landing gear is 250 knts) and altitude ( usually max FL200)

Using gear & F/S to achieve a higher rate of descent is used to get established on the final glide path when intercepting from above. This must be must carefully planned. Indeed any low level flight segment in landing configuration is very penalising in terms of fuel burn

Answer 5

It sure will descend faster in a high drag config in a dirty config in a high speed glide. Think of it this way: you’re depleting the energy state of the aircraft much faster. An airplane a energy state is the sum of its kinetic energy ie how fast the groundspeed is and potential energy ie how high it is above the ground. As you pointed out as speed increases, drag increases as the square of the speed, and energy is a product of the force applied to an airplane over a given distance, this means you are draining four times as much energy from the A/C’s energy state at twice the groundspeed. If airspeed is maintained ie pitching for a fast glide without additional power, the energy loss is coming from the potential energy reserves. As potential energy is directly proportional to altitude, you are going to lose 4x as much altitude descending at twice the given airspeed.

I’ve flown DA-42s and DA-62s, both of which allow the pilot to lower the gear as high as Vne. For emergency descents in that plane, you simply roll into a 30-45° turn and drop the gear, thence pitch for a speed as high as Vne (typically just short of Vno for simulated emergency descents). You will get down faster in a high drag state than a low drag state for a given airspeed.

As for doing that in a jetliner with the paying passenger load, well it is always an option in emergency, it would generally be considered a faux pas under normal conditions. Remember the idea is to keep the passengers comfortable and not spooked or frightened. A far superior solution to the problem would be flight management so that you arrive at certain way points at certain airspeeds and in certain configurations which minimize steep descents, rapid deceleration, etc.