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Often when approaching landing, I feel the aircraft slow down considerably, it hasn't landed yet, but kind of hovers around waiting for clearance to land. Thus I was wondering, is there any defined minimum cruise speed for airliners.

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It's called the stall speed. Below which the wings won't give enough lift to stay airborne.

Another minimal speed to adhere to is the minimal control speed. Below that the controls surfaces won't be able to counteract suddenly losing an engine.

However slowing down before landing has nothing to do with waiting on their turn/clearance. If they need to wait they go around in a circle instead.

The slowing down is so they touch down with as low speed as possible to limit the braking distance needed on the runway.

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The lift a wing is able to generate is proportional to square of speed¹. If the aircraft is moving through the air too slowly, it will stall², pitch down and start to descend rapidly.

So a fixed-wing aircraft³ must move at least some speed, called stall speed or $V_S$. And normally some margin error should be left, so the aircraft never flies slower than about $1.3×V_S$.

What the $V_S$ is depends on the aircraft and the weight. Because lift is needed to balance weight, less weight means less lift is needed and thus less air flow to generate it. For jet airliners the stall speeds may range from around 100 knots when light (~185 km/h, ~115 mph) to maybe 130 knots (~240 km/h, ~150 mph) when loaded⁴.

On the other hand, at altitude below 10,000 ft, maximum speed of 250 knots (~463 km/h, ~288 mph) is usually defined so the pilots have enough time to see each other when flying near the airports under visual flight rules or in case the controller makes a mistake.


¹ It is actually proportional to dynamic pressure, which is proportional to pressure and square of speed, so as the aircraft climbs, the minimum speed increases. Modern airliners fly very high (usually 32,000 to 42,000 ft) where the lower pressure and corresponding lower drag allows flying much faster, but the minimum speed is also higher.

² Lift is proportional to dynamic pressure and angle of attack. At slower speed the wing flies at higher angle of attack, which is how the nose of the aircraft still points up when it is approaching to landing. The wing is able to provide more lift up to critical angle of attack above which it stalls.

³ Rotorcraft (helicopters) create the air flow by rotating their wings instead and therefore don't need any forward speed. In fact they stall when flying too fast instead when the retreating blade is no longer moving aft fast enough.

⁴ These are values for high lift devices (flaps and slats) deployed. With them retracted, the stall speed is higher. The stall speed also depends on the relative wing area of particular aircraft. For example A318, A319, A320 and A321-200 all have the same wing and therefore the same stall speed at the same weight, but they have different size and therefore different typical weights. There is similar difference between say B737-700, B737-800 and B737-900.

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    $\begingroup$ Very nice! I like the simple answer with the footnotes to indicate that you've covered all your bases. $\endgroup$ – FreeMan Sep 1 '15 at 13:29
  • $\begingroup$ A stall won't necessarily result in a nose-down attitude (and of course, recovery usually needs to begin with the pilot pitching the aircraft down in addition to adding power for speed, then pulling back up). Take AF447 as just one well-known example; Koyovis' answer to On Air France 447, what would have been the lowest altitude to initiate recovery after the stall developed? quotes the accident investigation report as specifying a vertical drop of 10912 ft/min and a 16.2 degrees nose-up pitch attitude as the last values recorded by the FDR. $\endgroup$ – a CVn Apr 15 '18 at 11:44
  • $\begingroup$ @MichaelKjörling, well, stall will always create a large pitch-down moment. The pilot may, however, have just enough elevator authority and keep the plane stalled. On stable aircraft with normal controls, the large force needed to keep the nose up can be considered a telltale sign of speed decay, but some pilots still managed to stall their planes this way, not realizing what was going on (I've read about at least two such accidents, but don't remember enough to find them now). On AF447 the Airbus alternate law made things worse by auto-trimming all the way nose up as the speed decayed. $\endgroup$ – Jan Hudec Apr 15 '18 at 19:40
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The minimum speed at which the aircraft can fly (cruise) is called the stall speed. At this speed, the lift is equal to the aircraft weight.

The lift L is given by,

$L = \frac{1}{2}C_{L} \rho S V^{2}$,

where,

$C_{L}$ is the lift coefficient, $\rho$ is the density at that altitude and $S$ is the wing planform area. During level, unaccelerated flight, the lift is equal to weight.

$L = W = \frac{1}{2}C_{L} \rho S V^{2}$

This gives the stalling speed as,

$V_{s} = \sqrt{\frac{2 W}{\rho C_{L_{max}} S}}$

This is the minimum speed at which the aircraft can fly. Note that it depends on three things: the Weight of the aircraft, the density of the air and the $C_{L_{max}}$ of the wing section.

During landing, two things happen: the weight is lesser (compared to takeoff weight), and the flaps are deployed, which increases the $C_{L_{max}}$, which in turn reduces the minimum speed required for flight. This is the reason you're feeling a reduction in speed.

Boeing 777 Flaps
Source: assets.decodedscience.com

Boeing 747-200 Lift
Source: Stall Speed, Prof. Dr. Mustafa Cavcar

In general, aircraft are flown well over the stall speed. If the margin (between flight speed and stall speed) is maintained, then your flight speed will reduce when the stall speed is reduced.

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Arguably, an aircraft can cruise at any speed as long is it does not stall. However, sometimes, ATC will notify a flight crew and tell them to maintain a certain airspeed due to traffic or other factors. Keep in mind, stalling is not based on speed as much as it is based on AOA (angle of attack). Angle of attack is the angle that the air meets the wing (in case you did not know). Stalling is when the airflow over a wing becomes broken and turbulent or separates from the wing. To a certain point, a higher angle of attack will grant more lift (that is the point of flaps). However, it can be dangerous since it can also induce a stall.

https://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-StallFormation.svg.png

The above picture displays what I am trying to say. So as long as critical angle of attack is not reached, an aircraft can technically fly at any speed upon landing (although ATC will often request airplanes to keep airspeed relatively high or low so traffic runs smoothly). The book, "Stick and Rudder" has an enormous section on AOA, among many other things.

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