CABLE vs FBW

[DC-ATE]

FlightDetent -

"I like (so far) the tone of the thread. My opinion is that it DID matter. Rods/pulleys have no check valves or sort. FBW may, as per individual design. Pilot error forcing A/C to go out of certified envelope is mitigable on FBW w/ protections.
Could Cali CFIT had been avoided through last-stop measure of some SW logic auto-retracting speedbrakes when TO/GA thrust is set?"


Indirectly, you are correct. But I suspect a pilot could make a mistake on a FBW aircraft as well with the same results. I don't have the Cali report.

[Dani]

Lemurian, of course automation is a biggy, and I agree with you about whatever you say. But the topic is not about automation, and you are wrong in the - not mentioned - assumption that all electrically wired aircraft are heavily automated.

As others said, the Embraer E-Jets are not "fly-by-wired" like the Airbusses. Also the Saab 2000 is not. Also the 777 is not. Also most aircraft with Fadec and power-by-wire are not. In fact, the only aircraft that is - in your sense - heavily automated is the Bus itself. And it is a good thing. I like it.

But the topic starter was arguing about missing mechanical links between the controls and the actuators. But also this has been identified as not accurate, not safer and not better anymore.

So I guess all has been said and done?

Dani

[Lemurian]

Dani
One could define automation as the existence of a computer between the pilot's command and the control, be it flight controls or engine or navigation...etc...
That definition -which is by no means complete - therefore includes everything that has been included in an aircraft cockpit or systems and takes the pilots away from a "direct" link to the desired information or command.
As for the FBW, whether one could define different levels of protection or not, and through what means, the fact is that a demand by the crew goes through a set of computing steps before the result goes to the concerned system. (forget the loops, for simplicity's sake)
The old A-vs-B argument only lies on definitions of "soft" vs "hard" protection, but it becomes more and more a dispute on semantics ( See the tail strike protection of the T7 for instance ).
Another dispute is about moving/not moving T/Ls. To that one I'd just comment that the BA T7 which managed a glide into Heathrow had both throttles firewalled, which somehow proves that were not indicative of the engines'output...
I'm not familiar with the architecture of the Embraer products but the Airbus philosophy has been adopted by Dassault and now by Bombardier on their C series...and studied closely by the Russians. (Could mean something...).
All the above tell me that the level of automation reached by the modern airliners seems quite wide-spread and a simple quick look at the incident/accident statistics will show that the level of safety the air transport industry has achieved through automation is on another planet altogether compared with the pre-jet era.
Another quick look at the accident statistics of jet airliners also show that the last of the classically flight-controlled airplanes, the DC-9/MD8xxx has a safety record that can't compare with the more modern ones in the same period...(Might say something, too...)
To illustrate my point and although I don't agree with all the arguments presented, this is a paper from the USAF about cockpit automation and its "costs"
Identifying And Mitigating The Risks Of Cockpit Automation

[Dani]

Completly agree, Lemurian, I'm the wrong guy to be beaten up.

I just want to tell you that there isn't an imperative incorporation of FBW and automation. To the contrary, "soft-fly-by-wired" aircraft are following a more traditional approach of control technology: Although there is no mechanical linkage, the design is still very similar to "old" cockpit technology: a yaw damper or a stick shaker or a speed limiter is a stand-alone box somewhere alongside the signal stream, and does only interfere when its time has come. It's still not system integration like in modern military solution or like in the Airbus product.

I also agree that only Airbus' way is the way to go - and the longer Boeing is waiting to do something really innovative (I'm no insider of the 787), the more they lag behind.

I'm merely putting down what the topic starter asked or feared. If we can lift the anxiety of some more traditional Aviators we finally get where we want to go: More safety, at lower costs and more environmentally friendly.

Dani

[yowdude]

i've flown the dc9 and now i fly the bus. building time on the 9 was the best experience i got on how to fly the wing. it seems that i can only reminisce on my flights while flying the bus coz there nothing much to do after you set it up

[johnnyDB]

there are several recent airliners around with cable operated tabs which directly move controlsurfaces, no computer, no hydraulics.
I'm on about 100-150 passenger jets.

[FlightDetent]

Originally Posted by DC-ATE

But I suspect a pilot could make a mistake on a FBW aircraft as well with the same results. I don't have the Cali report.

Same mistake - yes, but with different, non-tragic results achieved through possibilities provided by FBW design.

My recollection on Cali is that after what is today a textbook CFIT scenario they hit treetops just by a few feet on a ridge during the escape manoeuvre, lost flaps during this intial impact but remained airborne. Only then succumbed down to ground on the other side of a valley. The speedbrakes were left extended throughout the manoeuvre.

On FBW a simple algorithm can be provided: IF ([any]THR LVR >= MCT/CON): SPDBRK 0. I am not saying a mechanical system with similar effect is impossible to engineer.

Also on FBW with protections (yet another category), you can have the pilot pull full back on pitch control and go to ALPHA(max) which can be set quite close to ALPHA(crit). Without protections, fbw or not (!), a StickShakeLimit becomes the pilots' ALPHA(max) and needs to be set further away from A(crit) to maintain margin for human performance.

Entry to the climb phase is significantly faster with protections and the performance available from the wing is utilized more effectively.

I suspect that comparison of contemporary FWB functions against the Little Rock scenario could also provide some credit to the argument.

Yours,
FD (the un-real)

[DC-ATE]

OK...thanks for that info.

re. "stick-shaker"; what is the stall warning indication on the A-320-type aircraft?

2nd question: Do the A-320 types have tail de-ice?

[NigelOnDraft]

what is the stall warning indication on the A-320-type aircraft?

In "Normal Law" there isn't one, because the FBW makes it hard to get near the stall, and "impossible" to stall...

In other laws i.e. when degraded, it shouts "Stall Stall" at you

Do the A-320 types have tail de-ice?

No...

HTH
NoD

[FE Hoppy]

Just to add the E-jet.

In both Normal and Direct mode we have a stick shaker at 1.05Vs-1g.
In normal mode when it's firing you can pull back to the stop but the fly by wire will limit the aircraft to Cl max. This is called AOA limiting so like the bus you cannot stall the wing. However, there is no automatic thrust increase at this point so unless you help out you can "stall" the aeroplane. i.e not enough V^2 for lift to equal weight.

In direct mode its just like a cable system and you can screw up as much as you like.


also no anti ice on the empennage.

[Lemurian]

FlightDetent

on FBW with protections (yet another category), you can have the pilot pull full back on pitch control and go to ALPHA(max) which can be set quite close to ALPHA(crit). Without protections, fbw or not (!), a StickShakeLimit becomes the pilots' ALPHA(max) and needs to be set further away from A(crit) to maintain margin for human performance.

Entry to the climb phase is significantly faster with protections and the performance available from the wing is utilized more effectively.

I suspect that comparison of contemporary FWB functions against the Little Rock scenario could also provide some credit to the argument.

Article by ALPA with NASA's help . Excerpts :

GPWS pull-up warnings require immediate crew action. Because most transport airplanes in CFIT accidents have hit within 200 feet of the top of the terrain, maximizing airplane performance at the beginning of the pull-up is crucial. The American Airlines B-757 that crashed on a ridge near Cali, Colombia, hit 100 feet below the ridge line--100 feet was the performance difference between survival and disaster.

A CFIT escape maneuver, a procedure usually performed in response to a GPWS warning, is designed to expediently remove an airplane from an impending collision with terrain. This maneuver is designed around the use of maximum or near-maximum aerodynamic performance. Typically, the airplane is in a descent. A CFIT escape maneuver can be started anywhere from a clean cruise descent (250 to 300 KIAS) to flying with full flaps and gear down at approach speed.

We looked at three cases: airplanes with conventional flight controls, FBW airplanes with "hard" protection features, and FBW airplanes with "soft" protection features.

Conventional flight controls

For an aircraft with conventional flight controls, the typical CFIT escape maneuver requires the pilot to apply maximum thrust and rotate at a smooth rate of 3 degrees per second to a pitch attitude of 15 to 20 degrees nose-up. This pitch attitude is maintained until the stickshaker activates or terrain clearance is ensured. The 3-degree-per-second rate was selected as a target pitch rate to avoid overstressing the airframe in the high-energy case and to avoid stalling in low-energy escape maneuvers.

An industry task force recommended this procedure, and it is the basis for most current CFIT escape maneuver procedures. Also, some operators had airplanes whose stickshaker may be activated by sensing AOA acceleration, and they believed a low-pitch rate was necessary to avoid activating the stickshaker. An operator selects for itself its CFIT escape maneuver procedure, and currently that may not be what the manufacturer recommends. Airlines often want to have a fleet-common procedure, regardless of aircraft type, for cross-training and standardization.

FBW flight controls

For the purpose of this investigation, we chose two airplanes that represent different FBW flight control design philosophies: the B-777 and the A330. We conducted test flights of the B-777 from Boeing Field in Seattle, Wash., and test flights of the A330 at the Airbus facilities in Toulouse, France. A company test pilot occupied the right seat in each case, and evaluation pilots took turns flying from the left seat. We operated both airplanes at a mid-CG with a takeoff weight that would permit the approach-configuration CFIT-avoidance maneuvers to be performed near the airplane's maximum (worst case) landing weight.

In preparing for the flight evaluations, we used the manufacturer's engineering flight simulator to work up a test card and to validate the profile. Simulator data allowed us to evaluate various maneuvers, determine the maneuvers with the best potential for optimal recovery, and practice the selected maneuvers.

Hard limits

Airbus, in the design of its FBW flight control system, incorporates "hard" limits, which prevent the pilot from exceeding a predetermined flight envelope. The FBW flight control system used in the Airbus design maintains an AOA margin to prevent the pilot from stalling the airplane. The aircraft cannot be commanded to exceed +2.5 gs or –1 g clean (regardless of gross weight). The pitch attitude is limited to the range between 30 degrees nose-up and 15 degrees nose-low. The bank angle is limited to 67 degrees. No approved or readily discernable method allows the pilot to override the flight envelope protections.

The A330-200's maximum takeoff weight is 507,100 pounds. For our flight, we weighed 402,600 pounds, just above the maximum landing weight of 396,800 pounds. Our fuel load was 134,000 pounds, with a maximum fuel capacity of 250,000 pounds. The A330-200's two P&W4168 engines are rated at 68,000 pounds of thrust each.

The procedure for Airbus's recommended CFIT escape maneuver in the A330 is for the pilot to pull full back on the stick, apply maximum thrust, and recover when terrain clearance is ensured. The speed brakes, if extended, will retract automatically. Control laws either stabilize the AOA at an optimum value or adjust pitch rate to obtain maximum allowed g.

Soft limits

Boeing, in the design of its FBW flight control system, incorporates "soft" limits, which warn the pilot when a limit is being approached, using such methods as increased stick forces and aural and visual warnings.

With soft limits, after the warning, the pilot is allowed to override and achieve maximum aerodynamic capability of the airplane; you could stall, overbank, overstress, or overspeed the airplane, if necessary or desired. The keyword for FBW soft limits is "overrideable."

The maximum takeoff weight of the B-777–300 is 660,000 pounds, and the maximum landing weight is 524,000 pounds. For our flight, the airplane weighed 501,000 pounds, of which 163,500 pounds was fuel. Two Rolls-Royce Trent 892 engines, each rated at 90,000 pounds of thrust, powered the airplane.

In a CFIT escape maneuver for the B-777, Boeing recommends that the pilot immediately select maximum thrust, rotate aggressively to 20 degrees of pitch, and retract the speed brakes (they do not automatically retract as do Airbus speed brakes), and recover when terrain clearance is ensured.

Flying the escape maneuvers

We flew simulated CFIT escape maneuvers near the maximum landing weight and at an approach speed of Vref +5 knots, with the gear down and flaps at maximum landing setting, at a descent rate of 1,500 fpm. We also flew simulated CFIT escape maneuvers at typical enroute descent speeds (250 and 300 KIAS as permissible), in the clean configuration, at a descent rate of 1,500 fpm. Splitting the task force's recommended 15–20-degree range for target pitch, we flew the maneuvers with a smooth pitch rate (target 3 degrees per second) to a pitch attitude of 17.5 degrees.

We then repeated these maneuvers with an aggressive pull-up (soft protections) or with full back stick (FBS) application (hard protections).

The benefit of the FBS maneuver for the A330-200 was quite apparent. Using full back stick resulted in an altitude loss of 40 feet, as opposed to 68 feet for the 3-degree-per-second recovery (see Chart 1). The evaluation team felt that time below entry altitude was as important a recovery parameter as total altitude lost. The time below the entry altitude was 3.4 seconds for the FBS versus 5.7 seconds for the aggressive pull-up. And finally, the FBS airplane was 172 feet above the 3-degree-per-second airplane, which was just getting back up to the entry altitude. Remember, most aircraft hit the ground within 200 feet from the top of terrain.

In the approach configuration, altitude loss during the FBS recovery was 35 feet, as opposed to 75 feet. The time below the entry altitude was 5.3 seconds versus 7.8 seconds (see Chart 2). And finally, the FBS airplane was 115 feet above the 3-degree-per-second airplane as the latter was just getting back up to the entry altitude.

As expected, data for the B-777 CFIT escape from an enroute descent showed that a rapid rotation method resulted in less altitude loss than did a slow rotation. Typical data showed about a one-third greater altitude loss with the 3-degree-per-second rotation. The test point showed 80 feet lost for 3 degrees per second (see Chart 4) versus 60 feet for the aggressive pull-up. The time below the entry altitude was also less, 3.1 seconds versus 4.7 seconds. The airplane with the rapid rotation rate was 180 feet above the slow-rotation-rate airplane as the latter passed through the entry altitude. Remember that in the Cali accident, 100 feet was the difference between clearing and not clearing the ridge.

The data for the B-777 configured for a power-on approach were interesting. The flight data showed a greater altitude loss for a rapid-rotation versus the slow-rotation maneuver--67 feet versus 50 feet shown (see Chart 3). Our simulator data predicted the opposite. While the data possibly have some scatter, the difference was attributed to the effects of the slow engine acceleration, which we first encountered while evaluating the airplane's stall characteristics.

The Trent engines demonstrated noticeably slow acceleration from low power settings, followed by a perceptible increase in engine noise and thrust surge as the engines "kicked in." After practicing, I could finesse the AOA rate of change during stall recovery to maintain optimum AOA during engine wind-up (I should mention, the test airplane had no dedicated AOA indicator).

In the low-speed CFIT escape maneuvers, however, the B-777's control laws apparently allowed overshoot of optimum AOA during this engine acceleration period, causing a "mush" effect that resulted in energy bleed and greater altitude loss. The 3-degree-per-second pitch-up was slow enough to allow thrust to catch up before the AOA increased to the same relative value.

We felt the Airbus pitch control laws prevented a similar AOA overshoot, even though the Pratt & Whitney engine acceleration times were similar. We did not see this effect in the high-speed escape maneuvers because enough energy remained so that AOA stayed low in both cases.

On the plus side, the B-777 rapid-rotation escape maneuver resulted in less exposure time below entry altitude than did the slow-rotation procedure (4.8 seconds versus 5.7 seconds), even though the altitude loss was greater. Passing through entry altitude, the rapid-rotation airplane was 50 feet above the airplane flown with the 3-degree-per-second rotation rate. On the line, different escape maneuver procedures might appear to be warranted for the high- and low-energy cases, with possibly a low-energy procedure addressing the Trent's slow spool-ups. As mentioned earlier, however, airlines tend to prefer "one size fits all" procedures. In addition, having an AOA indicator in the cockpit might help.

As in any flight evaluation, pilot comments are as important as the hard data. One of our most significant findings is that although we could achieve more consistent and repeatable performance with the "hard limit" design, this evaluation team philosophically preferred the flight envelope limiting features ("soft limits") of the B-777 design to the "hard limit" A330-200 design. This was a subjective judgment based on other handling quality evaluations that we performed and the premise that some situations might arise that the designers had not foreseen and for which the pilot might need to achieve full aerodynamic capability as opposed to being limited by software/control laws.

From our flight test work, we arrived at a number of conclusions and recommendations. First, the A330 full back stick CFIT escape maneuver gave better and more consistent performance than a 3-degree-per-second pull, without any increase in risk of exceeding flight envelope parameters. No additional or specific pilot training was necessary to perform the full back stick recovery technique because the FBW design provides excellent pitch rate and g control as well as excellent envelope protection for stall, overstress, or overspeed.

As a result, we recommend that Airbus FBW operators use the manufacturer's recommended full back stick CFIT recovery procedure. As I previously said, this may seem obvious; but until this report, none of the U.S. operators were following the Airbus-recommended procedure, and none felt that doing so was prudent. In addition, the ease of training and maneuver repeatability, in our opinion, outweigh the advantages of fleet-standardizing an airline's CFIT escape maneuvers for all airplane types.

In the case of the B-777, flight test results indicated that an aggressive pull-up as Boeing recommends yielded better CFIT avoidance performance than the 3-degree-per-second recovery procedure in all categories except total altitude lost in the case of the low-speed, low-power-setting situation. As a result, we recommend that B-777 operators follow the manufacturer's recommended CFIT escape maneuver procedure but consider modifying the procedure in the case of the low-speed, low-power-setting scenario.

As a final recommendation, based upon all of our previous conclusions, recommendations, and pilot comments, we feel that future FBW designs should consider protected flight-envelope limits with envelope-protection override.

I have flown all of the Airbus FBW airplanes and have been able to aggressively maneuver each airplane without worrying about overstressing the airframe. To pull the stick full back in an airplane that weighs half a million pounds and pull right to 2.5 gs is impressive (especially to the chief pilot if the airplane is loaded with passengers!). However, to be presented with a windscreen full of rocks and only a 2.5 g capability when more g is aerodynamically available is not comforting. This is when the capability to pull to the aerodynamic or structural limits of the airplane--as can be done in the B-777--is important. The B-777, however, does not have the carefree maneuvering capability of Airbus FBW airplanes. A combination of the best points from both these designs would be desirable.

Whole article "CFIT AVOIDANCE AND FBW", Here


NB : Article written in 1999...Ten years ago !

[DC-ATE]

Quote:
what is the stall warning indication on the A-320-type aircraft?
In "Normal Law" there isn't one, because the FBW makes it hard to get near the stall, and "impossible" to stall...

In other laws i.e. when degraded, it shouts "Stall Stall" at you


Quote:
Do the A-320 types have tail de-ice?
No...

Therefore, w/o tail anti-ice, the tail could stall before the wing and you would have no warning. I assume that in "other laws" the warning is based on some computer info from some source. But...same problem; tail stalling first.

[NigelOnDraft]

DC-ATE

Therefore, w/o tail anti-ice, the tail could stall before the wing and you would have no warning

Sorry - please expand

A320 etc. do not have tail anti-ice because they have been designed not to need it / have been certified as not requring it i.e. either the tail does not ice up (significantly) [in fact I know it does / can], or the design is such that it does not significantly affect the overall flying characteristics when iced.

Anyway, a tailplane typically produces a downforce, is at a lower alpha than the mainplane, has a different - in fact inverse - section, so if it did "stall" surely it would reduce the downforce, letting the nose drop...?

FEH

However, there is no automatic thrust increase at this point so unless you help out you can "stall" the aeroplane

Sorry - I do not understand why you need thrust to prevent a stall? If this is so:

In normal mode when it's firing you can pull back to the stop but the fly by wire will limit the aircraft to Cl max.

then so long as the FBW limits you to CL Max, you will not stall, power or no power? Might lower the nose a lot / lose a lot of height, but surely no stall?

NoD

[Lemurian]

NoD

A320 etc. do not have tail anti-ice because they have been designed not to need it / have been certified as not requring it i.e. either the tail does not ice up (significantly) [in fact I know it does / can], or the design is such that it does not significantly affect the overall flying characteristics when iced.

None of the "moving tailplane" equipped airliners needs it to be de-iced, as the authority of that big surface - covered or not with ice - on the control of the airplane is immense ( as witnessed by the "manual back-up" on the 320 : pitch control via the trim wheel necessitates absolutely minuscule corrections ).
See FAR 25 / 1419 and appendix C

FEHoppy

not enough V^2 for lift to equal weight.

That will cause only a descent. It certainly doesn't mean "stall" as you would be always below alpha max.

[DC-ATE]

NigelOnDraft & Lemurian -

Are you telling me that the A-320 cannot get ice on the tail plane? You can't be serious. Boeing said the same thing about the 737 and were wrong. If the wing can ice up, the tail can certainly ice up.

[FE Hoppy]

CS25

Vsr may not be less than the 1g stall speed.

Therefor if I'm at a speed lower than that required to maintain 1g flight I am by definition stalling.

There is also a reference somewhere that refers to the fact that if the control is on the rear stop the aircraft is stalled. I will print it when I find it.

I used the term Wing stalled to reference alpha above that which first achieves Clmax and Aircraft stalled when at Clmax weight is greater than Lift.

I understand the Airbus has Alpha floor which increases thrust to ensure that when Clmax is achieved V^2 is always high enough to arrest descent.

Sorry for the confusion.

Nigel. After flow analysis was complete your Airbus was flight tested with false leading edges on the empennage to simulate the maximum ice build up. the handling characteristics met all requirements so no anti ice system is required.

From CS 25.201 Stall Demonstration.
......
The aeroplane is considered stalled when
the behaviour of the aeroplane gives the pilot a clear
and distinctive indication of an acceptable nature that
the aeroplane is stalled. (See AMC 25.201 (d).)
Acceptable indications of a stall, occurring either
individually or in combination, are –
(1) A nose-down pitch that cannot be
readily arrested;
(2) Buffeting, of a magnitude and severity
that is a strong and effective deterrent to further
speed reduction; or
(3) The pitch control reaches the aft stop
and no further increase in pitch attitude occurs
when the control is held full aft for a short time
before recovery is initiated. (See AMC
25.201(d)(3).)
.....

So when AOA limiting is in effect the control is at the aft stop but the aircraft will no longer pitch up. This indicates the aircraft is stalled.

[Lemurian]

FE Hoppy

So when AOA limiting is in effect the control is at the aft stop but the aircraft will no longer pitch up. This indicates the aircraft is stalled.

Hmmmmm !That's the quickest syllogism I've ever read in aviation !
No, "stall" refers to a completely disturbed airflow state over an airfoil and as such an alpha that's past alpha max, and not to the effect of a device that would prevent the aerodynamics phenomenon to happen.

[FE Hoppy]

Lemurian

No, "stall" refers to a completely disturbed airflow state over an airfoil and as such an alpha that's past alpha max, and not to the effect of a device that would prevent the aerodynamics phenomenon to happen.

Hmmm...... not according to the regulation by which all modern aircraft are certified.

I know exactly what you mean by the classic definition but we now have FBW systems which prevent it happening. Even before FBW if an aeroplane had stall characteristics which were no deemed appropriate a stick pusher was used to prevent the aircraft from reaching the classical "stall".

[DC-ATE]

Even before FBW if an aeroplane had stall characteristics which were no deemed appropriate a stick pusher was used to prevent the aircraft from reaching the classical "stall".

And just what would those "characteristics" be? The 737 did not have a stick pusher and yet could stall.

[Lemurian]

FE Hoppy

Therefore if I'm at a speed lower than that required to maintain 1g flight I am by definition stalling.

That definition applies to a straight and level flight.
By definition, in the descent equation, weight is higher than the produced lift, at a reduced Cl, meaning a smaller α.

not according to the regulation by which all modern aircraft are certified.

I know exactly what you mean by the classic definition but we now have FBW systems which prevent it happening. Even before FBW if an aeroplane had stall characteristics which were no deemed appropriate a stick pusher was used to prevent the aircraft from reaching the classical "stall".

There is a difference between ”aerodynamic stall” and “stall recognition” and even more with "stall protection", of which the stick pusher is one.
3.3.2 DEFINITION OF STALL SPEED
“The stalling speed, if obtainable, or the minimum steady speed, in knots (CAS), at
which the airplane is controllable with.... (the words that follow describe the required
configuration).”
- FAR Part 23.45

“The stall speed (equivalent airspeed) at 1 g normal to the flight path is the highest
of the following:
1. The speed for steady straight flight at CLmax (the first local maximum of lift
coefficient versus which occurs as CL is increased from zero).
2. The speed at which uncommanded pitching, rolling, or yawing occurs.
3. The speed at which intolerable buffet or structural vibration is encountered.”
- MIL-STD-1797A

From a test pilot’s perspective, the task is to investigate how much lift potential can
be exploited for operational use, without compromising aircraft control in the process. The
definition of stall speed comes from that investigation.
The discussion of minimum speed includes the notion of maximum lift coefficient
(CLmax). To maintain lift in a controlled deceleration at 1 g, the lift coefficient (CL)
increases as the dynamic pressure decreases (as a function of velocity squared). This
increase in lift coefficient is provided by the steadily increasing during the deceleration.
At some point in the deceleration the airflow over the wing separates, causing a reduction
of lift. The lift coefficient is a maximum at this point, and the corresponding speed at these
conditions represents the minimum flying speed.
The speed corresponding to CLmax may not be a reasonable limit. Any other
potential limitations may prescribe a minimum useable speed which
is higher than the speed corresponding to CLmax. The higher speed may be appropriate due
to high sink rate, undesirable motions, flying qualities, or control effectiveness limits.
Influence of the separated flow on the empennage may cause instabilities, loss of control,
or intolerable buffeting. Any of these factors could present a practical minimum airspeed
limit at a lift coefficient less than the CLmax potential of the airplane. In this case, the classic
stall is not reached and a minimum useable speed is defined by another factor.