Va Maneuvering

[keith williams]

The material below outlines some of the FAA discussions on this subject.

Code of Federal Regulations Sec. 23.1507Part 23 AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES

Subpart G--Operating Limitations and Information
Sec. 23.1507[Operating] maneuvering speed.[The maximum operating maneuvering speed, VO, must be established as an operating limitation. VO is a selected speed that is not greater than established in Sec. 23.335(c).]Amdt. 23-45, Eff. 09/07/93 Comments Document HistoryNotice of
Proposed Rulemaking Actions:Notice of Proposed Rulemaking. Notice No. 90-18; Issued on 06/15/90.Final Rule Actions:Final Rule. Docket No. 26269; Issued on 07/28/93.



The following text is extracted from the above document.

Reference: Conference proposals 119 and 120.
No action is being taken to amend Section 23.335 Design airspeeds.

Explanation: Conference proposal 187 recommends revision of Section 23.335(c) to increase the design load factor to account for possible overloads resulting from maximum airplane maneuvers at speeds greater than VS√n for cases where the applicant chooses a design maneuvering speed greater than V=VS√n as allowed by Section 23.335(c). In support of conference proposal 187, the submitter states that the purpose of maneuvering speed (in addition to supplying a speed for design of control surfaces in accordance with Sections 23.423, 23.441 and 23.445) is to provide an operating speed where a pilot can be assured of not exceeding the design limits during maneuvers. If a design maneuvering speed in excess of VS√n is chosen (as currently allowed by Section 23.335(c)), and if the airplane is operated at that speed during maneuver, the potential exists for a pilot to exceed the design limit load factor unless that load factor is increased accordingly.

Post conference review indicates that the design maneuvering speed criteria provided in Section 23.335 is necessary and sufficient for control surface design. As such, design maneuvering speed selections greater than VS√n are appropriate, and requiring increases in load factor above those specified in Section 23.337 are unjustified.

However, the FAA recognizes that maneuvering speed is also used by the pilot as that airspeed below which full control surface inputs can be accomplished without structural damage. Maneuvering speed may also be used as a gust penetration speed to minimize the possibility of airframe damage. If the airplane is maneuvered at its maximum weight at airspeeds less than VS√n the airplane will stall prior to exceeding the maximum design load factor. If the airplane is operated at speeds greater than VS√n in the same conditions, the maximum design load factor can be exceeded.

The FAA recognizes the dual meaning given maneuvering speed and agrees that the maneuvering speed used to design the control surfaces and the maneuvering speed used by the pilot have different purposes, yet Sections 23.335, 23.1507, and 23.1563 use the same term, "design maneuvering speed, VA".

The FAA proposes to leave Section 23.335 unchanged but would establish an "operating maneuvering speed: VO" in Section 23.1507, and alter Section 23.1563 to require an airspeed placard listing a maximum operating maneuvering speed, instead of the design maneuvering speed VA. Since the operating maneuvering speed (that speed where the CNA max curve intersects the design load factor line) will reduce for weights less than maximum weight, the applicant may choose to placard operational maneuvering speeds for more weights than the maximum.

In the above text the term VSün means (Vs Square root n), depending on your font selection

[Lantirn]

Very nice input, thank you

[Brian Abraham]

My error, I see now what you're getting at. It didn't hit me even though I wrote

VA must be equal to or greater than VS√n

The design maneuvering speed is a value chosen by the applicant. It may not be less than Vs√n and need not be greater than Vc, but it could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.

VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits, nor should it be interpreted as a gust penetration speed. Only if VA= Vs√n will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.

Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507. VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits.

But I think confusion may remain. As a result of the American Flt 587 the FAA put out a "Maneuvering Speed Limitation Statement" addressing a FAR 25 amendment.

The NTSB’s investigation revealed that many pilots might have a general misunderstanding of what the design maneuvering speed (VA) is and the extent of structural protection that exists when an airplane is operated at speeds below its VA. VA is a structural design airspeed used in determining the strength requirements for the airplane and its control surfaces. The structural design requirements do not cover multiple control inputs in one axis or control inputs in more than one axis at a time at any speed, even below VA.

The NTSB found that many pilots of transport category airplanes mistakenly believe that, as long as the airplane’s speed is below VA, they can make any control input they desire without risking structural damage to the airplane. As a result, the NTSB recommended that the FAA amend all relevant regulatory and advisory materials to clarify that operating at or below maneuvering speed does not provide structural protection against multiple full control inputs in one axis or full control inputs in more than one axis at the same time.

This final rule adopts the proposed rule with minor changes that will resolve a longstanding inconsistency in the current requirements that would have been left in place by the proposed rule. This inconsistency, which goes back to at least the 1953 Civil Air Regulations Part 4b, concerns the reference to ‘‘maneuvering speed VA’’ in the existing § 25.1583(a)(3). Sections 1.2 and 25.335(c) define ‘‘VA’’ as the ‘‘design maneuvering speed,’’ not the ‘‘maneuvering speed.’’ Section 25.1507 defines the ‘‘maneuvering speed’’ as an operating limitation that must not exceed the design maneuvering speed, VA. Since the ‘‘maneuvering speed’’ can be less than VA, the reference to ‘‘maneuvering speed VA’’ in the existing § 25.1583(a)(3) is incorrect.

An applicant may wish to establish a maneuvering speed different from the design maneuvering speed, in order to make it easier for pilots to use. For example, the design maneuvering speed, VA, is an equivalent airspeed. Applicants might find it desirable to provide a maneuvering speed as a calibrated airspeed equal to or below the corresponding equivalent design maneuvering airspeed at all altitudes, in order to provide the information in a format that is consistent with that used on the flight deck airspeed indicator. In practice, the maneuvering speed has been identified as VA in AFMs even when it is not always exactly the same as the design maneuvering speed defined in § 25.335(c). We have no evidence of this being unsafe and see no reason to prohibit it in the future. However, in order to address the inconsistency in the regulations, for § 25.1583(a)(3), we have changed the reference to ‘‘the maneuvering speed VA’’ proposed in the NPRM to ‘‘the maneuvering speed established under § 25.1507’’ in this final rule. For new § 25.1583(a)(3)(i) and (ii), we have also changed the references to ‘‘VA’’ proposed in the NPRM to ‘‘maneuvering speed’’ in this final rule. We will continue to allow applicants to refer to this maneuvering speed as VA in AFMs.

For small airplanes, part 23 defines an operating maneuver speed (VO) to serve the same purpose as the maneuvering speed established under § 25.1507. The part 23 approach has one advantage in that there is a unique V-speed abbreviation for pilots to use that differentiates the maneuvering speed used operationally from the design maneuvering speed used to show compliance with the structural type certification requirements. We chose not to introduce a new V-speed term in part 25 because the VA term has historically been used for transport category airplanes for both the speed to be used operationally and for design purposes. Using a new V-speed term could also potentially lead to confusion if different speed terms and definitions are used for new airplane designs compared to current designs.

You go on to explain Vo and how it's related to the entire airframe (by ensuring that the limit maneuvering load factor isn't exceeded), but Va does not do that.

Seems to me that we have to be very specific as to what we're referring to now when mentioning "maneuvering speed". Is it FAR 23 (old), FAR 23 (new), FAR 25, or "corner speed" as referred to by fighter types.

And a question. Why would you have a VA (or whatever you want call it) where you have to check the control input to avoid exceeding "g" limit? All the old flight manuals that I have define VA as (direct quote out of a Cessna manual) "maximum speed at which full or abrupt control movements may be used" and lists the weight/speeds,
3800 lbs 130 kts
3050 lbs 117 kts
2300 lbs 101 kts
They are also the maximum recommended turbulence penetration speeds.

Piper manual quotes, "VA - Maneuvering speed is the maximum speed at which application of full available aerodynamic control will not overstress the airplane".

Convention wisdom has always been to reduce VA proportionally with reduced weight. Is anyone aware of an aircraft certified where VA>VS√n?

[Lantirn]

And a question. Why would you have a VA (or whatever you want call it) where you have to check the control input to avoid exceeding "g" limit?

From what I ve learned the last 2 days here, absolutelly nothing.
The only thing that I find helpfull, is that pilots have to be reminded by the presence of Va, that they can only do single full abrupt application of flight controls, below Va

Is anyone aware of an aircraft certified where VA>VS√n?

Yes. The AT3.
Stall speed in CAS (EAS anyway) is 50 knots. In manual, stated Va is 112kts, if you assume standard 4.4G limit. Calculated Vo=104 knots CAS, about 106 IAS

Even worse, AT3 has 3.8G positive load factor limit.
Calculated Vo=97 CAS, about 100 IAS

12 Knots margin!

[Brian Abraham]

Is anyone aware of an aircraft certified where VA>VS√n?

No wish to slight the VLA market, but was more interested in the mainstream GA aircraft, such as Cessna, Piper, Tobago etc

[italia458]

Brian,

Is anyone aware of an aircraft certified where VA>VS√n?

A Piper PA-28-140.

The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 2)

[westhawk]

Is anyone aware of an aircraft certified where VA>VS√n?

Not exactly, though it may appear so if one only considers the published Va for the normal category when the airplane was originally certified in both the normal and utility categories.

From the above linked The Flight Writer: Va: Not a Bad Speed, Just Misunderstood (Part 2):

At maximum gross weight, the Piper Cherokee (PA-28-140) has a stall speed (Vs) of 64 MPH, a limit load factor of 3.8 g’s, and a Va of 129 MPH.

The PA28-140, like several other production SEL piston airplanes, is FAA certified in both the normal (3.8g) and utility (4.4g) categories. The MGW is lower and the aft CG limit is further forward in the utility category. The aircraft may be operated in either category provided that the appropriate limitations for the applicable category are respected.

So, doing the math:

Vs = 64
Normal category load limit factor
sq rt 3.8 = 1.949
1.949 x 64 = 124

Utility category load limit factor
sq rt 4.4 = 2.0976
2.0976 x 64 = 134.247

134 + 124 = 258.
258 / 2 = 129 mph

See what it looks as though they did? Right, simply average the utility and normal category calculated maneuvering speeds to arrive a a single published Va applicable to BOTH categories.

Try this for the dual category certified C-172 and see if you don't find a similar result!

[Steve Pomroy]

Vs = 64
Normal category load limit factor
sq rt 3.8 = 1.949
1.949 x 64 = 124

Utility category load limit factor
sq rt 4.4 = 2.0976
2.0976 x 64 = 134.247

134 + 124 = 258.
258 / 2 = 129 mph

See what it looks as though they did? Right, simply average the utility and normal category calculated maneuvering speeds to arrive a a single published Va applicable to BOTH categories.

Interesting trick. I've never noticed this. But I think the numbers working out this way is just a coincidence. Or perhaps the manufacturer chose to do this for some reason, but it still leaves Va exceeding the regulatory requirement.

Remember that the stall speed changes with weight. So your calculated utility Va should be adjusted accordingly. Correcting for weight, the utility stall speed is 61 MPH, and the utility Va should be 128 MPH (rounded off). I think a more likely explanation is that the manufacturer identified the correct utility category Va and used it as the design speed (128 MPH v. 129 MPH could be simply a result of rounding error somewhere).

Try this for the dual category certified C-172 and see if you don't find a similar result!

Not in the models I've flown. The Va for the normal category that I've seen is the correct minimum-required Va. However, if you correct the Va for both new stall speed and new limit load factor at the max utility weight, you get an unchanged value for Va. Although if I recall correctly, the published Va is adjusted downward for weight (probably because you can still be in the normal category if the CG is outside utility limits, but that's just an educated guess).

In the Grob G-120A, the numbers are as follows:

Utility:
- Vs = 67 KCAS;
- LLF = 4.4 g;
- Minimum Va = 141 KCAS;
- Published Va = 145 KCAS.
Aerobatic:
- Vs = 66 KCAS;
- LLF = 6 g;
- Minimum Va = 162 KCAS;
- Published Va = 165 KCAS.

Interestingly, if you use the utility stall speed and the aerobatic LLF, you get to within rounding error limits of the aerobatic Va.

[westhawk]

I think a more likely explanation is that the manufacturer identified the correct utility category Va and used it as the design speed

I think you're right Steve. I see now that I neglected to use the lower utility category stall speed in my calculation and that skewed the resultant Va calculation upward. And yes it's probably fair to say a 1 knot difference is negligible.

[hvogt]

Brushing up on my knowledge from flight school, I came across this thread. I was at the point where I thought I had fully understood the difference between VA and VO, but now I'm not sure anymore. There is one aspect I don't yet understand.

Since VS sqrt(n), respectively VS1 sqrt(n) in CS-25, is the upper limit for VO and the lower limit for VA, I would have assumed to find VA drawn at the point of the V-n diagram where the +CNmax curve intersects the limit load factor line (at the 'corner speed') or to the right of it. Looking at the V-n diagrams in CS/FAR 25.333(c), I see VA is drawn to the left of the corner speed.

I could find references where VA is coincident with the corner speed [1,2], but only the Oxford POF textbook [3] provides an explanation for VA being left of 'the corner'. They say 'VA is slower than the speed at the intersection of the CLMAX line and the positive limit load factor line (point A) to safeguard the tail structure because of the higher load on the tailplane during the pitch manoeuvre'. Isn't this a direct contradiction of the design requirements in CS 23.335(c)(1) and CS 25.335(c)(1)? Lantirn has quoted the same source and mentioned their confusion of VA and VO. Still, I can't see why the flight envelopes in CS/FAR 25.333(c) put VA to the left of the corner. Is it possible the diagram in the certification specifications is from a time before the introduction of VO, and VA is meant to be an operational VA rather than a design VA? I gather from Keith's post this might be an explanation.

Am I missing something?

[1] Torenbeek E, Wittenberg H (2009) Flight Physics. Springer, Dordrecht, p 321

[2] Gallagher G et al. (1992) Fixed Wing Performance. In: U.S. Naval Test Pilot School Flight Test Manual. Veda Incorporated. http://www.vmihosting.com/MWS/Docume...PS_FTM_108.pdf. Accessed 30 Mar 2013

[3] Oxford Aviation Academy (UK) Limited (2008) Principles of Flight. OATmedia, Oxford, p 463

[hvogt]

Still thinking about it and hoping somebody might point me in the right direction.

[punkalouver]

Originally Posted by hvogt

Still thinking about it and hoping somebody might point me in the right direction.

This article came out recently in an aviation safety letter. It starts off slow but gets more interesting. Comments would be welcome


Turning to a page in the manual, the car salesman excitedly pointed out two newly developed features of the car I was interested in purchasing. “Now, as it says here on page 44, if you drive at or below this speed—48 km/hr—no matter how treacherous the pothole you hit, you will not damage the suspension or cause the steering to go out of alignment. Just drive at or below this speed.” He continued, with a voice of cautious tone: “Now, hit a pothole above this speed and, well, a wheel could fall off.” He pointed to the digital speedometer on the instrument panel. “And just to remind you that you better be on smooth pavement when you go above this speed; once you hit 49 km/hr, the numbers on the speedometer are in yellow, which means caution!”

After expressing my amazement, I asked about the other newly developed feature. “This one’s even better,” he smiled. “It has to do with a sudden change of direction in the event of an emergency, like swerving to avoid a deer that’s just jumped in front of you.” Living in the country, I was quite interested in this feature and invited his explanation. “Well,” he continued, “If you drive at or below this speed—60 km/hr—you can make a sudden turn, and the car is guaranteed to not roll over.” He drew my attention to the manual: “You’ll notice there’s a list of rollover speeds. The heavier you are, the faster you can go. So, if you’re on the way to the airport with passengers and some luggage, you can swerve to miss that deer at a faster speed and not wind up in the ditch. But once you drop them off: SLOW DOWN!”

These are, of course, imaginary safety features of an automobile, but they serve to illustrate the concepts that underlie two V-speeds when we fly: Vno—Normal Operating/Maximum Structural Cruising Speed—and Va—Maneuvering Speed. I find these automobile images help my ground school students understand the important limits of Vno and Va.

Prior to learning about and understanding the significance of these speeds, the class has learned about load factor and G-forces and the impact these have on an aircraft’s stall speed. This is a necessary context.

“Those nice, white, pretty, puffy clouds you see on a summer’s day usually mean there are potholes in the sky,” I tell my class (more on that when we study meteorology). We can often predict turbulent air, but sometimes we cannot, especially in the world of general aviation, where sophisticated on-board weather radar is usually not found in the array of cockpit instruments. The proficient pilot, however, would be aware of conditions that could lead to turbulence, especially CAT—clear air turbulence. I emphasize the importance of checking PIREPs to become aware of potentially turbulent conditions. Aviators in mountainous regions need to be particularly vigilant.

In January 1964, the B-52 Stratofortress pictured here unexpectedly encountered severe turbulence at 14 000 ft ASL over New Mexico. The stresses caused by the turbulence were too much, exceeding load factors, and the tail fin was sheared off. Fortunately, the crew was able to safely land the plane several hours later.

A key difference between these two speeds, Vno and Va, is found in their very definitions. While the “no” in Vno is conveniently remembered as “n”ormal “o”perating speed, the definition most commonly given is “maximum structural cruising speed.” The key word here is “cruising.” If we return to the car analogy, the pothole speed applies to the car cruising along a road. Presumably in somewhat of a straight line. Va, by contrast, offers another image in its definition: “design maneuvering speed.” This connotes some change in direction. For the automobile, it suggests turning; in the airplane, it implies movement around any axis: roll, pitch or yaw.

Respect for both of these speeds will keep the pilot from bending, breaking or cracking something on the aircraft.

The part of the definition of Va that befuddles many students is the explanation that if a pilot flies at or below the maneuvering speed, “the plane will stall before it breaks.” That is to say that the aircraft will stop flying before a load factor that will overstress its structure is imposed on it. An understanding of this will also help the student grasp why it is that Va decreases as weight decreases and vice versa. Rookie pilot logic says “If I’m lighter, I should be able to go faster!”

We’ll start with a couple of givens: first, doubling the angle of attack doubles the load factor. In other words, if I suddenly pull back on the stick and increase my AoA from 3° to 6°, I will experience a load factor of 2, or 2 Gs. Next, we’re going to say that the airplane in our lesson stalls at 16° angle of attack (the critical angle of attack for most small aircraft is between 15° and 18°). Let’s also give our aircraft a weight of 3000 lbs and a cruising speed

of 120 KIAS. To maintain level flight at this weight and speed, the AoA is 3°. Finally, the maximum rated load factor is +4 Gs. Beyond this load factor, the manufacturer—Mr. Cessna or Ms. Piper—tells us in the POH that the structure of the aircraft experiences stresses that are dangerous; something could bend, break or crack.

Now, if I were to really yank back on the stick and effectively quintuple the angle of attack from 3° to 15°, I will impose on the aircraft a load factor of 5: more than the maximum rated load factor of 4. But I would still be flying because the critical angle of attack is 16°. Something could bend, break or crack.

What if I slow down to 100 KIAS? Now, to maintain level flight at 3000 lbs, I need to increase my angle of attack to, let’s say, 4.5°. This time, I suddenly pull back on the stick and quadruple the AoA from 4.5° to 18°. Do I also quadruple the “weight” and impose a load factor of 4? Not quite! The plane would stall at 16°, 2 degrees before I got to four times the load factor. Falling forward in the stall, the stressful loading on the wings is relieved before reaching the critical load factor of 4, thus preserving the structural integrity of the aircraft.

Now, the question of weight. Let’s say I land and say goodbye to 300 lbs worth of passengers and their baggage before taking off again. At 100 KIAS, weighing now 2700 lbs, level flight only needs 3° AoA. Ah… now we’re back to the same problem we had initially. Solution: slow down even more. The generally accepted formula is 2% change in weight (increase or decrease) = 1% change in maneuvering speed. 300 lbs is 10% of the pre-unloading weight of 3000 lbs. So, we reduce our speed by 5% from 100 KIAS to 95 KIAS. This will necessitate an increase in AoA to, let’s say, 4.5°. And we know that’s safe.

The class has already learned that the stalling speed of an aircraft increases by a factor of the square root of the load factor. If the load factor is 2 (as in a 60° banked turn), then the stall speed is multiplied by 1.41, the square root of 2. We can use this same concept to understand how Va is calculated for an aircraft. Let’s say that the stalling speed of an aircraft is 55 KIAS and the maximum rated load factor, as in the example above, is 4. At this maximum load factor, the stalling speed would double; the square root of 4 is 2. Therefore, the maneuvering speed of the aircraft would be 110 KIAS: the stalling speed of 55 KIAS times 2. In other words, if the aircraft is flying at or below 110 KIAS–Va should it encounter a sudden gust of wind that quadruples the load factor, it will stall before it exceeds its maximum rated load factor of 4.

The important test question has to do with a change in weight on a multi-leg cross-country flight. Examiners routinely ask students to recalculate weight and balance to determine a revised center of gravity when fuel has been burned off and the aircraft payload changes. The same applies to a revised maneuvering speed.

For example, a Cessna 172N departs with a take-off weight of 1950 lbs. The Pilot Operating Handbook (POH) indicates that the maneuvering speed at this weight is 89 KIAS. The plane flies for 3 hours, burning 10 GPH (6 lbs per USG). After landing, 50 lbs of baggage is unloaded, as well as the rear passenger, who weighs 160 lbs. Before taking off again, the proficient pilot calculates the new maneuvering speed. Recall that 2% weight = 1% speed.

Loss of weight in fuel: 3 x 10 x 6 = 180 lbs + 50 lbs luggage + 160 lbs passenger = 390 lbs total weight loss.

Now we need to determine by what percentage the take-off weight has been decreased.

390 ÷ 1950 x 100 = 20%. So, if the weight has been decreased by 20%, then the maneuvering speed decreases by 10%. Taking off on the second leg of the cross-country flight at a weight of 1560 lbs, the new Va is 89 – 10% = 80.1 KIAS. And in the case of Va, we always round down; that’s the safest.

Student pilots’ awareness of the change in maneuvering speed (Va2) when aircraft weight configurations change is essential knowledge, as is the skill to determine it.

Failure to respect this could cause a new pilot to be really stressed out!

[zzuf]

The latest iteration of FAR 23 will not be much help, but below is a section of AC23.19A which shows that most of the assumptions here are questionable.

W48. What is the design maneuvering speed VA?
a. The design maneuvering speed is a value chosen by the applicant. It may not
be less than Vs√ n and need not be greater than V c, but it could be greater if the applicant
chose the higher value. The loads resulting from full control surface deflections at VA are
used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.
b. VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane structural limits, nor
should it be interpreted as a gust penetration speed. Only if VA = Vs √n will the airplane
stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where
VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would
exceed the limit load factor.
c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507.
VO is established not greater than VS√n, and it is a speed where the airplane will stall in a
nose-up pitching maneuver before exceeding the airplane structural limits.