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PA-28-181 Vno below Vc minimum design speed?

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PA-28-181 Vno below Vc minimum design speed?

Old 22nd May 2020, 19:36
  #21 (permalink)  
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Arghem... advent, please read post #2! (All of the words and numbers..) Thanks 733driver.

Oggers, kind of you to say and thank you for your input, now what is your response to the later clarification by the FAA in AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27
46. Why would I want to define VC as equal to 0.9 VH? Use this definition if you are designing an airplane that is capable of a sustained speed (VC) higher than that obtained by using the wing loading (W/S) formula.
FYI per the Archer III POH Speed Power chart 5-21 Vh is 133 KTAS = 153 MPH 0.9 = 135.75 MPH so a Vno of 140 MPH CAS is conceivable if we ignore the above AC but personally I would prefer not to ignore a later clarification. Understood from Piper's perspective this is hindsight.

I am still interested in a list of any other Vno < Vcmin designs besides the later/heavier Cherokees. I am sure there will be and it will be an interesting list for sure.

With regards to the problem taking care of its self, this issue is that if an aircraft design increases its MTOW on paper and then artificially reduce their Vno as their structures are not able to comply with the gust penetration requirements this is an indication of a weaker than required air frame which becomes relevant in descent and this is where it gets more interesting.

The Archer III POH specifies a Fuel, Time, Distance to Descend chart based on an engine RPM (2500) and KIAS (122) which puts it an additional 3 knots under Vno, limiting the descent rate to only ~450fpm whilst keeping the engine warm.
This configuration is mentioned again in the Descent subsection of Normal Procedures in reference to carb ice. (The Archer III has a cooler air pick up than previous 4cyl PA-28s) so its descent profiles now start to look limited by the low Vno.
These Fuel, Time, Distance to Descend charts are published for the later Pipers (Seminole, Malibu, Warrior 161s, Archer 181s etc) and not published by the other manufactures singles mentioned above including popular IFR trainers like the DA-40 and DA-42 where these tables would be more expected.
The exception to the above is the later Mooneys. However they configure on a 750fpm descent rate rather than an airspeed likely due to their published Vno being some 20% above Vcmin.

Are there any ex Piper engineers here that might know or do we have any friendlies in their current design team?
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Old 23rd May 2020, 00:43
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Originally Posted by AC103 View Post
I agree, "And further provided" has an "AND" operator. It looks like the FAA changed their mind or realized that they hadn't been clear and didn't want to invite a whole load of paper MAUW increases.
Yes, it seems to me that something has changed and I missed it. Perhaps they have been seeing faster airplanes wrt to their wing loading than earlier designs? I see this from the Light Aircraft Association http://www.lightaircraftassociation....0envelopes.pdf. I like what was quoted from VLA: "It is recommended that Vc should not be less than 0.9 Vh" Perhaps Francis Donaldson could fill in the history and perhaps explain why the AC mangles the text of the rule instead of changing the rule. Of course, we have a fancy new FAR 23 now and I eagerly await a new AC 21-19.

Originally Posted by AC103 View Post
Please do share any designs you know of that have Vcmin below 38 √ W/S. That would be a very interesting list. I have not managed to find any yet besides the later Cherokees.
The Aviat Husky series is one that I am very familiar with. The current model A-1C has W/S of 12 so 33 √ W/S = 114 kts but Vc used in design and Vno is 103 kts.

Originally Posted by AC103 View Post
Don't get me started on design vs operational Va... now that is a proper mess.
Sorry but I take more interest in this now, having retired from engineering and instruct aerobatics these days. I like the bit in AC 21-19A about "For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor." Sometimes my discussions with instructor trainees venture into that area. Perhaps the subject of another thread.
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Old 23rd May 2020, 21:47
  #23 (permalink)  
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Originally Posted by djpil View Post
Perhaps Francis Donaldson could fill in the history and perhaps explain why the AC mangles the text of the rule instead of changing the rule…
…The Aviat Husky series is one that I am very familiar with. The current model A-1C has W/S of 12 so 33 √ W/S = 114 kts but Vc used in design and Vno is 103 kts…
…I like the bit in AC 21-19A about "For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor."... Perhaps the subject of another thread.
Indeed as you say the VLA also provides the updated guidance on Vc. Adding the context.

You are not constrained to use these minimum values. Indeed, if your aircraft has high performance in terms of a high cruising speed, the value of Vc would be much greater than the minimum. Additionally:

It is recommended that Vc should not be less the 0.9 Vh
Forgive me if I am stating the obvious but as I understand it updating a rule is slow, onerous and expensive in comparison to issuing letters of interpretation https://www.faa.gov/about/office_org...terpretations/ or updating ACs. (NPRM, comments, replies to comments etc.) Here is a good example. The wording of Part 43 Appendix A and AC43-12A that conflict with the FAR 1.1 definition of preventative maintenance have still not been updated 11 years (ATOW) after the Coleal Inerpretation from 2009.

43 Appendix A: On the list of 32 preventive maintenance items.
(c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations:
AC43-12A:
If a task or maintenance function does not appear in the list, it is not preventive maintenance.
Coleal Interpretation: (David Coleal from Lear asking if crews are allowed to check the ~200psi tire pressures on a Lear 60 required daily under an Airworthiness Limitation)
https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/data/interps/2009/Coleal%20-%20(2009)%20Legal%20Interpretation.pd

Yes, crews can check the tire pressure under Part 91
AND
Even though the introductory text of subparagraph (c) states that "preventive maintenance is limited to the following work....", in view of the broader definition of preventive maintenance in section1.1, we believe that such limitation is not controlling.
AND
we also believe that the… sentence in Advisory Circular 43-12A, Preventive Maintenance … is overly restrictive
AND
the lists are better viewed as examples of the tasks in each category-they cannot be considered all-inclusive
AND
There are, no doubt, many "simple or minor preservation operations [tasks]" and many" replacement of small standard parts not involving complex assembly operations" performed daily, especially on small general aviation aircraft, that the agency would consider to be preventive maintenance, though they are not included in the 32 listed items. It is our understanding that Flight Standards' Aircraft Maintenance Division is planning to clarify this issue in a future revision to the AC.
So owners/operators have more scope to undertake work of comparable ease, complexity and risk as those on the list and make log book entries in accordance with CFR 43.9 with the date, the description of the work performed and signed with their pilot’s certificate number. So long, of course, as they follow the acceptable methods, techniques and practices out lined in CFR 43.13 eg. they have the relevant AMM, IPC, SB’s and tools at hand and the work is performed to a standard at least equal to its original or properly altered condition (with regard to aerodynamic function, structural strength, resistance to vibration and deterioration, and other qualities affecting airworthiness) and ‘should’ (though not required for pilots even though it is required under Part 66 for mechanics) have been supervised for the first time that they perform the task.

Yet the regs and the AC still say they cannot to this day.



Thanks for the Husky

You are totally on the right track with Va. Yes probably for another thread. It may well have already been flogged here due to the AA587 accident.

I sure would love to talk to a Piper engineer to understand which structure they believe would not be able to withstand loads at proper Vc min for its wing loading.

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Old 27th May 2020, 15:48
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Originally Posted by AC103 View Post
Oggers, kind of you to say and thank you for your input, now what is your response to the later clarification by the FAA in AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27

FYI per the Archer III POH Speed Power chart 5-21 Vh is 133 KTAS = 153 MPH 0.9 = 135.75 MPH so a Vno of 140 MPH CAS is conceivable if we ignore the above AC but personally I would prefer not to ignore a later clarification. Understood from Piper's perspective this is hindsight.
Yes I agree the AC indicates the Vc cannot be less than the wing loading formula. However, the regulations themselves do not say that and ".... this AC is not mandatory and does not constitute a regulation". Frankly, I think the AC is badly written on that point. It says, quite literally, if designing an aircraft with a cruise speed higher than the wing loading formula, make Vc equal to 0.9VH. But in para 45a it says the designer may establish the Vc in the range, "[wing load formula] ≤ Vc ≤ 0.9VH". So imo the AC manages to be both more ambiguous and more restricitive than the regulation it was based on. At the end of the day the FARs of the time said Vc need not be higher than 0.9VH, and it seems that is what Piper did.

With regards to the problem taking care of its self, this issue is that if an aircraft design increases its MTOW on paper and then artificially reduce their Vno as their structures are not able to comply with the gust penetration requirements this is an indication of a weaker than required air frame which becomes relevant in descent and this is where it gets more interesting.
I don't entirely agree with what you are saying there. The Vno of the Archer 3 didn't change from the Archer 1. The heavier aircraft complies with gust load factors at the same airspeed as before. I assume they had enough margin in the original structure to up the weight at the same design Vc (but for all I know they may have beefed something up). In anycase, to my mind if they needed to reduce Vno in order to achieve a higher weight this is not a weaker airframe, it is the same airframe optimised for a different task.
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Old 27th May 2020, 21:47
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Originally Posted by oggers View Post
"At the end of the day the FARs of the time said Vc need not be higher than 0.9VH, and it seems that is what Piper did.
It would seem that the majority of manufactures did not and many also provided a good buffer, which is generous of them as Vd is tied to Vc. More understandable in a Husky less so in a commuter.

The Vno of the Archer 3 didn't change from the Archer 1.
Precisely, this is what I mean by a 'reduced' or artificially low Vno.
To my mind if they needed to reduce Vno in order to achieve a higher weight this is not a weaker airframe, it is the same airframe optimised for a different task.
I would say that all the Cherokees share an equivalent task. The heavier ones are now more boxed in on descents.

Also note that the in Rev 44 of the PA-28 TCDS the Archer III was given a Va of 108KCAS, later changed to the 111KCAS at 2550lbs – 89KCAS at 1634lbs we have now (rev. 59). Unclear as to why. It looks like original Hershey bar winged PA-28s use 112KCAS for Va. When the longer winged Archers came out it went down to 110KCAS then 108 with the taper wing, then revised to the range we have now. Also noteworthy is the reduced aileron control surface movement limitations in the taper wing PA-28s even when compared with the long Hershey bar winged Archer. (25/12.5 vs 30/15) The first taper wing aircraft, the 1973/4 PA-28-161 Warriors, had the spar failure issues and early serial numbers that also ended up with reduced aileron control surface movement limits.

I appreciate your thoughts oggers but I think the later heavier Cherokees are an example of an outlier here.

Last edited by AC103; 27th May 2020 at 22:58.
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Old 28th May 2020, 10:37
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I would say that all the Cherokees share an equivalent task. The heavier ones are now more boxed in on descents.
Hello again. The objective fact remains that the aircraft structure meets or exceeds the regulatory strength requirements if, and only if, operated within the certified flight envelope, the same as any other aircraft. You have specifically mentioned descent being an issue twice now but I don't undertsand what you are getting at there?
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Old 28th May 2020, 19:34
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Originally Posted by oggers View Post
Hello again. The objective fact remains that the aircraft structure meets or exceeds the regulatory strength requirements if, and only if, operated within the certified flight envelope, the same as any other aircraft. You have specifically mentioned descent being an issue twice now but I don't undertsand what you are getting at there?
The majority of other manufactures including Piper for its other aircraft chose to design around the more onerous interpretation that is not in direct conflict with later advice from the FAA and provides a more useful and safer operational envelope.. So this is absolutely not an objective fact as outlined in the post regarding the Coleal Interpretation. To be fair I think I very clearly spelt out the operational considerations around descent with comparisons in post #21
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Old 29th May 2020, 15:09
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Originally Posted by AC103 View Post
To be fair I think I very clearly spelt out the operational considerations around descent with comparisons in post #21
Ok, so the quote below is what we are talking about from post 21:

With regards to the problem taking care of its self, this issue is that if an aircraft design increases its MTOW on paper and then artificially reduce their Vno as their structures are not able to comply with the gust penetration requirements this is an indication of a weaker than required air frame which becomes relevant in descent and this is where it gets more interesting.

The Archer III POH specifies a Fuel, Time, Distance to Descend chart based on an engine RPM (2500) and KIAS (122) which puts it an additional 3 knots under Vno, limiting the descent rate to only ~450fpm whilst keeping the engine warm.
This configuration is mentioned again in the Descent subsection of Normal Procedures in reference to carb ice. (The Archer III has a cooler air pick up than previous 4cyl PA-28s) so its descent profiles now start to look limited by the low Vno.
These Fuel, Time, Distance to Descend charts are published for the later Pipers (Seminole, Malibu, Warrior 161s, Archer 181s etc) and not published by the other manufactures singles mentioned above including popular IFR trainers like the DA-40 and DA-42 where these tables would be more expected.
The key point in there is where you say "descent profiles now start to look limited by Vno". Would you prefer the recommended descent speed was higher than Vno or where exactly? The structure does comply with the gust penetration requirements. You are reading too much into this. The recommended descent profile is 'nice to have'. It is not a limitation. It is not even an unduly limiting recommendation because it turns out to be one knot faster than a 75% cruise at 4000' DA (according to Fig 5-21 of the POH).

The majority of other manufactures including Piper for its other aircraft chose to design around the more onerous interpretation that is not in direct conflict with later advice from the FAA and provides a more useful and safer operational envelope.. So this is absolutely not an objective fact
The aircraft is certified. That is the fact. Its flight envelope provides the same safety factor of 1.5 between limit loads and ultimate loads as every other certified aircraft.

The AC is: "not mandatory.....not a regulation.....not the only means of compliance". Other manufacturers have complied with certification by using the other of two alternative and equally valid criteria in the regulations. The reg has since been reduced to something vague by the new FAA part 23, but the original certification basis is still extant in EASA land where:

"(3) VC need not be more than 0·9 VH at sea level."


This aircraft is fully compliant. There is no "deal with the FAA here to allow increases in gross weight for the Cherokees without complying with the Vno requirements?". because it complies with the pre-existing foreign regs too. You are barking up the wrong tree.
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Old 29th May 2020, 21:23
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Originally Posted by oggers View Post
Ok, so the quote below is what we are talking about from post 21:
The key point in there is where you say "descent profiles now start to look limited by Vno". Would you prefer the recommended descent speed was higher than Vno or where exactly? The structure does comply with the gust penetration requirements. You are reading too much into this. The recommended descent profile is 'nice to have'. It is not a limitation. It is not even an unduly limiting recommendation because it turns out to be one knot faster than a 75% cruise at 4000' DA (according to Fig 5-21 of the POH).
The aircraft is certified. That is the fact. Its flight envelope provides the same safety factor of 1.5 between limit loads and ultimate loads as every other certified aircraft.
The AC is: "not mandatory.....not a regulation.....not the only means of compliance". Other manufacturers have complied with certification by using the other of two alternative and equally valid criteria in the regulations. The reg has since been reduced to something vague by the new FAA part 23, but the original certification basis is still extant in EASA land where:
"(3) VC need not be more than 0·9 VH at sea level."
This aircraft is fully compliant. There is no "deal with the FAA here to allow increases in gross weight for the Cherokees without complying with the Vno requirements?". because it complies with the pre-existing foreign regs too. You are barking up the wrong tree.
I disagree with pretty much all of that oggers, as previously clarified. Also there is no tree, no-one is barking and reading and writing is all that we are doing here. Please don't drag this thread down there.
My objective when discussing Aviation via an HID is to stimulate deeper understanding and help protect the communities I care about from potentially risky dark corners. There are many of those in this world worth shining a light on, this is one of them. And your objective here?

There is no doubt the later heavier Cherokees are an outlier by a meaningful margin on declared Vno relative to wing loading that does impact operations and safety margins when compared to its peers, and there is also reasonable ground that the designers may have misinterpreted the regs.

If the regs were written as follows there would be no question on the interpretation,
§ 3.184 Design air speeds.
The design air speeds shall be chosen by the designer except that they shall not be less than the following values:
Vc (design cruising speed)
= 38 √ W/S (NU)
= 42 √ W/S (A)
except that:
(a) For values of W/S greater than 20, the above numerical multiplying factors shall be decreased linearly with W/S to a value of 33 at W/S=100:
(b) The required minimum value need be no greater than 0.9 Vh actually obtained at sea level.


But they do not say that. They says this:
§ 3.184 Design air speeds. The design air speeds shall be chosen by the designer except that they shall not be less than the following values:
Vc (design cruising speed)
= 38 √ W/S (NU)
= 42 √ W/S (A)
except that for values of W/S greater than 20, the above numerical multiplying factors shall be decreased linearly with W/S to a value of 33 at W/S=100: And further provided, That the required minimum value need be no greater than 0.9 Vh actually obtained at sea level.


This begs the question what are they 'providing' for.
What they were actually providing for is later clarified in AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27
Why would I want to define VC as equal to 0.9 VH? Use this definition if you are designing an airplane that is capable of a sustained speed (VC) higher than that obtained by using the wing loading (W/S) formula.
So the regs would have been better written as follows:
§ 3.184 Design air speeds. The design air speeds shall be chosen by the designer except that they shall not be less than the following values:
Vc (design cruising speed)
= 38 √ W/S (NU)
= 42 √ W/S (A)
except that for values of W/S greater than 20, the above numerical multiplying factors shall be decreased linearly with W/S to a value of 33 at W/S=100: And further provided,
for an airplane that is capable of a sustained speed (VC) higher than that obtained by using the wing loading (W/S) formula, the required minimum value need be no greater than 0.9 Vh actually obtained at sea level.


This is the interpretation that majority of design teams complied with including Piper teams for their other models.

As an operator this is reassuring as it requires designers to build stronger aircraft with wider envelopes that don't have their descent profiles as tightly boxed in between engine care and Vno as these later Cherokees. This has a significant real world impact on tactical options available during normal operations for dealing with weather, ATC etc.

Again please review the post on the Coleal Interpretation to understand why there are still may ambiguous and just plain off the reservation regs still in black and white today that the FAA do not want us to comply with as written.

If I was to guess I would say that there was some doubt about the longer tapper wings being able to cope with a normal >38 √ W/S based Vc combined with pressure to increase the MTOW for things like air con maybe, especially considering the number spar issues and changes to Va and control deflection limitations that occurred as this wing was introduced.

It would be very nice to hear from a Piper engineer involved though as that is just a guess.

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Old 30th May 2020, 00:47
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Originally Posted by AC103 View Post
What they were actually providing for is later clarified in AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27
Much later, after the fact. That AC wasn't written until after myself and others had gotten out of the game.

Originally Posted by AC103 View Post
If the regs were written as follows there would be no question on the interpretation
Interpretation of the rule by that AC means that text itself is only guidance so why is it in the regulation? If just guidance it should only ever be in guidance material.

Originally Posted by AC103 View Post
This begs the question what are they 'providing' for.
After calculating Vcmin by that formula read the next line "(2) For values of W/S more than 20, the multiplying factors may be decreased linearly with W/S to a value of 28.6 where W/S =100." So, if applicable, we can redo the calculation of Vcmin using a lower multiplier, despite (1) stating "Vc(in knots) may not be less than" the number we had first calculated. So move onto the next line (3) as another rule that allows us to change the number first calculated.

I distinctly recall my old Uni lecturer, Henry Millicer, telling me about FAR 23 flight envelopes as above. He was fresh from being Chief Designer of the Aircruiser back in the '60s. The CT/4 was developed from the Aircruiser and the current model, the CT/4E (aerobatic, higher weight than earlier variants and 50% more power), has a Vc of 147 kts (same as previous variants) per the NZ TCDS but my calculated Vcmin is 162 kts.
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Old 31st May 2020, 23:08
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AC103, you say:
This begs the question what are they 'providing' for.
What they were actually providing for is later clarified in AC 23-19A - Airframe Guide for Certification of Part 23 Airplanes p.26-27
It makes no sense for the FAA to use an AC issued in 2007 to clarify a phrase they removed from the regs in 1965. The phrase "further provided..." was dropped when CAR 3 became Part 23, in favour of:
"(3) VC need not be more than 0.9 VH at sea level"
.....clearly stated in the source regulation. Read in context with the rest of 23.335.a it means that if the chosen Vc is less than 33.√(W/S) it must be at least 0.9 VH. And in anycase "This AC provides information and guidance concerning acceptable means, but not the only means, of complying with [part 23]...Material in this AC is neither mandatory nor regulatory. AC methods may be freely chosen, or ignored, by an applicant who seeks to demonstrate regulatory compliance".
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Old 8th Jan 2021, 03:09
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Originally Posted by oggers View Post
.....clearly stated in the source regulation. Read in context with the rest of 23.335.a it means that if the chosen Vc is less than 33.√(W/S) it must be at least 0.9 VH.
Just happened to be looking at the old Appendix A for another reason CFR-2012-title14-vol1-part23-appA.pdf (govinfo.gov) and noticed the different text there .... Vcmin need not exceed 0.9Vh.

Furthermore, the old CAM400.pdf (faa.gov) is quite clear with "... the required minimum value need be no greater than 0.9Vh".

Last edited by djpil; 8th Jan 2021 at 03:27. Reason: added CAM
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