When do too many minor changes require a completely new design?

riggerrob

I really should change my personal text
Senior Member
Joined
Mar 11, 2012
Messages
2,058
Reaction score
1,503
This question arose during discussions about American Special Forces buying the Polish PZL MC-145B Wiley Coyote, twin-engined, light STOL transport with a bewildering array of sensors and weapons hanging from the nose, belly and wings. The belly pannier already makes it look fat.
By the third nose extension, it makes Cyrano de Bergerac's nose look dainty and petite in comparison.
At what point do engineers say "enough!" stop modifications at bulkhead XYZ and design an entirely new nose????????
I understand that the primary reason that the US military buys DHC-4 Cariboo, DHC-5 Buffalo, DHC-6 Twin Otters, Alenia G 111, Shorts Sherpa, Spartan C-27, etc. is because no American factories want to compete in the light twin STOL market.

As an aside, the US military gained foreign military sales approval for the similar-sized, brand-new, Cessna 408 Sky Courier mere days after it received FAA certification (spring 2022). Does this mean that the Cessna is better-sized for the mission or that the USA wants to "buy American," etc.

For an aside, we can continue the debate about when Boeing made too many minor changes to their 737 Max.

P.S. My professional background is in the parachute industry and I can tell you tales about dozens of minor changes forced upon the FAA to the point that no original parts will fit on the updated parachute harness/container.
 

Foo Fighter

I came, I saw, I drank some tea (and had a bun).
Senior Member
Joined
Jul 19, 2016
Messages
2,839
Reaction score
1,713
I suppose it SHOULD come down to external control of which changes are made and how often, from a safety POV. Internal audit and lack of proper oversight caused the 737 max fiasco.

There are military 737 variants suffering (Or suffered) from the same lack of accountability with tools left in wings for example and toilets that do not fit.
 

Stovepipe

ACCESS: Restricted
Joined
Apr 22, 2022
Messages
39
Reaction score
24
There are military 737 variants suffering (Or suffered) from the same lack of accountability with tools left in wings for example and toilets that do not fit.
That kind of thing is down to gross lack of inspection by the inspectorial staff in the hangars where the aircraft is being built or serviced. back when Inspectors meant something, no panel was ever closed until the Inspector had checked inside first. I did some time on 737 overhauls and routinely found swarf and rags and the odd drill bit and discarded rivet shanks. It turned out to be a wrangle between full timers and contractors (I was a Connie but knew a lot of the fulltimers well) about who's job it was to clean up/aircraft cleaners refused to clean out swarf as it was considered by them to be the bashers' job/crew chief wouldn't rein in the abuse being given by his full time mates to the connies. Some of it was just down to laziness and unprofessionalism and the worst kind of Union mentality.
 

riggerrob

I really should change my personal text
Senior Member
Joined
Mar 11, 2012
Messages
2,058
Reaction score
1,503
Bear in mind too that a new type will require a great deal more in the way of certification than an existing type that has been modified. I suspect this is the driver.
Yes. Sometimes the new certification data requirements are based upon more accurate measuring equipment.
For example, back in 1994 I was helping Rigging Innovations do drop tests on their P124A/Aviator Pilot Emergency Parachute system. We could have simply applied for certification as a "minor change" to the existing TSO C23C certification for the Talon series of "Talon" sport parachute harness/containers, but we were also certifying a new parachute canopy and the boss wanted to be able to sell to customers who weigh more than the 254 pound limit (on suspended weight) specified in the old TSO C23C (circa 1985).
Since the newer FAA TSO C23D required measurements of opening shock, we had to partner with another manufacturer who had the specialized instrumentation to measure opening shock hundreds of times per second.
 

BB1984

I really should change my personal text
Joined
Aug 21, 2011
Messages
102
Reaction score
114
This question arose during discussions about American Special Forces buying the Polish PZL MC-145B Wiley Coyote, twin-engined, light STOL transport with a bewildering array of sensors and weapons hanging from the nose, belly and wings. The belly pannier already makes it look fat.
By the third nose extension, it makes Cyrano de Bergerac's nose look dainty and petite in comparison.
At what point do engineers say "enough!" stop modifications at bulkhead XYZ and design an entirely new nose????????
I can think of two possibilities:
  1. Requirements are added incrementally rather than "all from the start." In this kind of situation each change is the minimal modification from where you were after the last change and, after a few iterations, you wind up with something radically different than what you'd have had if the initial design had included everything. It works from the customer end too: they get a budget for each modification that is always enough for the current work but never enough to go back and make big changes for overall efficiency and effectiveness, even if that would be more cost-effective in the long run.

  2. Designers are working around re-certification requirements, doing things in a way that is sub-optimal on purpose because the best solution requires so much regulatory work that it would sink the program (i.e., what you and Sabrejet have already gone back and forth about). Might be misremembering, but might even have been on this board that the story was a modernized helicopter was using antiquated machine guns, used no where else in the inventory, because qualifying a new machine gun for helicopter use was so time consuming and difficult that it would have caused program issues and no one wanted to accept the increased program risk and cost to deal with it.
 

pathology_doc

ACCESS: Top Secret
Joined
Jun 7, 2008
Messages
1,121
Reaction score
554
Wasn't the Spitfire II intended to incorporate as factory-built all the alterations and improvements the Spitfire I received from introduction to service? (Plus a beefier Merlin to maintain performance.)
 

Stovepipe

ACCESS: Restricted
Joined
Apr 22, 2022
Messages
39
Reaction score
24
The Mk1 was probably becoming a bit past it when you compare the 109 E4. I'd say it was tied in to rationalising the accumulated changes, as you say with an eye to future Merlin power changes coming down the track, as well as further refining the airframe for mass production
 

riggerrob

I really should change my personal text
Senior Member
Joined
Mar 11, 2012
Messages
2,058
Reaction score
1,503
Wasn't the Spitfire II intended to incorporate as factory-built all the alterations and improvements the Spitfire I received from introduction to service? (Plus a beefier Merlin to maintain performance.)
Mark I, A Model and 100 series are usually crude, weak and sluggish. Mark I is usually only short production run with numerous updates incorporated in the B Model.
Mark I usually retires after only a few years in service.
My question is more about the Mark VI or Mark VII that adds sensors or weapons that had not been invented yet when the Mark I first flew.
 
Last edited:

pathology_doc

ACCESS: Top Secret
Joined
Jun 7, 2008
Messages
1,121
Reaction score
554
I'd say it was tied in to rationalising the accumulated changes,
We are trying to say the same thing. You just said it better.

adds sensors or weapons that were not invented yet when they Mark I first flew.
Sensors aren't a thing for (the majority of) single-seat fighters in the Second World War - the main one was still the Mk I eyeball. (Yes, I know some American carrier fighters had a radar pod.) The question of adding sensors was one that tended to plague jets from the second generation onwards.

Weapons are of course their own special problem; we all know that the introduction of cannon to the Spitfire did not initially go smoothly, for example. The problems multiplied when missiles came along that demanded their own particular avionics. Sidewinder is a fairly easy retrofit, at least in its early forms, but I recall reading that the Australians played with the idea of fitting Firestreak to their Avon-powered Sabres, only to find that the only place they could fit the missile's support hardware was the gun bays - and while this was acceptable for a test aircraft, it was ruled as a no-go for combat. The pilots understandably and justifiably wanted to keep their guns.
 

riggerrob

I really should change my personal text
Senior Member
Joined
Mar 11, 2012
Messages
2,058
Reaction score
1,503
The original question was aimed at original factories introducing multiple modifications to the same basic airframe.

For civilian airplanes, the FAA issues Supplementary Type Certificates for major modifications, for example the various STOL kits offered by Robertson, sportsman, Ultimate Wing Beaver, etc.. STC kit designers had to demonstrate no degradation of weight, balance, strength, etc. AND improved performance in at least one corner of the envelope.
At what point does an STC change so many things that the FAA requires a completely new TSO certification?
 

Dynoman

ACCESS: Top Secret
Senior Member
Joined
Jul 29, 2009
Messages
1,146
Reaction score
652
AC 21-40A

"However, if we find that the proposed change is so extensive that it will require a substantially complete investigation of compliance with the applicable regulations, then the applicant will have to apply for a new type certificate."

14 CFR Part 21.19

"Each person who proposes to change a product must apply for a new type certificate if the FAA finds that the proposed change in design, power, thrust, or weight is so extensive that a substantially complete investigation of compliance with the applicable regulations is required."

There does not appear to be a percentage of change in weight, performance, or another specification (or a combination of changes) that would necessitate a change from an STC to a TSO. It appears to me that the tipping point for the decision is based on the FAA's ability to ascertain whether the modification are safe or if the changes are significant enough to necessitate operating under an STC or if requires the process for a new TSO.
 
Last edited:

Stovepipe

ACCESS: Restricted
Joined
Apr 22, 2022
Messages
39
Reaction score
24
In the case of the Spitfire, it would also involve changes to manufacturing processes in the factory and things like dimensional changes to the airframe and things like changes in riveting and bolting as the aircraft was changed. By the time it got to the Spitfire V, that was about as much could reasonably be extracted from the 1939 production standard airframe and more powerful Merlins, a taller fin, different radios, different gun layouts and so on were coming along. You can imagine the difficulty of keeping up with the drawing requirements alone.
 

Foo Fighter

I came, I saw, I drank some tea (and had a bun).
Senior Member
Joined
Jul 19, 2016
Messages
2,839
Reaction score
1,713
MK V onwards was basically a different aircraft, not the only time this happened. I recall a major change about MK XX and on but would need to check facts.
 

Stovepipe

ACCESS: Restricted
Joined
Apr 22, 2022
Messages
39
Reaction score
24
Bulkhead moved for heavier engine, last frame altered for bigger fin and rudder, undercarriage moved outwards, universal wing and multiple minor changes.
 

pathology_doc

ACCESS: Top Secret
Joined
Jun 7, 2008
Messages
1,121
Reaction score
554
I recall a major change about MK XX and on
Mk 21 was the variant designed from the ground up to take the two-stage, two-speed Griffon, and had a new, more rigid wing with much higher aileron reversal speed (the speed at which the ailerons cause twisting of the wing which nullifies their effect); the Mk21's aileron reversal speed was technically 850mph, much faster than ANY Spitfire could ever be expected to go. They also took the opportunity to design it for four 20mm cannon and nothing else. The rest as Stovepipe says.

It was almost called the Supermarine Victor I. Very few of them saw active service and none shot down an enemy aircraft in WW2. The stop-gap Mk 14, based on the Mk VIII airframe, was ready first and served in far greater numbers.

Mk XX is contentious, and I've read various accounts of what this airplane actually was. One says that it was the renumbered Griffon Spitfire IV, when that mark was reassigned to the PR variants. Another says that it was the F.21 with single-stage, two-speed Griffon.

Spitfire marks and their connection to the aircraft's evolution could be a book all by themselves, especially since the aircraft's service history was characterized by the lash-ups (e.g. V, IX, XII, XIV) being built and used in far greater numbers than any of the aircraft in the planned development pathway (III, IV, VIII, 21).
 

Foo Fighter

I came, I saw, I drank some tea (and had a bun).
Senior Member
Joined
Jul 19, 2016
Messages
2,839
Reaction score
1,713
Thanks for the clarification, much appreciated. I for one would buy that book.

I would like to see an archeological dig on the housing estate now located where the runway for the Castle Bromwhich factory was to reclaim the tooling for these later marks. Apparently still in the big holes they dumped everything in PW.
 

pathology_doc

ACCESS: Top Secret
Joined
Jun 7, 2008
Messages
1,121
Reaction score
554
I would like to see an archeological dig on the housing estate now located where the runway for the Castle Bromwhich factory was to reclaim the tooling for these later marks.
Translation: "I would like to assemble, using the discarded tooling, the 494mph Spiteful and use it to troll some of the slower jet airliners." :p
Wouldn't we all, mate... wouldn't we all.
 

pathology_doc

ACCESS: Top Secret
Joined
Jun 7, 2008
Messages
1,121
Reaction score
554
I for one would buy that book.
It could be fairly short, but might easily be huge - I need to go reference my huge Spitfire book tonight, actually.

The thumbnail sketch.
Mark I - the original classic.
Mark II - rationalized rebuild with all Mark I changes, more powerful engine.
Mark III - the first true attempt to make alterations/improvements to the airframe, including Merlin XX engine, cannon wing. Not proceeded with.
Mark IV - Initially the first Griffon Spitfire, one example; later reassigned to unarmed PR variants.
Mark V - Evolved Mark II offshoot to get Merlin 40-series engines into the air quickly.
Mark VI - First pressurized variant, single-stage, single-speed Merlin optimized for higher altitudes.
Mark VII - Pressurized airframe development with two-stage, two-speed Merlin.
Mark VIII - Unpressurized equivalent of VII. Supposed to be the definitive Merlin version, but eclipsed in production and service by:
Mark IX - Mark V-derived lash-up with Merlin 60-series engine.
Mark X - Essentially a PR version of Mark VII, limited numbers built
Mark XI - Essentially a PR version of Mark IX
Mark XII - First Griffon variant (single-stage two-speed), derived from Vc.
Mark XIII - Limited production armed PR with cannon removed and Merlin 32 for low level recon.
Mark XIV - First two-stage Griffon variant, derived from Mark VIII airframe.
Mark XV - Not used for Spitfires; was used for first, Mark XII-like Griffon Seafire.
Mark XVI - Designation for Mark IX with Packard Merlin, many had a bubble canopy.
Mark XVII - Not used for Spitfires, was used for a developed Seafire XV with bubble canopy.
Mark XVIII - Rationalized Mark XIV with bubble canopy, slightly strengthened airframe.
Mark XIX - PR variant of Mark XIV.
Mark XX - See above; seems to be the Mark IV renumbered.
Mark 21 - Definitive improved airframe with stronger, 4-cannon wing for Griffon 60-series.
Mark 22 - Bubble canopy variant of 21.
Mark 23 - F.21 variant with high-speed wing section, not proceeded with.
Mark 24 - Basically a 22 with a different electrical system.
 

Foo Fighter

I came, I saw, I drank some tea (and had a bun).
Senior Member
Joined
Jul 19, 2016
Messages
2,839
Reaction score
1,713
Much appreciated. Thank you.
 

Similar threads

Top