LSA Weight waivers?

[BrianL99]

A friend of mine works for Terrafugia and was telling me about a request they made to the FAA this week, for a "weight waiver". I questioned whether the FAA had the power to grant the waiver and he told me the Icon got one.

http://www.aero-news.net/index.cfm?do=m ... 2b1ddcc4c4

http://www.aopa.org/News-and-Video/All- ... ?CMP=ADV:1

Where does the FAA get the power to issue a waiver from ASTM standards and what's next on the waiver horizon?

Why isn't everyone with "added safety components" (i.e. parachute) requesting waivers?

[CTLSi]

......

[BrianL99]

ICON got one because they convinced the FAA that their so called 'anti spin' capability was compelling. ICON also gets to be heavier because they are a water plane.

Terrafuga won't get one just because they can't 'fit' in the LSA category. And they are not a water plane.

I don't like the FAA making exceptions. I think the ICON is now a violation of the category. Why make some abide and not others? It's utterly arbitrary.

As was once said about me at a Town Meeting, I don't really want anyone to know I agree with him, but I agree.

[MrMorden]

The 1320lb limit is set by the FAA in the FARs. ASTM only sets the methods and standards for LSA construction. The performance and weight limitations is regulatory and set by FAA.

That said, exemptions are BS. Making everybody abide by the same limits, unless they can sweet talk the FAA smacks of favoritism. Every LSA maker mounting a ballistic chute should sue the FAA on equal protection grounds. The chute adds at least as much safety as Icon's anti-spin wing but gets no exemption.

[BrianL99]

The 1320lb limit is set by the FAA in the FARs. ASTM only sets the methods and standards for LSA construction. The performance and weight limitations as regulatory and set by FAA.

That said, exemptions are BS. Making everybody abide by the same limits, unless they can sweet talk the FAA smacks of favoritism. Every LSA maker mounting a ballistic chute should sue the FAA on equal protection grounds. The chute adds at least as much safety as Icon's anti-spin wing but gets no exemption.

That's what I was looking for.

I've been told there's very good chance that Terrafugia is going to get the waiver.

[drseti]

As I've stated before, these exemptions degrade safety, rather than enhancing it. Terafugia is asking for increases in both weight and stall speed. This increases three ways the kinetic energy to be dissipated in the event of a takeoff or landing accident: once because KE varies with m, a second time because KE varies with v, and a third time because KE varies with v squared. I will be filing an oposition to the waiver request, which will include my engineering assessment of KE.

[drseti]

Here is a letter to the editor on the subject, just sent to EAA Sport Aviation:

As a PhD aerospace engineer specializing in GA safety (and a onetime recipient of the EAA Safety Achievement Award), I find Terrafugia's request for a further LSA weight and stall speed exemption (Sport Aviation, February 2015, p. 14) disturbing on several levels. Don't get me wrong; I'm enthralled by the concept of a roadable aircraft, and wish the company nothing but the greatest success. But, simple physics shows the folly of their request.

First off, it's important to dispel the myth that the LSA weight and stall speed restrictions are arbitrary. They are in fact based upon solid science. Kinetic energy is a function of mass and velocity. In any accident, whatever KE the aircraft is carrying has to be dissipated as the aircraft comes to a sudden stop. In a takeoff or landing accident, some fraction of that KE will be absorbed by the occupants. So, in the interest of maximizing survivability, we strive to keep that KE low. (That's why many of us choose to fly light aircraft with low stall speeds).

In Terrafugia's case, it is no surprise that they have sought relief from both the maximum gross weight and the maximum stall speed restrictions of the LSA category. That is because the two are highly correlated. As every student pilot learns, stall speed increases with weight (actually, by the square root of load factor, which itself increases linearly with changes in weight). So, if you make a plane heavier, its wing will naturally stall at a higher speed. The 1800 pound maximum gross weight being sought by Terrafugia is 40% above the 1320 pound LSA limit, so its wing, if unchanged, would naturally stall at...53 knots (just marginally below the 54 knot exemption they are seeking).

But what has all this to do with accident survivability? In a takeoff or landing impact at the mandated 45 knot stall speed, a fully loaded (1320 pound) LSA has 160 kiloJoules of energy to dissipate. If we increase weight to 1800 pounds, and allow stall speed to increase to the requested 54 knots, that KE nearly doubles, to 316 kJ. One could reasonably expect that doubling KE would roughly double the fatality rate in a given accident scenario.

Those who choose to fly Light Sport Aircraft do so with a certain expectation of a relative safety advantage. Waive the rules, and that advantage goes out the window. So, just as a fat ultralight was no longer an ultralight (a major impetus for the LSA rules in the first place), so should a fat Terrafugia no longer be considered an LSA.

H. Paul Shuch, EAA 1017932
Lock Haven PA

[snaproll]

As one who has designed, built, and tested aircraft, I am in complete agreement with Paul. As for my opinion, the waiver should be denied and they should be thrown into Part 23 if they want to go forward. VR.. Don

[deltafox]

Thanks Paul. Very well written, scholarly. Unfortunately, no good deed goes unpunished. Standby.

[dstclair]

FWIW. I completely agree that the KE of an 1800lb plane is way outside the intent of the LSA class. That being said, I do think a precedent was set by ICON by getting a waiver for up to 1680lbs. I didn't agree with FAA on this one either but now that the cow is out of the barn, I would think a waiver to the same level (1680lbs) would be logical provided Terrafugia can show their design provides an equivalent level of safety as the average 1320lb LSA. I know, logic and the FAA don't mix but i can dream

[drseti]

I would think a waiver to the same level (1680lbs) would be logical provided Terrafugia can show their design provides an equivalent level of safety as the average 1320lb LSA.

I concur, Dave. To provide a safety level equivalent to a compliant LSA, an aircraft weighing in at 1680 pounds would have to have a calibrated clean stall speed of 39.8 knots or less. Only if a manufacturer can accomplish that, would I support a weight waiver.

It would have been even better if FAA had set a KE limit instead of a weight plus stall speed limit, and then left it up to the manufacturer to determine how to meet that spec. However, compliance would be more difficult for the non-physicist pilot (or more to the point, a non-physicist FAA employee) to verify.

[FastEddieB]

Professor,

I'm following you to a point.

I can see the hazards that extra speed leads to, literally squared! You've seen me argue for landing as slowly as possible for just that reason. No problem there.

But I don't quite see how extra weight, in and of itself, is hazardous to the pilot.

Imagine two planes, one at 1,320 lbs and the other at 2,640 lbs. but through advanced airfoil design, or mumbo jumbo, or whatever, they both stall and land at the same speed.

I can see in an accident, the increased KE would be a huge factor in damage or injury to whatever they hit. It's less clear why the pilot and passengers would be at increased risk. Seems like the extra mass might even provide some modicum of protection.

By analogy, I'd rather be hit by a Smart Car than a Land Cruiser. But all things being equal, does not the increased mass of the Land Cruiser actually aid the occupants, KE notwithstanding?

I assume if I'm fuzzy on this, others may be as well.

[SportPilot]

.......

[drseti]

does not the increased mass of the Land Cruiser actually aid the occupants, KE notwithstanding?

Not in and of itself, Eddie. Sure, if the increased weight is comprised of safety items such as airbags and crush zones, the occupants may actually incur a safety advantage. But, if the extra weight is just more of the same (engine, airframe, passengers, avionics, or baggage), then it does nothing to protect the occupants. If some fraction of the KE which those items add will end up being dissipated by the occupants, then fatality rate logically increases.

Bottom line: more weight, by itself doesn't improve crash-worthiness. A more crash-worthy design (which might require more weight) can. It all depends upon what you do with that weight increase!

[BrianL99]

does not the increased mass of the Land Cruiser actually aid the occupants, KE notwithstanding?

Not in and of itself, Eddie. Sure, if the increased weight is comprised of safety items such as airbags and crush zones, the occupants may actually incur a safety advantage. But, if the extra weight is just more of the same (engine, airframe, passengers, avionics, or baggage), then it does nothing to protect the occupants. If some fraction of the KE which those items add will end up being dissipated by the occupants, then fatality rate logically increases.

Bottom line: more weight, by itself doesn't improve crash-worthiness. A more crash-worthy design (which might require more weight) can. It all depends upon what you do with that weight increase!

I assume the Terrafugia has to meet all DOT standards, so it's going to have airbags and crush zones, isn't it?