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Regulatory Brief
Lake Aircraft Wing Spar AD
Contents:
- Summary
- The Issue
- Required Action
- Update
Summary
The FAA has issued an Airworthiness Directive requiring inspection and repair of wing spar
components on all Lake LA-4, LA-4-200, and Lake Model 250 amphibians. This brief provides
details about the issue and the AD.
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| Docket #: |
99-CE-27-AD |
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| AD #: |
AD-2000-10-22 |
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| Reference: |
Notice of Proposed Rule Making
REVO Service Bulletin B-79 (PDF, 85KB) (superceded by B-79 R1, see below)
SPA Comments on NPRM 99-CE-27-AD (PDF, 202KB)
REVO Service Bulletin B-79 R1 (PDF, 205KB)
Final Airworthiness Directive w/Request for Comments (PDF, 133KB)
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| Applicability: |
Lake LA-4, LA-4A, LA-4P, LA-4-200, and Lake Model 250 |
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| Procedure: |
Visually inspect spar doubler plate; replace if cracked.
Install REVO Kit B-79. |
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| Cost: |
$4,650 ($3,590 parts, $960 labor/16 hrs) for Lake LA-4 and LA-4-200 models
$7,000 ($5,800 parts, $1,200 labor/20 hrs) for Lake Model 250 |
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| Compliance Deadline: |
Whichever comes first:
(1) 50 hours time in service accumulated beginning June 20, 2000
(2) June 20, 2001 |
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| Comment Deadline: |
July 28, 2000 |
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| Comments Address: |
Send comments in triplicate to:
FAA, Central Region, Office of the Regional Counsel
Attention: Rules Docket No. 99-CE-27-AD
901 Locust, Room 506
Kansas City, MO 64106 |
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| FAA Contact Person: |
Richard B. Noll, Aerospace Engineer, FAA Boston Certification Office, 781/238-7160. |
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| Parts Contact: |
Aerofab (Lake factory), 207/324-3916. $500 deposit per kit, 2 kits per airplane. |
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| Shops: |
Amphibians Plus (Harry Shannon); Bartow, Florida; 863/534-8025.
Aircraft Innovation & Repair (Paul Furnee); Winter Haven, Florida; 863/299-4655
Lake Central (Elton Townsend); Gravenhurst, Ontario; 705/687-4343 |
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The Issue
Cracks have been detected in the wing spar cap angles of Lake LA-4-200's (Buccaneers) and Lake Model 250
(Renegades and Turbo Renegades), and in a doubler plate of a Lake Model 250. These cracks have been determined to
be related to fatigue, not corrosion or accident damage. There are four spar caps and doubler
plates per aircraft, one set each on the lower and upper terminus of each wing spar where the wing is attached
to the fuselage fitting with six bolts. The wing spar attachment points of a Lake Buccaneer are shown at right,
as seen looking up and back into the wheel well. The six bolts and fuselage attachment fitting are visible, as well
as the doubler plate. The spar cap angle is sandwiched between the doubler plate and the spar web, which runs vertically
between the upper wing surfaces. The fuselage is on the right hand side of the photo.
Figures 1 (left) and 2 (below) illustrate the lower left hand wing attach point. The most common crack is found in
the spar cap angle near the second bolt hole (marked in both figures). This crack originates in the radius where
the spar cap angle flange ends, and propagates towards the second bolt hole. Although a crack at the second bolt
hole is important, the spar cap angle need not be replaced if this crack is found.
A crack was found in one aircraft (a Renegade) at the #6 bolt hole (marked in Figure 2). Cracks in this location are
much more serious, and could result in wing separation. The #6 bolt carries 42% of the structural load, compared
to 5-10% of the structural load carried by the #1 and #2 bolts combined. The cause of the crack at the #6 bolt
hole is uncertain, although theoretically a crack at the #2 bolt hole could transfer load to the #6 bolt
hole and accelerate fatigue. If a crack is found at the #6 bolt hole in either the spar cap angle or doubler plate,
the cracked part must be replaced.
Several mechanics speculate that the cracks at the #2 bolt holt are caused by the wing twisting moment generated
during braking, beaching (gear down) and taxiing over rough surfaces. The flange termination radius on the spar
cap angle, the location where the #2 bolt hole crack originates, is the weakest point under torsional stress.
Further, imprinting on the doubler plates of aircraft with cracked spar cap angles confirms that torsional stress is
applied to the spar assembly.
Subsequent to the original discovery of the wing spar cap and double plate cracks, 28 aircraft (13 Lake Model 250's
and 15 LA-4-200's) were inspected with a borescope at Amphibians Plus, an shop located in Bartow,
Florida. Cracks were found in the spar cap angle near the #2 bolt hole (shown at right) in 92% of Renegades and
46% of Buccaneers inspected. Approximately 40% of all aircraft inspected showed cracks in the upper spar cap
angle, while 92% of Renegades and 26% of Buccaneers displayed cracks in the lower spar cap angle. The difference
in the failure rate of the lower spar cap angle is presumably due to gross weight and brake strength differences
between the two models. Renegades are heavier and have more powerful brakes.
No other aircraft has been found to have cracks at the #6 bolt hole.
Although none of the affected Lake models have a history of in-flight wing separation, the cracks discussed
above have the potential to result in wing separation in due time.
Required Action
The Airworthiness Directive requires that that all Lake model aircraft (excluding the
Seawolf, which is not a certified model) comply with the AD within 50 hours time in service
after June 20, 2000, or before June 20, 2001, whichever comes first. Aircraft that do not comply
with the AD within the specified time become grounded.
The final AD requires that the spar doubler plate be inspected visually, which can be accomplished
without removing or disassembling the wing. If cracks are found in the spar doubler plate, the part
must be replaced, which effectively means replacing the entire wing.
Kit SB-79 (which includes reinforcing doubler plates, attachment hardware, and spacers for the Renegade)
must be installed on each spar where the spar is attached to the fuselage. Each kit is sufficient for one
wing; two kits are required to repair one aircraft. Aerofab (the Lake factory) is requiring a $500 deposit
per kit ($1,000 per airplane) to secure a delivery slot for kits. The kit price is not presently gauranteed,
but the per-airplane price for orders received by May 15, 2000 was $5,800 per Renegade and $3,590 per Buccaneer.
Installation of the kits does not require wing removal. However, because of the position of fuel tanks on the
Renegade, those models must be defueled and partially disassembled before the kit can be installed. Labor
is estimated at 16 hours (both wings) for the Buccaneer and 20 hours (both wings) for the Renegade. At a shop
rate of $60/hr, that results in costs of $960 and $1,200, respectively.
Update
Airtech Canada has received FAA and Transport Canada approval for a wing doubler kit that
satisfies the
requirements of the Lake Amphibian wing spar AD. The alternate kits are priced at $1,700
U.S. ($2,500 CDN) for Buccaneers and $3,500 U.S. ($5,200 CDN) for Renegades.
Aerofab has reduced prices to match Airtech Canada's prices. The Airtech Canada kit is
sold as an STC, approved both in the United States and Canada. The STC has been further
approved in both countries as an Alternate Means of Compliance (AMOC), eliminating the
need for a determination of AMOC for each installation. Airtech Canada can be reached
at 705/743-9483, and Aerofab can be reached at 407/847-9000.
Most shops with experience in the kit installation report that the average installation
requires 5 to 6 man hours.
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