Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
NM-08-27
June 6, 2008
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) advises you,
registered owners and operators of Transport Category Airplanes
equipped with carbon brakes and operated to/from airports where runway
de-icing (RDI) fluids are used, that the use of carbon brakes in
aircraft since the 1980s and the concurrent switch to more
environmental friendly organic salt RDI fluids have led to a concern
that is possibly safety related, and that corrective actions may impose
additional cost.
The current FAA-recommended SAE AMS (Aerospace Material Specification)
runway deicer specifications were developed with the endorsement of the
SAE G12 aviation industry representatives, which included both domestic
and foreign airlines, airframe manufacturers, and regulators. For
better protection of aircraft material and equipment, the FAA will
modify the SAE AMS specifications once the affected parties formalize
new testing protocol that has been formally endorsed by the SAE G12,
Aircraft Ground De/Anti-icing Committee.
Background
During the course of the last 18 months, aircraft manufacturers
have informed airworthiness authorities, including the FAA, that RDI
fluids containing organic salts (mainly potassium formate and acetate,
but other alkalis as well) are sprayed by the wheels, mainly during
aircraft take-off and landing runs. The fluid remains on the underside
of the aircraft and can be collected as ice and slush on the landing
gear. The worst condition is the spray between wheels, which drives the
RDI fluid directly into the brakes and, particularly, coats the
(carbon) heat sink plates, which are also used as the pressure plates
to provide braking. During landing gear retraction, the ice and slush
on the gear (now in a horizontal position) melt into the brake units
where they further absorb into the carbon discs. The presence of the
alkalis creates a catalytic condition, which lowers the temperature at
which oxidation occurs. This softens the carbon, causing it to flake
and crumble over time, reducing the life and long-term efficiency of
the brakes themselves.
Note: Thermal oxidation is a normal condition of brakes, which
occurs at elevated temperatures (400 - 500°C is the threshold at
which measurable oxidation can be observed). This is a known phenomenon
and does not impinge on brake life. Catalytic oxidation is a condition
where the alkalis lower the temperature of oxidation significantly and
lower brake life by crumbling, etc.
As a result, there is a danger of possible brake failure during
high-speed aborted take-off or dragged brake during normal take-off
(and subsequent overheat, once airborne) or excessive vibration during
any ground operation. It should be noted here that the center of the
brake unit cannot be easily inspected, and this is where its stator
couplings are indexed to the torque tube, mechanically linked to the
axle, thus transmitting the braking torque to the wheels. If the stator
couplings fail, the brake effectiveness will be diminished.
The FAA is evaluating the aforementioned information with regard to
potential continued airworthiness concerns on U.S.-registered aircraft
(e.g., the loss of braking during emergency situations, a rejected
take-off operation is potentially catastrophic). At this time, the
airworthiness concern is not an unsafe condition that would warrant
airworthiness directive (AD) action under Title 14 of the Code of
Federal Aviation Regulations (14 CFR) part 39.
Compliance with the U.S. Environmental Protection Agency (EPA)
Regulations (Clean Water Act/stormwater management) has led airport
operators to use environmental friendly RDI fluids such as potassium
acetate/formate. The resulting interaction of these fluids with
aircraft equipment [electronics and carbon brakes] is detrimental and
costly for the airlines. The FAA Airport Operational Regulations allow
use of the aforementioned fluids to maintain runway safety. Depending
on latest developments and advice from industry, a revision to Brake
and Wheel minimum performance standard (e.g., TSO-C135) could be
considered, if necessary.
In June 2006, the SAE G12 Fluids Committee established an SAE-G12-F
working group to address the specific issue of “Catalytic Oxidation of
Carbon Brakes” with members from Boeing, Airbus, brake vendors, runway
deicer fluid vendors, several airlines, airport authorities, and
airworthiness authorities (FAA, Transport Canada, EASA). The working
group has been meeting twice a year since November 2006 and has been
using a monthly telecon for updates.
In the meantime, the FAA:
Informs operators of
transport category airplanes by way of this Special Airworthiness
Information Bulletin to raise awareness of these issues;
Will continue to monitor the situations and associated developments; and
Will evaluate the need to
issue mandatory continuing airworthiness actions (i.e., airworthiness
directives) if airport measures alone are found unable to mitigate the
risk.
Recommendations
For owners/operators of transport category airplanes equipped with
carbon brakes and operated to/from airports where runway de-icing (RDI)
fluids are used, we recommend you do the following. During scheduled
maintenance of the landing gear wheels, carry out a detailed visual
inspection of the carbon heat pack for obvious damage, distortion,
missing elements or corrosion.
Reference publication: EASA Safety Information Notice No. 2008-19,
issued March 13, 2008, which can be found at:
http://www.easa.eu.int/ws_prod/c/c_sin.php.
For Further Information Contact
Mahinder Wahi, Aerospace Engineer, Transport Standards Staff, 1601
Lind Avenue SW, Renton, Washington 98057-3356; telephone: 425-227-2142;
fax: 425-227-1320; e-mail: mahinder.wahi@faa.gov.