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Background
Thunderstorms and lightning
are part of a global electric circuit. In compliance
with nature’s plan to maintain an electric
potential between the Earth surface and the ionosphere,
thunderstorms are a natural occurrence. The total
number of thunderstorms occurring at any given
time around the world is approximately 2,000. These
thunderstorms average about 100 lightning strikes
per second. To the flying public, lightning represents
one of the most terrifying environmental phenomena.
This apprehension to lightning is naturally founded
on the frequently occurring property damage to
ground-based objects and the human fatalities traditionally
experienced during severe storms. The National
Weather Service publication, Storm Data, recorded
3,239 deaths and 9,818 injuries from lightning
strikes between 1959 and 1994. Only flash floods
and river floods cause more weather-related deaths.
However, lightning strikes
to aircraft have not recently been a major cause
of aircraft accidents, though the potential of
damage or upset to electronic systems that perform
flight critical functions, to fuel systems, and
to structures made of composite materials remains
an important safety issue. Although commercial
aircraft experience a direct lightning strike approximately
once per year per aircraft, the damage is usually
confined to burn marks on the aircraft skin and
the trailing edges of wings or tail surfaces. The
minimal damage experienced by most aircraft can
be attributed to the widespread use of aluminum
(an excellent electrical conductor) for the skins
and primary structure, careful attention to ensure
that electrical paths are not disrupted by gaps
in the skin, and the use of mechanical and hydraulic
flight control systems, which are relatively immune
to the adverse effects of lightning. Initially,
the lightning will attach to an aircraft extremity
such as the nose or a wingtip. The aircraft then
flies through the lightning flash, which reattaches
itself to the fuselage at other locations while
the airplane is in the electric “circuit”
between the regions of opposite polarity. Most
of the current will travel through the conductive
exterior skin and structures of the aircraft and
exit off some other extremity such as the tail.
Lightning currents, therefore, do not usually enter
critical systems within the aircraft, and personnel
are protected from electrical shock hazards by
the highly conductive aluminum skins and structures.
Typical lightning strikes;
cloud to ground (left) and cloud to cloud (right).
Photograph courtesy of NOAA.
Nonetheless, certain aircraft
components and systems are of special concern because
of potential lightning effects. For example, some
strikes have splintered the nonconductive plastic
radar domes on the nose of some aircraft. Current
flowing through the aircraft structure can also
result in isolated arcing or sparking and heating.
If this occurs in a fuel tank, explosion, fire,
and catastrophic structural damage can result.
Fuel vapor ignition has been identified as the
cause of over 10 fatal lightning accidents in the
past. In 1958, a Lockheed Constellation experienced
fuel tank explosions after departing Milan, Italy,
for Paris, France, and in 1963, a Pan American
Boeing 707 exploded near Elkton, Maryland, with
82 fatalities. Several other Air Force and commercial
transport airplanes experienced similar lightning-related
accidents in succeeding years; and in September
1976, an Iranian Air Force Boeing 747 was destroyed
near Madrid, Spain, with a loss of 17. In both
accidents, ignition of fuel vapors caused explosions
which in turn resulted in structural failure of
the wing. Since those accidents, much has been
learned about how lightning can affect aircraft,
and protection design and verification methods
have improved. Civil aircraft now undergo a rigorous
set of lightning certification tests to verify
the safety of designs so that accidents such as
those just described are very rare today.
The expanding use of lightweight,
performance-enhancing composite materials for aircraft
structures and the use of low-voltage digital avionics
for flight controls, engine control, cockpit displays,
and systems management have resulted in new challenges
to ensure adequate lightning protection in the
design of new airframes and control systems in
order to maintain today’s excellent lightning
safety record enjoyed by civil transport aircraft.
For example, composite materials are relatively
poor electrical conductors; advanced techniques
such as metallization of exterior surfaces or fine
metal wires interwoven into carbon fiber composite
skins are required to provide adequate conductivity
for lightning currents. Despite this protection,
indirect lightning effects, including magnetic
fields and potential differences that occur between
different parts of the airframe during lightning
current flow, may induce transient voltages in
electrical and avionic systems. These effects may
upset or damage electronic control and display
systems that have not been lightning protected.
In addition to developing
appropriate protection design and verification
methods for the emerging advanced aircraft of the
future, an ongoing need has also been to continue
to update the characteristics of lightning that
affect aircraft structures and systems. To minimize
lightning strikes to airplanes, such fundamental
information as how lightning strikes are initiated
and how they interact with airplanes, electrical
properties, and operational techniques represent
key factors to continue to improve design practices
and operational safety. In-flight experiments have
shown that there are two types of aircraft lightning
strikes. Probably the most frequent type is lightning
triggered and initiated by an aircraft in a region
with an intense electrostatic field created by
cloud electric charges. The other type is the interception
of a branch of a naturally occurring lightning
leader by an aircraft.
Langley Research and Development
Activities
In the summer of 1977, an
unusually large number of thunderstorm-related
commercial airliner accidents prompted the chairman
of the National Transportation Safety Board (NTSB)
to issue an urgent message to U.S. airline management,
airline manufacturers, and researchers in government
and academia that highlighted the seriousness of
the problem and requested a concerted effort to
find solutions and methods to avoid such environmental
problems in the future. In particular, causal factors
such as hail, turbulence, wind shear, and lightning
were identified as key factors that should be addressed
by a national organized research program to improve
methods of detection of such phenomena and to define
the operational methods for coping with them if
they could not be avoided. The message carried
extreme urgency and focused national attention
on the hazards associated with severe storms.
At NASA Headquarters, Allen
R. Tobiason and John H. Enders enthusiastically
supported Langley’s plans to attack the storm
hazard problems. Within Langley, Joseph W. Stickle
interfaced with headquarters, industry, and academia
to lead the management of the overall effort. Langley
researcher Norman L. Crabill defined a broad technical
program that addressed each of the causal factors
(lightning, wind shear, turbulence, and precipitation)
in terms of prediction, detection, operational
procedures, and design standards.
From 1978 to 1986, the Langley
Research Center conducted the lightning element
of the NASA Storm Hazards Program to improve the
state of the art in storm hazards detection and
avoidance; additional efforts were directed toward
the protection of aircraft components against lightning-induced
damage. In 1978, a commercially available airborne
lightning locator was flown on a Langley DHC-6
Twin Otter aircraft to obtain preliminary information
on lightning characteristics by flying on the periphery
of thunderstorms. Project Manager Norman L. Crabill
and Project Engineers R. Earl Dunham, Jr., and
Bruce D. Fisher planned the flights, analyzed information
from the data system, compared measurements with
ground-based measurements of precipitation at NASA
Wallops Flight Facility, and reported their findings
at the 1980 Conference on Aircraft Safety and Operating
Problems held at Langley. The results of the study
showed no significant correlation of turbulence
and lightning to the in-flight measurements, contrary
to the initial claims for such systems.
Following the DHC-6 experiments,
Crabill formulated and led a more comprehensive
research program on storm hazards by using a specially
instrumented and lightning-protected NASA F-106B
aircraft. Prior to this program, the lightning
environment included in FAA and European standards
for aircraft lightning protection certification
and most military aircraft qualifications was based
upon the known aspects of the cloud-to-Earth lightning
strikes; many measurements of these strikes had
been made over the previous 50 years, mostly by
researchers interested in lightning effects on
electric power systems. Very little was known of
the characteristics of intracloud lightning strikes
that aircraft were (and are) believed to encounter
most frequently. This program subsequently became
internationally recognized for its unique contributions
to the knowledge base of the aircraft intracloud
lightning environment and interaction technology.
Langley DHC-6 Twin Otter
Storm Hazards research aircraft used in 1978.
In the F-106B program, Langley
researchers intentionally attempted to encounter
intra-cloud lightning strikes to quantify the electrical
characteristics of the intracloud lightning environment,
to determine aircraft lightning-triggering mechanisms,
and to identify atmospheric conditions conducive
to such strikes. Two basic questions were addressed:
What are the mechanisms that influence lightning
strike attachments to an aircraft? What are the
electrical and physical effects of these in-flight
strikes? The lightning electromagnetic effects
quantification research program, formulated and
led by NASA researcher Felix L. Pitts, was designed
to provide data from in-flight measurements of
direct-strike lightning characteristics to assess
the lightning environment for aircraft electrical/electronic
systems.
Langley’s flight research
programs, initially for the DHC-6 and later for
the F-106B aircraft, were conducted with flights
in Oklahoma and Virginia in cooperation with ground-based
guidance and measurements by the National Oceanic
Atmospheric Administration (NOAA) National Severe
Storms Laboratory and the NASA Wallops Flight Facility.
F-106B (NASA 816) research
aircraft during Storm Hazards Program in 1982.
Note paint spots applied to aircraft that denote
lightning attachment points.
Throughout its research studies
on lightning phenomena, Langley maintained close
working relationships with industry, the FAA, academia,
and unique commercial organizations with significant
experiences in lightning protection and characterization.
For example, a key participant in the Langley research
activities was J. Anderson Plumer of Lightning
Technologies, Inc. (LTI), specialists in eliminating
lightning hazards to advanced systems through research,
development, engineering, and testing services.
In addition, coordination of research efforts and
results was maintained with other lightning research
activities conducted by the U.S. Air Force and
the FAA.
Langley F-106B Research Aircraft
As Langley developed its
research plan for lightning studies in the late
1970s, a high-priority item was the acquisition
of a rugged, lightning-hardened aircraft capable
of extended flights within severe thunderstorms.
In January 1979, a two-seat NASA F-106B was transferred
from the NASA Glenn (then Lewis) Research Center
to Langley to serve as NASA 816 in the Storm Hazards
Research Program conducted from 1979 to 1986. During
the 1980–1986 thunderstorm seasons, the F-106B
aircraft made 1,496 thunderstorm penetrations,
during which an astounding 714 direct lightning
strikes were experienced. The F-106B was selected
because of its metal framed canopy, dual inlet
to single-engine, and delta wing configuration,
which minimized the potential for lightning effects
on the crew and engines, and the number of extremities
that would have to be instrumented to capture important
lightning data.
The flight project was managed
by Crabill, and the lead researcher was Bruce Fisher.
In addition to his analysis and research roles,
Fisher flew onboard the F-106B as the test engineer
in the rear seat of the aircraft during thunderstorm
penetrations and was in the aircraft for 216 of
the 714 strikes obtained in the program. Harold
K. Carney, Jr., the lead technician for electromagnetic
measurements, also flew on numerous flights as
test engi-neer. Project pilots included NASA pilots
Perry Deal and Philip W. Brown and Air Force pilots
Maj. Gerald L. Keyser, Jr., Maj. William R. Neely,
Jr., Lt. Col. Michael R. Phillips, and Maj. Alfred
J. Wunschel. The research program was designed
to provide data from in-flight measurements of
direct-strike lightning characteristics to assess
the lightning threat to aircraft with digital systems
and composite structures. The program also provided
data for the correlation of the relative location
and strength of the various severe storm hazards
of precipitation, wind, turbulence, and lightning
during the life cycle of severe storms.
Under the leadership of LTI,
NASA 816 was protected against the hazardous effects
of lightning by installing surge-protection devices
and electromagnetic shielding of electrical power
and avionics systems, improving protection of the
fuel tanks, and using JP-5 fuel instead of the
more volatile JP-4. A simulated lightning safety
survey test was performed on the aircraft prior
to each thunderstorm season. These on-ground tests
were performed with the aircraft manned, the engine
running, and all flight systems operating on aircraft
power. The instrumentation system measured key
electrical properties induced on the aircraft in
response to an intentional current pulse of known
amplitude and waveform that was generated by a
high-voltage capacitor discharge apparatus attached
to the nose boom. The current exited from the aircraft
tail and was returned to the generator using symmetrical
return wires. To further enhance hardening against
the effects of sustained lightning attachment to
the airframe, the aircraft exterior was stripped
of paint in 1983 to minimize lightning attachment
dwell times and melting damage. Electromagnetic
sensors installed throughout the aircraft and a
shielded recording system in the weapons bay recorded
the electromagnetic waveforms from direct lightning
strikes and nearby flashes. Several video, movie,
and still cameras captured the lightning attachment
and subsequent swept-stroke attachment patterns
along the exterior of the aircraft. An X-band,
color, digital weather radar displayed both airborne
and ground-based images of the weather systems
to the crew. An air sampling system was carried
in the weapons bay of the aircraft to obtain atmospheric
samples of air during the strikes, and a composite
research fin cap was also used to evaluate the
impact of lightning damage to composite materials.
Storm penetrations were flown at altitudes from
5,000 to 50,000 ft for a variety of atmospheric
conditions.
Damage to composite fin
cap on F-106B. Note burn areas
around inspection panel and near trailing edge
of cap.
A specially developed lightning
instrumentation system was developed in-house at
Langley. Felix L. Pitts, Mitchel E. Thomas, Robert
M. Thomas, Jr., and K. Peter Zaepfel conceived
and developed a unique system with ultrawide bandwidth
digital transient recorders housed in a sealed
power isolated enclosure in the missile bay of
the F-106B. For use in acquiring the fast lightning
transients, they adapted and devised electromagnetic
sensors based on those used for measurement of
nuclear pulse radiation. To aid understanding of
the lightning transients recorded on the F-106B,
Rod Perala led a team at Electromagnetic Applications,
Inc. (EMA), in mathematical modeling of the lightning
strikes to the aircraft.
NASA Storm Hazards Program
The objectives of the Storm
Hazards Program for Crabill and Fisher were focused
on three factors relative to aircraft lightning
strikes: electrical activity and aircraft initiated
(“triggered”) lightning, altitude and
ambient temperature effects, and turbulence and
precipitation effects. The lightning research community
was especially interested in the manner in which
lightning strikes occurred to aircraft. Two theories
existed, including one which hypothesized that
aircraft lightning strikes occurred because the
aircraft was approached by a naturally occurring
lightning leader. The second theory assumed that
the aircraft itself could initiate a lightning
flash when it enters an electric field associated
with cloud electric charges. The research conducted
by Langley with the F-106B, using onboard camera
systems and ground-based radar, provided the first
instrumented proof of aircraft-initiated lightning
flashes originating at the aircraft. Most aircraft
strikes were initiated by the F-106B at altitudes
above 20,000 ft. The data also confirmed that intercepted
lightning strikes could occur, with most of the
intercepted strikes occurring at altitudes below
20,000 ft.
Rearward view showing Bruce
Fisher in rear seat during lightning strike to
F-106B.
Note plasma streamers exiting from wingtip of aircraft.
Data on lightning strike
incidents as a function of altitude gathered before
the NASA program indicated that most of the lightning
strikes to operational civil and military aircraft
(regardless of geographical location) occurred
within 10∞C of the freezing level (0∞C).
However, when the F-106B flight program began in
1980 and 1981, intentional flights at ambient temperatures
within 10∞C of the freezing level resulted
in very few lightning strikes. In subsequent years,
radar was used to provide the flight crew guidance
to electrically active regions in the upper levels
of thunderstorms, resulting in hundreds of high-altitude
direct lightning strikes. In the NASA program,
the ambient temperature values for lightning strikes
ranged from 5∞C to -65∞C, and the peak
strike rates occurred for ambient temperatures
colder than -40∞C. During one research flight
through a thunderstorm anvil at 38,000 ft in 1984,
the aircraft experienced 72 direct lightning strikes
in 45 min of penetration time, with the rate of
strikes reaching a value of 9 strikes/min. Lightning
strikes were encountered at nearly all temperatures
and altitudes in the Storm Hazards Program; therefore
the indication is that there is no altitude or
ambient temperature at which aircraft are immune
to the possibility of experiencing lightning strikes
in a thunderstorm.
The most successful piloting
technique used during the NASA Severe Hazards Program
in searching for lightning was to fly through the
thunderstorm cells that were best defined visually
and on the airborne weather radar. Frequently,
heavy turbulence and precipitation were encountered
during these penetrations. However, the lightning
strikes rarely occurred in the heaviest turbulence
and precipitation, and occasionally there was no
lightning activity whatsoever. Most lightning strikes
(approximately 80 percent) occurred in thunderstorm
regions in which the crews characterized the turbulence
and precipitation as negligible or light. During
penetration of thunderstorms at low levels, lightning
strikes were found to occur in areas of moderate
or greater turbulence at the edge of and within
large downdrafts. Conversely, lightning strikes
experienced in the upper areas of thunderstorms
and in the vicinity of decaying thunderstorms most
frequently occurred under conditions of little
turbulence or precipitation.
The objective of the lightning
electromagnetics quantification research program
was to statistically determine the electrical parameters
of the intracloud lightning environment for aircraft.
The key finding of this research was that lightning
strikes to aircraft actually include multiple bursts
of current pulses that are significantly shorter
in time duration but more numerous than previously
believed. The bursts are also more numerous than
the more well-known strikes that occur in cloud
to Earth flashes (that aircraft are also required
to tolerate). This finding proved particularly
important from the standpoint of devising protection
of digital computers and other avionic systems
against upsets which might occur in response to
bursts of pulses that could be caused by lightning
on new airframes and control systems. These findings
are now reflected in lightning environment and
test standards used to verify adequacy of protection
for electrical and avionics systems against lightning
hazards. They are also used to demonstrate compliance
with regulations issued by airworthiness certifying
authorities worldwide that require lightning strikes
not adversely affect the aircraft systems performing
critical and essential functions.
This remarkable 8-year research
program peaked the interest of the international
lightning community and rapidly disseminated its
information via international symposia and industry
and government technical committees responsible
for updating environment and test standards applied
for design and certification purposes. For example,
the electromagnetics quantification research program
provided focus for the U.S. civil and military
lightning communities culminating in the National
Interagency Coordinating Group (NICG) on Lightning
and Static Electricity. The NICG consists of representatives
from NASA, FAA, U.S. Air Force, U.S. Navy, and
U.S. Army who coordinate research programs in these
agencies and sponsor symposia and conferences.
In recognition of the accomplishments of the electromagnetics
quantification research program, the Flight Safety
Foundation lauded the program for outstanding Contributions
to Flight Safety in 1989, and peers at Langley
chose the technical report on the activity as the
outstanding paper of the Center in 1991 (the H.
J. E. Reid Award).
In addition to its pioneering
efforts to obtain critical data for the commercial
and military operational fleets, the unique assets
operated by the program were used for other national
purposes. For example, in 1984, the NASA F-106B
was used in a cooperative NASA and Air Force Weapons
Laboratory test to compare the electromagnetic
effects of lightning with those produced by nuclear
blasts. Felix L. Pitts had generated cooperative
interests with the Air Force early in the F-106B
flight program; this led to the Air Force loaning
Langley an advanced 10-channel recorder that had
been developed by the Air Force for the measurement
of electromagnetic pulse data. Langley utilized
the advanced recorder in the F-106B flight tests,
vastly expanding the capability to measure magnetic
and electrical rates change as well as currents
and voltages on electric wires inside the aircraft.
In addition, the Air Force provided a researcher
to fly in the back seat of the aircraft and operate
the advanced equipment in July 1993, when 72 lightning
strikes to the F-106B were obtained. In the subsequent
electromagnetic pulse effort, the aircraft was
subjected to the output of a nuclear electromagnetic
pulse simulator at Kirtland Air Force Base, Albuquerque,
New Mexico, while mounted on a special test stand
and during flybys. Crabill and Plumer participated
in the Air Force Weapons Laboratory review of these
data.
NASA 816 during nuclear
electromagnetic pulse simulation testing at Kirtland
Air Force Base. Photograph on left shows aircraft
being hoisted to test platform and one on right
shows aircraft in place for tests.
Following the Storm Hazards
Program, the aircraft was used by Langley for flight
evaluations of an advanced aerodynamic concept
known as the vortex flap. In 1991, NASA 816 (the
F-106B) was retired after 25 years of NASA research
programs and transferred to the Virginia Air and
Space Center in Hampton, Virginia, for public display.
Applications
Aircraft certification and
flight safety authorities internationally require
that aircraft structures and systems critical or
essential to the safe flight of an aircraft must
be protected from significant lightning-induced
damage or system functional upset. These requirements
are fulfilled through a certification plan that
details the methods to be used to prove that the
lightning protection designs are adequate and in
accordance with applicable standards and regulations
through verification testing.
Data gathered by the Storm
Hazards Program provided vital information for
the designers of future advanced aircraft systems.
The lightning protection design and certification
testing of future aircraft will reflect a more
complete understanding of the in-flight lightning
environment than was available prior to this program.
The program also provided valuable guidelines on
the probability of lightning occurrences at various
altitudes and within various cloud conditions.
The airborne data, in conjunction with ground-based
data from the NASA Wallops Flight Facility, provided
the first verification that aircraft frequently
trigger their own lightning strikes in regions
where there is no lightning activity until the
airplane gets there. Additionally, atmospheric
science benefited from an experiment designed by
Langley’s Joel S. Levine to capture samples
of air directly struck by lightning. Analysis of
the samples disclosed a significant amount of NO2
in thunderstorms with electrical activity above
approximately 30,000 ft; this disclosure impacted
conventional wisdom regarding the relative amounts
of NO2 caused by natural and man-made sources.
This information has become
increasingly critical for emerging modern aircraft
that use low-voltage digital controls which might
be susceptible to system upsets or advanced composite
materials, which by themselves are significantly
less conductive than aluminum. These aircraft use
composites embedded with a layer of conductive
fibers or screens designed to carry lightning currents.
These designs are thoroughly tested before they
are incorporated in an aircraft.
Prototype of Glasair III
LP manufactured by Stoddard-Hamilton
Aircraft, Inc., undergoing direct-effect testing
at LTI’s laboratory.
The growing popularity of
kit-built composite aircraft and the growing desire
of some kit manufacturers to manufacture and sell
completed (and therefore FAA-certified) airplanes
also raise some concerns over lightning protection.
Because owner-assembled aircraft kits are considered
by the FAA to be “experimental,” they
are not subject to lightning protection regulations.
Many owner-built aircraft are made of fiberglass
or graphite-reinforced composites. Pilots of unprotected
fiberglass or composite aircraft should not fly
anywhere near a lightning storm or in other types
of clouds because non-thunderstorm clouds may contain
sufficient electric charge to produce lightning.
In response to these concerns, Langley sponsored
a Small Business Innovation Research (SBIR) project
for the development of cost-effective lightning
protection for kit-built aircraft. Conducted by
Stoddard-Hamilton Aircraft, Inc., and Lightning
Technologies, Inc., the program designed and tested
lightning protection against severe in-flight strikes
for Stoddard Hamilton’s fiberglass composite
Glasair III LP, a small high-performance, kit-built
aircraft. The Glasair III LP was the world’s
first composite kit aircraft to achieve lightning
protection to the level of FAA FAR 23.
The Langley lightning research
and development activities have made a very significant
contribution to improvement of the safety of modern
aircraft in the lightning environment, and this
is one reason that accidents caused by lightning
strikes are very rare today. The Langley research
followed other important research at the NASA Lewis
Research Center (now Glenn), begun in the 1960s,
to understand the causes of lightning-related fuel
tank explosions. This small (as compared with other
NASA aeronautics programs), but consistent, effort
within NASA to understand the effects of one of
the most dangerous flight environments has had
an impact on flight safety that is far out of proportion
to the resources that NASA has been able to devote
to this technology area.
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