Onera researchers analyze lightning strike data collected in the European FULMEN program for a better understanding of its interaction with aircraft.
Information was provided by P. Lalande,
A. Bondiou-Clergerie, and P. Laroche, Onera, Chantillon, France.
The French aerospace company, Aerospatiale has compiled lightning strike data from the FULMEN program into a public database, which contains data on in-flight and ground measurements, in-flight incidents, and manufacturer transfer functions. To understand aircraft lightning strikes better, researchers from Onera have extracted some of the FULMEN data for further analysis. Inflight data was taken from Convair 580 and C-160 Transall flights performed during the summers of 1985 and 1988, respectively.
Lightning is one of the natural threats that have to be considered for safety reasons in the design and the certification of aircraft. In the 1980s, several joint programs with instrumented aircraft (F-106 for NASA program, CV580 for FAA program, and C-160 for DGA program) were performed to investigate the interaction between lightning and aircraft.
Aircraft lightning strike phenomenon
In-flight experiments have shown that there are two types of aircraft lightning strikes. The most frequent case (90% of events) is lightning triggered by the intrusion of an aircraft in a region with an intense electrostatic field. The other case is the interception of a branch of a natural lightning by an aircraft.
For lightning triggered by an aircraft, the main chronological sequence of events can be investigated by using the typical E-field variation measured on all E-field aircraft sensors. These measurements show that there are two phases in the lightning strike to aircraft.
The first phase is characterized by the bi-directional leader inception and development. This phase begins when an aircraft flies into a region of a thunderstorm in which the electrostatic field reaches a critical value in the range of 50-100 kV/m. An electrical discharge (positive leader) is initiated from the aircraft and propagates in the direction of the ambient field. During the development of this leader (from ta to tb, Figure 1), a negative charge is injected into the aircraft, producing a positive variation of the E-field on the aircraft surface. Consequently, a few milliseconds later, the electrical conditions for the inception of a negative-stepped leader are reached. It develops from the aircraft and propagates in the opposite direction of the ambient field vector and positive discharge. The negative leader development injects positive charge, which reduces the negative net charge of the aircraft and leads to a negative variation of the E-field from tb to tc. Consequent to the negative leader development, the positive leader accelerates and branches, which produces a positive increase of the E-field.
Figure 1. E-field and current variation during the leaders phase. |
The second phase begins with the propagation of the bi-directional leader by the development of recoil streamers. It is characterized by groups of high-impulse currents, called bursts, separated by a few tens of milliseconds (Figure 2).
Figure 2. E-field and current variations during the recoil streamer phase. |
When a test aircraft intercepted lightning, slow E-field variations were recorded from t1 to t2 (Figure 3). The sign of these variations was different between sensors, indicating that this first phase corresponds to a variation of ambient field (leading to a polarization effect) rather than a variation of the aircraft net charge (as in the ab phase of triggered strikes). From t2 to t3, the E-field variations on all sensors indicate that the aircraft has acquired a net positive charge. This could be attributed to the simultaneous inception of a positive and a negative leader, the latter injecting a higher charge in the aircraft, or even to the inception of a single negative stepped leader. From t3, the E-field on all sensors increases toward a positive value as observed from time tc in triggered cases. The second phase, beginning at time trs1, is similar to the one observed in the triggered case.
Figure 3. E-field variation before the recoil steamer phase during a lightning strike intercepted by an aircraft. From t1 to t2, E-field variations depend on the sensor location. |
