Summer Flying Hazards: Lightning Strikes and Radar Shadows

Mobility Forum, Jul/Aug 2005 by Rainey, Kevin

Lightning is seen clearly from the flightline at Joe Foss Field, Sioux Falls, S.D., during a storm. Short and forceful storms with lightning and thunder are a frequent occurrence during the summer months in South Dakota.

With summer thunderstorms rumbling through most of the U.S. and the rest of the northern hemisphere, the potential for damage to aircraft from lightning, turbulence, and hail increases. The following discussion summarizes current guidance and provides historical data on lightning strikes. A second related subject warns aircrews about the dangers of flying towards embedded thunderstorms or "radar shadows".

The most common reported weather hazard to aircraft is lightning strikes or electrostatic discharges (ESD). Since Dec. 1992, there have been 144 reported lightning strikes' or ESDs to Air Force aircraft. The average repair cost was approximately $103,000. This adds up to a pretty expensive "electric bill" for the Air Force.

So what can aircrew do to avoid these electric encounters? Good weather briefings are a start, but having a better understanding of the conditions favorable for strikes and discharges is critical. Further analysis reveals the vast majority of these strikes occurred below FL230, near the freezing level. Specifically, KC-135 aircraft have experienced 23 strikes in the same time period, and 21 have been below FL230, within 20NM of a thunderstorm or heavy rain shower, and operating within 5,000 feet of the freezing level. Research aircraft have shown that penetration of the upper reaches of a thunderstorm 35-40,000 feet, with temperatures below -40 degrees Celsius, provide the greatest potential for strikes and discharges.2 However, the majority of Air Force and civil incidents have occurred at lower altitudes in non-stormy clouds and in areas outside of active thunderstorm cells. It is thought that strikes/discharges of this type are triggered by the aircraft since they would not have occurred naturally in the absence of the aircraft. In most of these cases, the aircraft was operating in one or more of the following conditions.

a. Within 8 degrees Celsius of the freezing level

b. Within approximately 5,000 feet of the freezing level

c. In light precipitation

d. In clouds

e. In light or negligible turbulence

The above conditions are exactly what AFH 11-203 Volume 1, Weather For Aircrews, states are the conditions likely for lightning strikes or ESDs. The final ingredient is the aircraft. From AFH 11-203 Vol 1:

13.6.4.1. Electrostatic discharges are very similar to natural lightning but are triggered by the aircraft itself. Charges build up on aircraft when they fly through clouds or precipitation (liquid or frozen) or even solid particles such as dust, haze and ice. The aircraft's electrical field may then interact with charged areas of the atmosphere resulting in an electrostatic discharge. This discharge does not have to occur in a thunderstorm. Aircraft have reported damage from electrostatic discharges occurring in cirrus downwind of previous thunderstorm activity, in cumulus around a thunderstorm's periphery, and even in stratiform clouds and light rain showers. Electrostatic discharges usually cause minor physical damage and indirect effects such as electrical circuit upsets.

During the weather briefing, make a mental note of the freezing level and apply good risk mitigation measures to alter your route of flight to avoid flying through clouds or precipitation when passing through the freezing level. If mission requirements allow (i.e., a training sortie), heed the cautions in AFI 11-202 Vol. 3, General Flight Rules and delay your takeoff:

5.23.3. Flight Planned Route. When observed or reported thunderstorm activity adversely affects the flight plan route, pilots will delay the scheduled mission, alter the route of flight to avoid the thunderstorm activity, or proceed to a suitable alternate. Pilots shall use all available information including radar, PMSV, and PIREPs to avoid thunderstorm activity.

NOTE: Induced lightning strikes and electrostatic discharges can occur in what may look like benign conditions; a thunderstorm does not have to be present for these discharges.

If you are the victim of a strike or discharge, be wary of your electrical systems and instruments. Damage to aircraft electrical systems, instruments, avionics, and radar is possible. Transient voltages and currents induced in the aircraft electrical systems, as well as direct lightning strikes, make the accuracy of electronic flight control navigational systems questionable. After an electrostatic discharge or a lightning strike, consider all instruments invalid until proper operation is verified.3 Speaking of instruments, the airborne weather radar on most mobility aircraft is capable of providing excellent warnings of thunderstorm activity. However, there is a potential pitfall-the radar shadow.

An aircrew observes a line of thunderstorm cells along their route of flight. The crew requests to fly toward a very small line of yellow and red rain showers in between two larger red areas that appear to be about 10 miles apart. Seems logical right? Well if one understands the phenomenon of a radar shadow, it's possible that this may be a disastrous course of action. There is potential for this area to be the most hazardous part of the storm due to radar attenuation. This situation occurs when the electromagnetic energy emanated from the radar is completely absorbed by intense rainfall. Since all of the energy is absorbed, reflected back, and displayed on the multi-function display (MFD), no energy gets through the initial edge of the storm. Therefore, the display shows a "clear" (black) area just beyond the heavy rain. This is called a radar shadow. The KC-135R WXR 700X has a feature that can warn of this situation. It's called a Path Attenuation Correction (PAC) Alert.