Industry safety groups advise flight crews to avoid areas with lightning conditions as much as possible by using airborne weather radar. What did the sample of lightning strike incidents described in Part 1 of this article reveal about the use of airborne weather radar?
Nearly half of the flight crews observed no significant adverse weather on the weather radar at the time of the lightning strike. Aircraft were observed to be more than 10-to-15 mi. from the nearest indicated cell in 15% of the lightning strikes.
Roughly 20% of the incidents occurred when the aircraft were immediately adjacent to or within “green.” Only 10% occurred within “yellow” cells. None of the lightning strikes were reported to have occurred within a “red” cell. On reflection, this is probably a good sign that pilots are detecting and avoiding adverse weather when indicated on their weather radar display.
There are some caveats looking at these flight crew reports. An aircraft will tend to attract lightning when it is in proximity of a storm’s high electrical field, whereas radar is effective at detecting rainfall and wet hail. Heavy droplets (thunderstorm droplets) will return the majority of the energy in radar waves. According to Honeywell’s Weather Radar Systems User’s Manual with Radar Operating Guidelines, large wet hail creates the strongest reflectivity of any kind of precipitation particle. Rain can also give rise to high radar reflectivity.
Weather radar, if not correctly used or interpreted, may mislead the flight crew when dry hail precipitation returns a relatively weak echo. Dry hail (a hail pellet that is totally frozen without a thin layer of water on the surface) is a very poor reflector of energy and can lead to underestimation of a storm’s intensity.
The following NASA Aviation Safety Reporting System (ASRS) report indicates a flight crew’s problems with management of their weather radar and problems with attenuation due to rain during a lightning strike.
“We were vectored through heavy rain. Onboard radar showed green returns only. After landing, I looked at the satellite radar picture. We had flown through a well established line of weather showing a lot of yellow returns along its center. Our onboard radars are very, very outdated limited tools because of attenuation in heavy rain and difficult tilt management (as demonstrated by many incidents/accident).”
[ASRS Report 512539. May 2001]
Other ASRS narratives revealed a similar trend. The flight crew of an Airbus A300 was unable to explain why their weather radar failed to see a significant weather cell that led to an inadvertent penetration of the cell in the following ASRS report:
“Aircraft had 1 hr., 20 min. ground time air traffic control hold for thunderstorms east of DFW. On climb-out, had to navigate around numerous thunderstorms. After level-off, we appeared to have cleared the weather. We were in cirrus when aircraft pitched up 2000 ft. and plunged down the same. I selected 30 nm on navigation display and saw we were on a large cell. ATC asked me what altitude we were at and I advised them of severe weather encounter and that we were busy recovering. After clearing the cell, aircraft performed ok. Upon arrival, maintenance advised me of damage to nose cone, lightning strikes and other damage to the leading edge of engine #1 and engine #2 pylon panels. Entered event in log. Unable to explain why radar failed to see the cell on 240 range. Aircraft currently undergoing numerous inspections.”
[ASRS Report 591977. August 2003]
Avoiding Bad Weather
It is worth noting that one-third of the ASRS reports in this sample experienced lightning strikes while attempting to obtain alternate routing and/or altitudes to avoid hazardous weather, but were unable to do so because of traffic congestion.
This is a continuation of a previously cited ASRS report in the aftermath of a lightning strike.
“Other issue: air traffic control congestion during bad weather is becoming a safety issue. Several times we heard, ‘don’t call me, we’ll call you’ from busy controllers. It becomes difficult to request weather deviations in a sky crowded with airplanes. Several times we were denied our request for deviations.”
[ASRS Report 512539. May 2001]
Approximately 24% of the ASRS reports indicated that flight crews were able to detect adverse weather either visually or with weather radar but that air traffic control was unable to see the same weather.
An example of this is contained in the following ASRS report.
“The controller on Washington Center frequency 124.55 stated, ‘he wasn’t picking up anything greater than Level 1 activity, and that other aircraft traversed the area, so why couldn’t we?’ Flight conditions were rapidly deteriorating, with continuous moderate turbulence and lightning in the distance. Time was critical since onboard radar depicted cell activity ahead, and any further delay would have compromised safety. We were apparently denied a turn due to overlapping sector airspace responsibility.”
[ASRS Report 542806. February 2002]
Pre-flight and post-fight inspections should include grounding and bonding straps that protect the aircraft from lightning strikes, in Part 3 of this artice.
