They begin to “paint” on the weather radar and rear their ugly, ominous heads. Thunderstorms are on the horizon while flying in a remote area — perhaps over the Pacific or the Atlantic Ocean or maybe cruising the thunderstorm-infested Brazil’s Amazon jungle. VHF radios are out of range, and HF (High Frequency) is nothing but unreadable static. Your company promised CPDLC (Controller Pilot Data Link Communication – think text messaging) but didn’t install it, or it’s unavailable in your present area. (As I write, only a few American ATC centers have implemented CPDLC using KUSA as the logon. Canada is up and running using specific identifiers for each center). Maybe you are a new hire and lucky to hold the right seat on a wide-body aircraft globe-trotting the world, and the crusty captain is in the bunk on crew rest. Do you wake him up? Will you try to fly through the wall of convective cloud and hope for the best by not bouncing the captain’s head on the crew bunk ceiling or, worse yet, injuring passengers and flight attendants? This same scenario may be asked by a check airman deciding whether to upgrade you to captain. Is there a procedure for getting around weather when ATC is not reachable?
You try to get a weather deviation clearance, but those thunderstorms get closer and closer. You need to decide! The ‘pucker factor’ is escalating as you recount anecdotes of Air France flight 447 that penetrated the ITCZ (Intertropical Convergence Zone) and didn’t come out the other side. Maybe Tokyo Control will reply, stating their usual, “unable due to traffic.” This is unacceptable. Remember, their office won’t knock about in severe turbulence.
Here is what’s expected:
- Alert all listening parties by declaring a “PAN, PAN” three times. (Paperwork will ensue after the flight because of it).
- Broadcast your intentions on the emergency VHF frequency 121.5 and back up on 123.45 with your flight number (identification), flight level, aircraft position, track code if on an oceanic track, and intentions.
- Watch for conflicting traffic both visually and on TCAS. Turn on all exterior lights and remain at your assigned flight level if within five miles. Any farther, you must change altitude, but NOT by much.
- If the deviation is perhaps 30 miles to the right, you must change altitude by 300 feet depending on your heading. Luckily during my entire career, I have yet to perform this weather step maneuver.
- If eastbound (000° to 179°), you descend 300 feet if you deviate left and ascend 300’ if you steer right. When westbound (180-359°) and turning left, you climb 300’ and descend if heading right. You may hear the acronym SAND (South of track – Ascend, North of track – Descend). You may also learn “follow the tropopause.” As a weatherman, I like this, i.e., follow the tropopause height. The tropopause is generally higher to the south, so you ascend, and lower to the north, so you descend. But be forewarned, this does not apply in the southern hemisphere.
You must also know about deviating to alternate airports due to weather, medical situations, or engine failures during ETOPS (Extended Range Twin Engine Operations), but that is another story.