Thunderstorms — Page 257, Lesson 216

I want to walk you through the thunderstorm encounter procedures, starting with the in-flight actions during the storm itself. First, rule number four: do not correct for altitude gained or lost through up and down draughts unless absolutely necessary. Up and down draughts are the powerful vertical air currents inside a thunderstorm — they can push the aircraft up or drop it suddenly. Your natural instinct might be to adjust the controls to hold your exact altitude, but the book tells you to resist that unless it's absolutely necessary. Fighting those vertical gusts wastes energy and can overstress the airframe. Rule five: maintain the trim settings and avoid changing the power setting except when necessary to restore margins from stall warning or high-speed buffet. The target pitch attitude should not be changed unless the mean indicated airspeed differs significantly from the recommended penetration speed. Let me unpack that. "Trim settings" are the stabiliser and control surface adjustments that hold the aircraft in a steady flight attitude without constant control pressure. "Power setting" is the thrust you've selected. You want to leave both alone. The only reason to touch power is if you get a stall warning — meaning the wing is about to lose lift — or high-speed buffet, which is the vibration from airflow separation at high Mach numbers. "Target pitch attitude" is the nose-up or nose-down angle you're aiming for. Don't change it unless the average indicated airspeed — the mean IAS — is significantly different from the recommended penetration speed, which is the specific speed published in your aircraft's manual for flying through turbulence. Rule six: if trim variations due to the autopilot — that's auto-trim — are large, the autopilot should be disengaged. Auto-trim is the autopilot's function that automatically adjusts the stabiliser trim to maintain the commanded attitude. In severe turbulence, the autopilot may make large, rapid trim changes trying to hold altitude or heading, which can be dangerous. So if those variations become large, you disconnect the autopilot. However, movement of the Mach trim, where it occurs, is necessary and desirable. Mach trim is a different system — it automatically adjusts the stabiliser to compensate for Mach-number-related pitch changes at high speed. That movement is normal and helpful, so you leave it alone. Also, check that the yaw-damper remains engaged. The yaw-damper is a system that automatically opposes unwanted yawing oscillations — it keeps the aircraft directionally stable. You want it on. Rule seven: if negative 'G' is experienced, temporary warnings — for example, low oil pressure — may occur. These should be ignored. Negative G means the aircraft is experiencing downward acceleration, like being pushed down in a downdraught. That can cause oil to slosh away from the oil pump pickup, triggering a low oil pressure warning light. It's temporary and not a real failure — so you ignore it. Rule eight, and this is a hard rule: on no account climb in an attempt to get over the top of the storm. Do not try to outclimb a thunderstorm. The vertical development of a thunderstorm can extend far above your aircraft's service ceiling, and the turbulence, icing, and hail at those altitudes are extreme. You stay at your penetration speed and ride through it. Now, after the thunderstorm encounter — still in flight. First point: if hail has been encountered, considerable damage to the airframe, not visible from the cockpit or cabin, may have occurred. Hail can dent leading edges, crack radomes, and damage antennas — damage you cannot see from inside. So you should consider diverting to a suitable and nearby aerodrome where the aircraft can be inspected for damage. If the damage is to aerodynamically significant areas — for example, a nose radome — the increased drag will affect fuel burn. That means the aircraft, if continuing to its destination, may burn considerably more fuel than expected or planned. So actual fuel usage should now be monitored very closely. Bear in mind that some Flight Management Systems calculate 'expected overhead destination' fuel based on data that assumes normal planned conditions and a normal — fully clean — aircraft aerodynamic state. The FMS doesn't know you've been damaged, so its fuel predictions will be optimistic. You must track real fuel burn yourself. Second point after the encounter: if the aircraft has been struck by lightning, treat all magnetic information — for example, from direct or remote indicating compasses — with extreme caution. The large electric currents associated with a lightning strike can severely and permanently distort the magnetic field of an aircraft, rendering all such information highly inaccurate. So your compasses, standby compass, and any heading references derived from magnetic sensors may be unreliable. Do not trust them. Finally, Air Traffic Control considerations. First: modern ATC radars in general do not display the build-up of weather that may constitute a hazard to aircraft, and ATC advice on weather avoidance may, therefore, be limited. Controllers see aircraft returns, not weather intensity. So don't rely on ATC to warn you about thunderstorms. Second: if, as recommended, a pilot intends to detour round observed weather when in receipt of an Air Traffic Service that involves ATC responsibility for separation, clearance should first be obtained from ATC so that separation from other aircraft can be maintained. You need permission to deviate. If for any reason you cannot contact ATC to inform the controller of your intended action, any manoeuvre should be limited to the extent necessary to avoid immediate danger, and ATC must be informed as soon as possible. In oceanic airspace, the weather deviation procedure in PANS-ATM, ICAO Doc 4444, paragraph 15.2.3 should be followed — that's a specific international standard for deviations over the ocean. Third: because of the constraints on airspeed and flight path and the increased workload of the crew when flying in a Terminal Manoeuvring Area, pilots should consider making a — and the excerpt cuts off there, but the point is clear: in the busy, high-workload environment near an airport, you should consider delaying or avoiding the thunderstorm encounter altogether rather than trying to handle it while also managing approach procedures.