Could a 30 knot direct crosswind really make a Learjet roll over 90 degrees?
A recent incident has prompted this question, but it's not explicitly about that incident (with its ongoing investigation).
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It only ever makes sense to talk about cross-wind when the ground is involved. While the aircraft flies, it moves relative to the air and how fast the air is moving relative to ground only affects ground speed and track. If the air is moving smoothly, it can't turn anything over. Only thing that could bank an aircraft over is strong turbulence.
Natural turbulence of a 30 knot wind is unlikely to be strong enough to bank a Learjet 35 significantly, but a wake vortex close behind a heavy aircraft possibly could. The separation minima are there to prevent encountering wake turbulence on approach.
The original question was could the wind roll the Learjet 90 degrees and cause it to crash while landing in gusty winds.
PART 1 THE DANGERS OF WINDY CONDITIONS CAUSING AN AIRCRAFT TO STALL - LEADING TO A ROLL AND CRASH
Ground speed is affected by wind speed and direction, which can make holding a ground track (landing pattern) in wind problematic.
Gusting wind presents a danger with tailwinds as a gust can lower airspeed and cause a stall. For example, an aircraft that stalls at 50 knots Indicated Air Speed will approach at around 1.2 - 1.3 V stall, around 60-65 knots IAS.
A 15 knot tailwind gust will put IAS at or below stall in level flight. An alert pilot will pitch down and add power to avoid stalling.
What compounds the problem is holding the ground track in the wind. A 20 knot tailwind will make 65 knots IAS 85 knots ground speed. A pilot who tries to hold the track with a steeper banked turn risks an accelerated stall. Stall speed for a 45 degree turn is 20% higher, now 60 knots. A 15 knot tailwind gust while turning would disastrously lower IAS to 50 knots.
Note also that a lull in headwind similarly would create a loss of airspeed.
So close monitoring of airspeed is essential in these situations, as well as an awareness of wind speed, direction, and gusts. While there are steady breezes, calm air will never gust or change direction.
PART 2 EFFECT OF CROSSWIND ON AIRCRAFT IN FLIGHT
An aircraft in flight is unaffected by any CONSTANT wind as it moves with the wind. The only motion relative to the wind is velocity caused by thrust and control inputs. Imagine the entire air mass and the plane moving through the sky. One way of looking at it is the motion of the earth and moon. The moon orbits the earth, but BOTH share the same motion around the sun.
There for a constant cross wind will not roll an aircraft. However, a GUST or change in wind from the side will put a side force on an aircraft as follows:
Gust strikes side of aircraft, pushing it sideways and rolling it away from wind.
Plane moves sideways at speed equal to gust, or gust begins to pass. Nose will start to point into gust as a result of change in relative wind.
When gust has passed, plane now has side force on OTHER SIDE due to "push" from gust and slip from roll and rights itself. Nose will point back to original course.
It is unlikely a 30 knot gust will roll a Learjet 90 degrees while it is in cruising flight, or even when it has reduced speed to land.
But, tragicly, the 90 degree roll is the death knell of a stalled aircraft near the ground. This is what we work to avoid.
PART 3 WAKE TURBULENCE
Wake turbulence from a large aircraft would also be capable of rolling a Lear Jet if it followed too closely. The vortex flows, especially when the aircraft is pitched up into a high induced drag configuration (even more so with slats and flaps fully deployed) is very dangerous to smaller aircraft.
PART 4 TURBULENCE CREATED BY TERRAIN OR LARGE BUILDINGS
This one is for mountain flyers, who must beware of rotors that form in the lee of the peaks, as well as strong updrafts on the windward side. This will turn a smooth wind flow into an unpredictable pattern that can suddenly roll an aircraft. Lenticular clouds over the mountains warn of the danger.