In Close and Gusty

“We all know to increase airspeed in gusty potential wind-shear conditions. But the question is how much? When experiencing a large and abrupt decease in airspeed, the ground speed stays constant. If an airplane has a normal approach reference airspeed of 120 knots and is flying into 20-knots headwind, its ground speed is 100 knots. If that same airplane loses 20 knots of airspeed due to sudden decrease of 20 knots in the headwind, it loses no groundspeed. Even though the airspeed has decreased to 100 knots, the aircraft is now flying into a zero wind, which results in a 100-knots groundspeed. This doesn’t mean that nothing is wrong. Airplanes fly based on airspeed, not ground speed. Airspeed should be increased until the groundspeed is equal to the desired reference speed of 120 knots. When flying into a 20 knots headwind, it will result in airspeed of 140 knots. When the airplane suddenly loses the headwind, it would be flying at airspeed of 120 knots, the normal reference speed. Using this procedure, if a shear is encountered, care must obviously be taken in the application of the required back pressure, but the point is that there will be enough airspeed left to make the required correction without stalling the airplane.”

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In a landing accident of a Gulfstream business jet (Global 5000) with a weight of around 77,000 lbs on Nov 11, 2007 at Fox Harbour Aerodrome, Nova Scotia in Canada on a runway 4500 feet long and served only by an Advanced Precision Approach Path Indicator (APAPI), the aircraft undershot runway 33 by seven feet and six inches. The captain adopted the wing down method for cross wind correction and continued in this manner even after flare to correct for the crosswind, stretching the glide by not adding power as the auto throttles came off at 35 feet. He also used an incorrect Vref by not sufficiently correcting for the wind of approximately 18 Knots headwind and gusting to 25-30 Knots. Vref was corrected to only plus 5 knots for the wind. The accident did not result in fatalities but the aircraft was destroyed.

The investigation accident board however took the view that the eye to wheel height at touchdown mattered and few crew are aware of that. The eye to wheel height is when the aircraft is in the landing attitude and not in the horizontal plane sitting on the runway as is commonly thought. Also the APAPI glideslope was incorrectly aligned by a .04 degree from the classic 2.5 degrees. Previously the crew flew the Cessna Citation series which has a lesser eye to wheel height.

 By courtesy: In Close and Gusty by Ross Detwiler, Business and Commercial Aviation Magazine Oct 2010

1 nautical mile = 1.2 statute miles

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Wind reported by Air Traffic Control Tower on the runway is in degrees magnetic and is coming from that direction

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Crosswind Landings: I have only been taught of crabbing (nose of airplane is not aligned with the runway) into wind as the desired method of approaching a runway for a crosswind landing in a big commercial jet airplane, and after flare (just very near the ground), aligning the longitudinal axis with the runway centreline by application of a tap on the rudder bar and opposite aileron application if the wing drops and is necessary.

Another technique such as the wing down method with opposite rudder application (nose aligned with the runway but one wing is down and opposite rudder is held) is the favoured method for light airplanes. This technique is risky for jets with underwing engines as one can contact the engine with the runway.

Correcting the speed bug or Vref for the headwind component is controversial when gusts are affecting the wind velocity. In PIA we used 1/2 of the headwind component plus full gust as the addition to Vref. Wind blowing on/across the runway has two components, the headwind and the crosswind component, assuming the airplane is taking off into the wind. However, this does not apply universally in North America, as only the gust component is added to the Vref for business jets.–Mohammad Syed Husain

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