Page 22 - Volume 21 Number 11

Under Pressure
NASA has tested the hydroplaning phenomenon extensively. It publishes a table that correlates tire pressure to the speed at which dynamic hydroplaning will occur. Note the main wheel tire pressure for the airplane you’re f lying, and compare that pressure to the actual touchdown speed, which should be very close to the stalling speed as adjusted for airplane weight. You may find that your normal landing puts you close to a hydroplaning speed.
There is extremely little margin between the full-stall, essentially short- field landing touchdown speed and the NASA hydroplaning speed. You need to be very careful and land much slower than many pilots routinely land to avoid hydroplaning on a wet runway in an airplane like that!
Note that these are groundspeeds, the speed the airplane’s tires are traveling across the wet surface. This becomes important as we return to the case of the Midway light jet.
Wet Runway
METARs around 1836Z, the time of the mishap, are listed below. I’ve highlighted some important information.
KMDW 121853Z 18009G17KT 10SM -RA FEW060 BKN110 BKN130 OVC200 23/21 A2991 RMK AO2 LTG DSNT NE SLP120 P0007 T02280211 $
K MDW 121753Z 21011KT 10SM -RA FEW040 SCT090 BKN120 OVC200 23/21 A2996 RMK AO2 LTG DSNT ALQDS RAB37 SLP135 P0001 60060 T02280206 10256 20206 51006 $
KMDW 121653Z 20011KT 10SM FEW045 SCT060 BKN110 OVC150 23/21 A2995 RMK AO2 LTG DSNT W AND NW RAE41 TSE09 SLP133 P0001 T02280211
The light jet, landing on Midway’s Runway 31C, touched down with a variable and at times gusty wind. 100 degrees to as much as 130 degrees off the runway heading, a crosswind-to-tail wind landing. The trend established by the NASA Tire Pressure versus Hydroplaning Speed table, suggests this to be a potential contributing factor, and certainly enough to remind us to think about dynamic hydroplaning in any airplane we fly.
Mitigation
To minimize the hazard of hydroplan- ing when landing on a wet runway:
• Fly the proper touchdown speed as slowly as possible to just above the stall speed at the moment of touchdown to preserve a hydroplaning speed margin.
• Land aligned with the runway centerline with zero sideslip using
Tire Pressure (psi)
Hydroplaning Speed (kts)
30
49
40
57
50
64
60
70
70
75
80
81
(above) Tire Pressure versus Hydroplaning Speed (NASA)
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20 • TWIN & TURBINE November 2017
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