Aviation Safety Corner
this month's topic, I would like to conclude our discussion on, "The Deadly Deluge." Research has shown
that heavy rain can rob an aircraft of lift quickly and without much warning. This is Part IV of IV.
Now, let's start our discussion with, "Brief Events". The worst decreases in aerodynamic performance are caused by torrential downpours associated with showers. This type of shower is confined to a fairly small geographical area and has a short duration.
The world record rainfall rate is 73.8 inches per hour, recorded at Unionville, Maryland. Typically, ground-level rainfall rates are much lower than this, with the heaviest rainfalls occurring for short periods of 30 seconds or less. During a thunderstorm, significantly higher rain intensities than those at low levels can be expected at a higher altitude.
Can radar help us avoid the heaviest rainfall rates? Tests were conducted with ground and airborne radars of Great Plains thunderstorms, but the radar returns conflicted, possibly because of the small size of the region of extremely intense rain. Weather radar measurements are still not sufficiently accurate for measuring rainfall intensities and should be used by pilots and accident investigation teams with caution. Radar measurements of rainfall rates often are in error by more than a factor of two.
Now, Let's Discuss, "Avoiding the Phenomenon". Obviously, the research community doesn't yet have many answers regarding the hazard of heavy rain. As with many weather phenomena, avoidance remains the key to safety of flight operations. No aircraft should penetrate heavy rain cells while landing, taking off or going around.
The only problem with this sage advice is that no one has the magic crystal ball to predict exactly when and where a heavy rain cell will suddenly develop. Remember that heavy rain cells tend to be very short-lived and small in size. The ability to predict which cell will suddenly dump buckets of water does not exist.
Heavy rain can come down very suddenly, catching any prudent and wary pilot completely by surprise. We don't have the weather-detection capability to give us anything more than just a general warning of convective activity. Being extra cautious around convective cells is always necessary. At the moment, this is a phenomenon against which we are merely rolling the dice, and some of us will, unfortunately, have the bad luck of encountering a cell producing very heavy rain.
All pilots should be alert to the possibility of a significant increase in descent rate and decrease in airspeed when penetrating a heavy-rain cell. Please remember that an aircraft will stall at a lower angle of attack if roughness elements are present on the wing. We should be aware of the possibility that an aircraft may stall prior to activation of the stall-warning devices.
According to the heavy-rain research groups, if execution of a go-around in heavy rain is necessary, high angles of attack and rapid climbs that result in bleeding airspeed should be avoided. Rather, an in-crease in airspeed with slower climb-out, to assure an adequate stall margin, is suggested. It should be noted that this suggestion is in opposition to recommended procedures for an aircraft exposed to verti-cal wind shear, where rotation to a high angle of attack is recommended until the descent rate is ar-rested. This conflict is yet another reason why pilots should avoid convective activity if at all possible.
Now in conclusion, remember, as with all scientific endeavors, we are continually in search of the true underlying principles. Our knowledge progresses with more study; and, sometimes, our earlier theories are found to be incomplete or incorrect.
Such is the nature of examining complicated problems. In time, we will learn more about the hazardous effects of heavy rain and how to avoid them.
Don't miss next month's, Aviation Safety Corner, when I'll start a new topic, "Open-Door Debacle." The result of a door opening in flight too often is a crash. It shouldn't be that way. This will be another Four Part Series. SAFE FLYING!
MERRY CHRISTMAS, AND A HAPPY NEW YEAR!
Larry G. Harmon
FAA AVIATION SAFETY COUNSELOR
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