An air tragedy in Brazil, wing icing in the tropics, supercooled droplets, Bernoulli effect, airfoils and Newton’s Third Law.
August 10, 2024 Sao Paulo, Brazil
On Friday afternoon, August 9, 2024 a VoePass ATR 72-500 aircraft, with 62 passengers and crew on board crashed just 45 miles short of its destination in the City of Vinhedo, while on its way to Sao Paulo City’s International Airport at Guarulhos, Brazil. Sadly there were no survivors. The ATR 72-500 turbo prop plane was manufactured in Europe by a ATR, a French-Italian consortium of the Airbus and Leonardo firms. Since the early 1990s this plane has had 15 air crash incidents with almost 500 fatalities. A similar plane, an ATR 72-200 crashed in 1994, in Roselawn, Indiana in which the probable cause was icing of wings. The crash caused 68 fatalities. A US Federal Aviation Agency investigation concluded with a warning to pilots not to use autopilot in icing conditions when flying these planes.
This VoePass ATR 72 was on a domestic flight from Parana State to Sao Paulo City with 62 passengers and crew on board, when at 11.21 PM the flight, cruising at 19,000 feet, began losing altitude over Vinhedo City about 45miles NNW of Sao Paulo City. The plane dropped 250 feet in 10 seconds, then climbed back up 400 feet in 8 seconds. Seconds later it lost 2000 feet and then it began a rapid spiraling downward fall, dropping 17,000 feet in one minute. It crashed and burned among houses in the City of Vinhedo. No local residents were hurt as the plane plummeted to the ground in a clear area among homes and burst into flames.
Video photographs published by onlookers on the ground show the intact ATR turboprop, falling, with no horizontal speed, plunging almost straight down. The video is frightening to watch since it is clear that the plane had lost all ability to fly and was in an uncontrolled deadly death spiral to the ground. The plane provided no visible reason to fall, it appeared unaltered, there were no flames or smoke, the wings, tail and body were intact. The ATR 72 was simply plunging straight down like a huge piece of silvery metal.
What could have caused such a crash?
It is winter in Brazil which is in the southern hemisphere. Though Sao Paulo and Vinhedo are located in a tropical-temperate zone, at about 23 deg south latitude, close to the Tropic of Capricorn, winter is relatively mild there. (Sao Paulo is far south of the equator as semi tropical Tampico, Mexico is north of the equator.) Average southern hemisphere “winter” temperatures for August in Sao Paulo range from 60-70 degrees F during mid day.
Since for the most part, air is heated from the bottom up, not by the sun’s rays passing through air, but indirectly by the sun heating the earth, and then the warmed earth transferring that heat to the air. For this “bottom up heating” reason the air cools at a regular rate at higher elevations. The Normal Lapse Rate is average rate at which the air temperatures drop when measured from the ground upward. That rate is about 3.5 F per 1000 feet.
Yesterday, (Friday, August 9, 2024) mid-day temperatures on the ground at Vinhedo were about 70F. But the air temperature at 19,000 feet was much cooler. Using the Normal Lapse Rate to calculate the approximate temperature at the ATR 72”s cruising height indicates that the air temperature aloft where the VoePass airliner was flying was probably around 4.5F.*
Thus where the ATR 72 was cruising the air temperature at 4.5 F was well below freezing (at 32F). Clouds at this level often contain moisture in the form of ice crystals, and water droplets which have been cooled below freezing but remain in the liquid state, these tiny liquid drops are called “supercooled water droplets”. These tiny drops of water less than 0.05mm in diameter, are so small that even at temperatures well below freezing (32F) they simply can not organize their relatively small number of water molecules within the drop into the typical ice crystal lattice. So they remain in the liquid state. However, if they come in contact with a solid surface, an ice crystal, dust, or pollen particles which may act as nuclei of crystallization, they freeze instantly.
When the leading edge of an airplane wing flies through such a cloud with supercooled water droplets, the drops freeze instantly to the plane wing, tail, sensors like pitots, antennae and other parts. This ice—called “rime ice”—is heavy and adds weight to the plane but it is most dangerous on the wings where it can interfere with lift.
As you would expect, the wing is what permits the plane to rise up off the ground and also provides the “lift”which keeps the plane aloft. The process is complex and probably not fully understood, but in simple terms we can think of the wing as an “air foil” with a cross sectional shape characterized by a flat undersurface and a slightly rounded or cambered upper surface. The wing functions as an airfoil (to create lift) only when it is moving forward through the air. In simple terms, the air flows smoothly over the top and bottom of the wing in cross section. The flow of air over the top surface is forced to speed up as it traverses the curved surface (a longer distance). It is this more rapid flow which creates lift. The Bernoulli Principle states that the pressure of a fluid in motion is inversely proportional the speed of the flow. Thus the pressure above the wing (air foil) is lower on the upper, curved, cambered surface than that of the bottom flat surface where pressure remains unaltered. Imagine “stretched out” or lower density molecules generating a zone of lower pressure along the upper wing surface. This does not occur on the undersurface where pressure remains at its original higher level of pressure.
As a result, the moving aeroplane wing generates a region of lower pressure all along its upper surface.The longer the wing the more lift it can develop. In a definitely non-scientific sense, the wing can be imagined as being “sucked” upward* by the air flowing smoothly over the upper cambered surface. As long as the air can flow smoothly in streamline flow over that surface the wing provides lift (or is “sucked” upward ) and this disparity in pressure helps to keep the plane aloft. (It is noteworthy that the angle of attack of the wing also creates lift. The angled upward wing tends to direct air downward creating a force (Newton’s Third (equal and opposite) Law) that pushes wings upward).
Thus it is clear that any disturbance to this smooth air flow or “streamline flow” over the upper cambered wing surface, such as turbulence caused by rime ice, can reduce the speed of the flow and destroy the Bernoulli (“suction”) effect of the upper curved surface. This is the likely problem which caused this tragedy.
How does ice form on wings?
As noted above high clouds at temperatures far below freezing have moisture in both solid and liquid form. When planes encounter supercooled droplets aloft ice can form along the forward or leading edge of the wing. Rime ice has a rough surface. It causes the air to flow irregularly in what is called “turbulent flow”. This form of air flow destroys the lift effect. Rime ice thus destroys the rapid smooth flow over the upper wing surface. The slowed flow of air along the wing edge causes loss of lift, or when excessive may cancels out lift altogether. On both wings, icing may cause the plane to lose elevation instantly. When only one wing is impacted, that wing loses lift and the plane may bank sharply in that wing’s direction, causing the aircraft to begin an uncontrolled downward spiral.
In the early years of flight, most planes flew well below levels of high clouds where supercooled droplets would likely be encountered. When aircraft began flying at high altitudes, methods to control icing on wings became essential for safe flight.There are several methods used on the most modern airliners. One more modern solution is to direct heated air or other fluids derived from the jet engines or the prop engines to the wings when required .These “hot wing” planes melt rime ice which may accumulate on the wings and leading tail edge. Another method common on many older types of aircraft is a rubber boot which encases the forward edge of the wing. This flexible boot can be pumped up with air when necessary which causes the boot to expand and crack the adhering ice which then is driven off the wing by air flow.
The ATR 72 model has this latter design. One problem with this system is that in cases when icing conditions are extreme or when such conditions are repeatedly encountered, multiple use of the system may form ice encrustations which break off and slide rearward only to refreeze onto the central cambered portion of the wing, beyond the rubber boot. When that occurs the aircraft loses lift rapidly and plunge to lower levels.
Though "icing" seems the likely cause of the tragedy in Brazil, what seems compelling when looking at the facts and circumstances from afar is often only the first hypothesis in perhaps a much more complex circumstance. We must await the full technical report on this sad air tragedy.
One can only feel pain and sympathy for the families of the more than 60 passengers and crew who lost their lives in this terrible crash. One hopes those who investigate these tragedies can find the cause and make recommendations regarding flight safety that will make such tragedies very much less common.*
(Though in relatively uncommon meteorological circumstances such as “ice storms” supercooled clouds may produce rain drops cooled to the freezing point. These liquid drops at freezing temperatures (not supercooled) fall into lower warmer levels of the atmosphere close to ground level. There they strike power lines or tree limbs and branches and as they strike these surfaces minor evaporation further drops the temperature causing the drops to freeze on contact, and over time building up thick layers of ice with often devastating consequences to trees and power-lines.)
* More recent (September 2024) reports indicate that indeed the Sao Palo crash was caused by wing icing. To this author, it seems reasonable to request that the ATR 72-500 turbo prop plane manufactured in Europe by ATR, a French-Italian consortium of the Airbus and Leonardo firms which in most circumstances appears to be a safe practical airship, should be grounded and refitted with more modern deicing equipment than its original design system before it can be safely flown again. Icing is not a mid latitude or cold climate issue. Rime icing can occur even close to the equator….. in Brazil.
*There is of course no such thing as “suction” but it may work as an idea aid.
*Normal lapse rate is about 3.5F per 1000 feet.
- (19 X 3.5F)- 70 F = Temp at 19,000 feet)) or 19 X 3.5 =66.5F, 70F-66.5 = 4.5F.
- Brazilian meteorological company reported severe icing in Sao Paulo State at the time of the crash.
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