The prototype for the first commercial jet airliner, the De Havilland Comet,
first flew in July of 1949. And it actually turned out to be a trouble design, we still
had a lot to learn. It was briefly withdrawn from service after three highly publicized accidents
occurred just in the year 1956, but some 40 years after the Comet first started flying, you’d think
we would have learned our lessons and would have understood things like basic jet engine design,
or how weather interacts with engines. But the terrifying tale of TACA International Flight 110,
May 24th of 1988, reminds us that mother nature still had some things to teach us about the
weather and how it impacts a jet engine. And it is a story that deserves to be remembered
TACA International Airlines Flight 110 was a regularly scheduled passenger flight between
San Salvador El Salvador, to New Orleans Louisiana, with an in route stop in Belize
City Belize. The airplane was a Boeing 737-300. The 300 model was a second-generation derivative
of the venerable Boeing 737, a tested and reliable aircraft that had been in service
for nearly two decades. In fact this particular Boeing 737 was nearly new,
having first flown in January of the same year, and only having had been in service
with TACA Airlines for two weeks. As a testament to how tested the design was,
this 737 was the one thousand five hundred and fifth to be manufactured. The plane was piloted
by a veteran crew. 29 year old Captain Carlos Dardano had amassed over 13,000 flight hours,
more than 4,000 of those on the Boeing 737. Veteran First Officer Dionisio Lopez
had more than 12,000 hours of flight time. And in addition, a third pilot, Pilot Instructor Captain
Arturo Soley, was in the cockpit to monitor the performance of the nearly new airplane.
In short this was a routine flight, with a veteran aircrew,
flying a nearly new airplane, but of a venerable and tested design. That's
as safe as it gets. Problems with this fight simply should not have happened.
The airplane, with 38 passengers, and a crew of seven on board, departed as scheduled from Belize
City and the flight was uneventful until descent into New Orleans from 35,000 feet. During the
descent from 35,000 feet the aircraft encountered severe weather…very severe weather. The plane had
in fact, encountered an area of intense rainfall followed by 30 seconds of heavy hail. The storm
intensity, stated in terms of liquid water content, was estimated to be 25 to 30 grams
per cubic meter, which equates to rainfall at a rate of approximately 30 inches per hour.
That should not have happened. The aircraft, like most all modern jet transports, was equipped with
weather radar that displays information regarding the intensity of weather in the vicinity of the
airplane. Weather radars intended to provide the pilots with the depiction of weather in front of
the plane, and may influence flight path decisions if the weather in front of the airplane is
sufficiently intense. This radar looks very much like the radar that you see in the evening news,
with green area showing lighter precipitation, with yellow and red area showing more moderate
or severe precipitation. The veteran crew, seeing light to moderate weather in their path,
adjusted their flight approach to go between the more severe weather, choosing an air approach
that brought them on a due East approach to New Orleans. The pilots had done everything right,
and had to use their radar to avoid exactly the weather they encountered. So why had
they hit such severe weather? It might be that the thunderstorm developed quickly,
a phenomenon called a Rapidly Growing Thunderstorm, but that also might have been
something called Radar Shadow. On the weather radar screen, areas where no precipitation is
seen by the radar are displayed is black, or the same color as the display background. However,
when displaying extreme precipitation levels the radar may display the highest intensities,
that is, worse than red, as clear on the radar screen. That is commonly known as Radar Shadow and
as a result of attenuation of the radar signal. In other words, it is possible for the worst
weather to appear on the radar screen as the best weather. As they encountered the severe rain and
hail at about 16,500 feet, the pilot radioed New Orleans Aircraft Control declaring an emergency;
both engines had flamed out. That means that the approximately 140,000 pounds jet aircraft
had suddenly become an unpowered glider, and the pilots were flying by dead sticking, The term
is dated and it's not referring to the flight controls, but to the period of propeller-driven
aircraft, where a loss of power would turn the propeller into nothing more than a dead stick.
Trying to dead stick landing with a passenger airliner can be described as not ideal. Moreover,
it should not have happened. The engine stalled due to something called a flame out,
and that has to do with the basic idea of a jet engine. Every internal combustion engine has a
combustion chamber, the place where the fuel-air mix is burned. A jet engine basically includes a
rotating air compressor powered by a turbine, with the leftover power providing thrust via a
propelling nozzle. The process is known as the Brayton Thermodynamic Cycle. In short,
the burning gasses in the combustion chamber rushed to the open back of the engine at high
speed providing thrust. Lose ignition and you lose thrust. The engine on TACA International
Flight 110 had ingested so much water that the flame in the combustion chamber had been doused.
But that still should not have happened. The risk of water causing a flame out was
understood at the time, and so the US Federal Aviation Authority had established a Water
Ingestion Certification procedure for all jet engine designs. Manufacturers reportedly tested
their engines by shooting them with a fire hose. The rain the TACA Flight 110 encountered
was supposedly well within these certification standards. Moreover, knowing the threat of weather
the pilots had taken appropriate precautions, switching the engine to continuous ignition and
activating the engine anti-icing system. And yet both engines still had flame outs. Why?
It all has to do with hail. According to the National Transportation Safety Board
prior to TACA Flight 110, the risk of hail was seen to be from the hail causing damage
to engine components. Essentially acting like a foreign object hitting the engine. Hail was not
considered for its effect on engine operation. Jet engine fan designs were built with the
idea of preventing water ingestion because the turbine fan blades centrifuged water away from
the engine core. It was assumed that hail would work pretty much the same as water. But testing
after the accident determined that it doesn't. In fact hail falls a much more ballistic path,
that at the right airspeed and turbine speeds, allows the hail to shoot straight into the
engine core. In fact, the FAA realized at some fan speeds and aircraft airspeed combinations,
ingestion of the core is greatly increased as hail misses the fan blades, and goes directly into
the core. Something commonly called the Venetian Blind Effect. Standards designed to prevent water
from entering the engine failed because of the combination of airspeed, turbine speed, and hail.
But even the double engine flameout should not be catastrophic, because an engine that
is flamed out can be restarted just by the wind going through the engine turning the turbine,
something called a Windmill Restart. And even if you don't have enough airspeed to do a
Windmill Restart, you can fire up the engine's starter motors and restart the engine that way.
Just the previous year an Air Europe 737 descending through rain and hail over Greece
had also suffered a double flame out, but the crew managed to restart the engines and land
without trouble. Likewise the crew of TACA Flight 110 attempted to restart the engines. When the
Windmill Restart failed they were able to get the turbine spinning using the starter motor,
but the engines could not get to idle speed and were not producing thrust.
There was still too much water in the engines. As the FAA explains, “Once the engine fails,
it cannot be restarted until the water to air ratio decreases to a point where
the fuel to air ratio can once again maintain combustion.” Now the situation was desperate,
the plane was without power and low altitude and turbulence from the storm gave them a limited
glide ratio. Air traffic control quickly came up with options, including Lakefront Airport whose
runway was minimally sufficient and US Interstate 10, which has one straight mile for every five
miles of highway. Unable to make it to either of these the pilots opted for a vector to Lake
Pontchartrain attempting a dangerous dead stick water ditching. New Orleans Air Traffic Control
lost the transponder signal as the aircraft reached 1,600 feet. They assumed the aircraft
had crashed and quickly dispatched a Coast Guard helicopter from Naval Air Station Joint Reserve
Base New Orleans assuming the water rescue would be critical. What those helicopters found frankly
should not have happened. TACA Flight 110 was parked on a narrow muddy New Orleans levee,
right next to NASA's Michoud Assembly Facility. At just 700 feet, First Officer Lopez had seen
the levee and identified it as being wide enough for a landing. The crew had managed a
maneuver called a Side Slip, something generally reserved for a small craft not an unpowered 737,
and had landed perfectly with no injuries to passengers or crew, and minimal damage
to the aircraft. As one New Orleans Air Traffic Controller said, “After more than two decades of
air traffic control, I still say this is the most splendid piece of air traffic teamwork,
and the most incredible piece of flying, I've ever been witness to or heard of. The flight crew had
mere seconds to assess the situation as to whether it's the lake or the field, and they nailed it.”
After the accident the National Transportation Safety Board mandated a change to the Water
Ingestion Standards and certain changes to the CFM56 engine that's used on the Boeing 737.
Those included changing the engine spinner from conical to a combination elliptical and conical
or coniptical shape that testing shows helps to guide the hail radially outward. A change in the
spacing of the fan blades to better deflect hail away from the engine core. A sensor to
force the combustor to continuously ignite under heavy rain. And extra bleed doors to help drain
water from the engine. The plane was minimally damaged. Repairs to the engine were made on site,
and the plane was towed to NASA's Michoud Facility nearby. where after further repairs, had used the
facility's main road, Saturn Boulevard to take off and fly to a full repair facility. Saturn
Road had at one time been a factory airfield, as airplanes were manufactured on the site during
the Second World War. After repairs, that 737 was put back into service with TACA Airlines,
eventually it was sold to Southwest Airlines that a continued service until it was retired
in December of 2016. But despite 28 years of service, it was really that one extraordinary
landing, a landing that frankly should not have happened, that most deserves to be remembered.
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