Aircraft De-Icing Systems

Posted by PITHOCRATES - October 23rd, 2013

Technology 101

A build-up of Ice on Airfoils causes a Reduction of Lift and a Loss of Stability

In the classic movie Airport (1970) after the guy pulled the trigger on his briefcase bomb the plane suffered a massive decompression.  When Dean Martin got back to the cockpit he told the flight engineer to give them all the heat they had.  Because it’s very cold flying above 10,000 feet without pressurization.  That’s why World War II flight crews wore a lot of heavy clothing and thick mittens in their bombers.  As well as oxygen masks as the air was too thin to breathe.  The B-17 even had open windows for the waste gunners.  Making it very cold inside the plane.  Because the air is very, very cold at altitude.

There is another problem at altitude.  Because of these very frigid temperatures.  Water droplets in the air will freeze to any surface they come into contact with.  They can reduce engine power for both propeller and jet engines.  They can freeze on ports used for instrumentation and give inaccurate readings of vital aircraft data (such as engine pressure ratio, aircraft speed, etc.).  And they can freeze on airfoils (wings, rudder, tail fin, etc.).  Disturbing the airflow on these surfaces.  Causing a reduction of lift and a loss of stability.

Ice and airplanes are two things that don’t go together.  As ice forms on a wing it disturbs the airflow over the surface of the wing.  Increasing drag.  And reducing lift.  Causing the plane to lose speed.  And altitude.  If the ice continues to form on the wing eventually it will stall the wing.  And if the wing stalls (i.e., produces no lift) the plane will simply fall out of the sky.  In the early days of aviation pilots were highly skilled in flying their planes where there were no icing conditions.  Flying over, under or around masses of air containing water droplets in subfreezing temperatures.  Today we have anti-icing systems.

The most common Anti-Icing System on Commercial Jets is a Bleed Air System

One of the most common anti-icing systems on turboprop aircraft is the use of inflatable boots over the leading edge of the wing.  Basically a rubber surface that they can pump air into.  When there is no ice on the wing the boot lies flat on the leading edge without interrupting the airflow.  When ice forms on the leading edge of the wing the boot inflates and expands.  Cracking the ice that formed over it.  Which falls away from the wing.

Commercial jets have larger airfoils.  And require a larger anti-icing system.  The most common being a pneumatic manifold system that ducts hot air to areas subject to icing.  Which works thanks to a property of gas.  If you compress a gas you increase its temperature.  That’s how a diesel engine can work without sparkplugs.  The compressed air-fuel mixture gets so hot it ignites.  This property comes in handy on a jet plane as there is a readily available source of compressed air.  The jet engines.

As the air enters the jet it goes through a series of fast-spinning rotors.  As the air moves through the engine these rotors push this air into smaller and smaller spaces.  Compressing it.  Through a low-pressure compressor.  And then through a high-pressure compressor.  At which time the air temperature can be in excess of 500 degrees Fahrenheit.  It is in the high-pressure compressor that we ‘bleed’ off some of this hot and pressurized air.  We call this a bleed air system.  The air then enters a manifold which ducts it to at-risk icing areas.  From the engine cowling to the wings to the instrumentation ports.  Using the hot air to raise temperatures in these areas above the freezing temperature of water.  Thus preventing the formation of ice.

The Drawback of a Bleed Air System is Reduced Engine Efficiency

The bleed air system does more than just anti-icing.  It also pressurizes the cabin.  As well as keeps it warm.  Which is why we don’t have to dress like a crewmember on a World War II bomber when we fly.  It also powers the air conditioning system.  And the hydraulic system.  It provides the pressure for the water system.  And it even starts the jet engines.  With the source of pressurized bleed air coming from the auxiliary power unit mounted in the tail.  Or from an external ground unit.  Once the jets are running they disconnect from the auxiliary source and run on the bleed air from the engines.

There is one drawback of a bleed air system.  It bleeds air from the jet engine.  Thus reducing the efficiency of the engine.  And a less efficient engine burns more fuel.  Raising the cost of flying.  With high fuel costs and low margins airlines do everything within their power to reduce the consumption of fuel.  Which is why pilots don’t top off their fuel tanks.  They’d like to.  But extra fuel is extra weight which increases fuel consumption.  So they only take on enough fuel to get to their destination with enough reserve to go to an alternate airport.  Even though it seems risky few planes run out of fuel in flight.  Allowing the airlines to stay in business without having to raise ticket prices beyond what most people can afford.

To help airlines squeeze out more costs Boeing designed their 787 Dreamliner to be as light as possible by using more composite material and less metal.  Making it lighter.  They are also using a more efficient engine.  Engines without a bleed air system.  In fact, they eliminated the pneumatic system on the 787.  Converting the pneumatic components to electric.  Such as using electric heating elements for anti-icing.  Thus eliminating the weight of the bleed air manifold and duct system.  As well as increasing engine efficiency.  Because all engine energy goes to making thrust.  Which reduces fuel consumption.  The key to profitability and survival in the airline industry.

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LESSONS LEARNED #56: “It’s competition in the private sector that makes life better. Not government regulation.” -Old Pithy

Posted by PITHOCRATES - March 10th, 2011

Deregulation makes Air Travel Cheaper and Safer

A lot of people bitched about the deregulation of the airlines.  Mostly union people.  Because before they were deregulated it was very expensive to fly.  Ticket prices were out of the reach of most of middle class America.  But, with those high prices, the airlines made a lot of money.  And the unions got a lot of that money.  Union members warned about safety with deregulation.  If they lowered ticket prices so anyone could fly there wouldn’t be enough money to maintain the highly skilled personnel to fly and maintain the airplanes.  If you put profits before safety planes could start falling out of the sky.

Well, they deregulated the airlines in 1974.  Government no longer controlled the price of tickets, air traffic routes or the number of airlines allowed to operate.  Ticket prices fell.  More airlines began operations.  More cities built airports.  More people were flying than ever before.  All people.  Not just the rich.  Deregulation was a huge success.  Except for unions.  For them it wasn’t quite the gravy train it was before deregulation.  So did safety suffer?  No.  Quite the contrary.

In 1959 there were 40 fatal accidents per million departures (at the beginning of the jet age).  That number fell to about 10 in 1960.  During the Sixties it was at or below 5.  The number fell by approximately half during the Seventies.  It fell to about 1 after the Seventies with a spike of about 1.5 in 1988.

At the beginning of the jet age, few in the government bureaucracy knew anything about jets.  So it was mostly the manufacturers and the airlines policing themselves as they developed jetliner travel.  And they did a pretty good job.  After deregulation air travel exploded with the new jets.  They were safe enough that people weren’t afraid to fly on them.  And they did.

Boeing and Douglas lead the way in the Jet Age

Competition drove early jet travel.  Air travel was growing and the airlines needed planes that could carry more people, fly farther and faster.  If they had the planes they could fly the people.  Two of America’s manufacturers stood up in a big way.  Boeing built the 707.  And Douglas built the DC-8.  This competition produced two jetliners that were safe to fly and they moved more people farther than any propeller plane.  There were some accidents in the beginning but they were less compared to the propeller planes they replaced. 

Air travel continued to grow.  There was a demand for bigger airplanes.  A bigger plane could move more people at a lower cost per person.  This meant even lower ticket prices.  And made air travel more affordable to the less rich.  Boeing rolled out the 747.  McDonnell Douglas (the merger of Douglas with McDonnell Aircraft Corporation) rolled out the DC-10.  The first of the wide-bodies.  The Boeing 747 went on to become a huge success with an incredible safety record.  It still flies today.  Few airplanes make people feel safer.  The DC-10, on the other hand, did not make people feel as safe.  For a period of time.  And that marked the beginning of the end of McDonnell Douglas.

McDonnell Douglas was a very successful company.  They built thousands of DC-9s and MD-80/90s.  Over 2,000.  These are very reliable and safe aircrafts.  The DC-9 had only 0.76 fatal accidents per million departures (PMD).  The MD-80/90s had only 0.31 fatal accidents PMD.  You’ll still see a lot of these flying today.  It has proven to be a very reliable airframe.  The DC-10, though, had a bumpier road with less than 500 built.  It, too, was a good airplane.  But it was involved with some very high-profile accidents.  And it got a reputation as an unsafe design.

A Close Call with the Cargo Hold Door of a DC-10

The cargo hold door on the DC-10 opened outward.  This allowed room for more cargo.  Doors that open in take up cargo space.  Which reduces revenue.  The more cargo you can carry, the more revenue you make and the lower ticket prices can be.  It’s just one in many ways to reduce the cost of air travel.  And it was yet another thing that made the DC-10 profitable to fly.

Airlines bought the DC-10 and put it into service.  It performed well.  But that cargo door would become an issue.  Doors on an airplane typically open inward.  For a good reason.  Once a plane reaches an altitude of 10,000 feet, it has to be pressurized so people can breathe normally.  That places a lot of pressure inside the passenger and cargo compartments.  The only ‘holes’ in the aircraft have doors that seal tighter at these higher interior pressures.  Because they open inward.  The cargo door on the DC-10, though, needed a special latching mechanism to withstand those pressures without opening in flight.  Because it opened outward.

Closed properly there was no problem.  But sometimes it wasn’t.  In 1972, a DC-10 departing from Detroit suffered an explosive decompression as it climbed above 10,000 feet.  The cargo door failed.  The sudden decompression collapsed the passenger floor and damaged the aircraft’s control cables and hydraulics.  The rudder was deflected full left.  The engines throttle levels slammed back to idle.  The tail-mounted engine control cables were severed completely.  The elevator provided little control.  The pilots varied the thrust on the wing-mounted engines to maneuver the aircraft back to the airport.  They compensated for the deflected rudder with asymmetric thrust on the wing engines.  Without a functioning elevator the nose dropped at lower speeds.  So they landed at a higher speed than normal.  As they slowed the force of the rudder declined and the asymmetric thrust took over, pulling the aircraft off the runway.  It was a tremendous piece of flying by the crew that brought that plane back without loss of life.

A Pair of Crashes Threaten the DC-10 and McDonnell Douglas

The rear cargo door was studied and some changes were made.  Issues with floor strength in the new wide-bodies were questioned.  They just started flying.  This was new territory for everyone.   No significant change was made.  Other DC-10s were flying safely.  This may have just been an isolated incident of human error (closing the cargo door incorrectly).  Then, in 1974, it happened again.  In a series of human errors that doomed a Turkish Airlines plane leaving Paris for London.  A different seat configuration put more people over the floor that collapsed on the Detroit flight.  The explosive decompression tore through the cabin floor, causing greater damage to the control cables and hydraulics than on the Detroit flight.  There was nothing the flight crew could do.  The plane was uncontrollable.  It crashed, killing all 346 aboard.  The largest loss of life to date.  And the first crash of a new wide body.

The subsequent investigation painted the DC-10 as unsafe.  Then in 1979 another catastrophic accident at Chicago’s O’Hare airport.  During takeoff.  After passing V1 (the speed the aircraft could no longer abort and stop safely on the runway) the left wing-mounted engine and pylon tore away from the wing.  The pilots had no idea what had happened other that an engine had lost all thrust.  They couldn’t see the wing from the flight deck.  So they followed procedures for a two-engine takeoff.  But the damage to the leading edge of the left wing was severe.  The leading edge slats retracted with the severing of the hydraulic lines.  The left wing now had a slower stall speed than the right wing.  But they didn’t know.  And they had no indication in the cockpit.  The plane was flying.  They climbed out per procedure.  They powered back from take-off power.  And when they did, the left wing started to dip.  In the few seconds they had to understand what was happening it was too late.  The wing stalled.  The plane rolled left and pitched down.  And crashed.  Killing all 271 aboard.

Was this a design flaw?  No.  Again, it was human error.  The maintenance crew did not follow published maintenance procedures.  The left engine and pylon was replaced after routine maintenance.  The maintenance manual called for the engine removal first.  Then the engine pylon.  Some airlines were replacing the engine and pylon as an assembly.  This saved maintenance hours (and cut costs).  And was safer because it reduced the number of fuel, hydraulic and electrical wiring that had to be disconnected and reconnected.  Or so they thought.  Lifting the engine and pylon assembly to the underside of the wing attachment point was a delicate procedure, though.  That’s a lot of mass pressed against the mounting flange.  And in this case, they pushed the assembly up too high into the flange, deforming it and causing a fracture.  No one knew this as they accelerated down that O’Hare runway.  As they approached take off speed the flange broke completely, sending that engine up and over the wing.

Plane Crashes don’t help Sell Planes or Tickets

With these high-profile accidents the DC-10 got a reputation for being unsafe.  Orders fell.  While orders for the Boeing 747 remained strong.  Even though they had similar safety records.  The early 747s had 1.41 fatal accidents per PMD (the later 747-400 had 0.19 fatal accidents per PMD).  The DC-10 had 1.36 fatal accidents per PMD.  It was as safe if not safer as the 747s that were flying during the same time.  But the public relations damage was done.  Boeing sales grew.  McDonnell Douglas sales fell.  The business founded by Donald Douglas in 1921 is no more.  Unable to compete with Boeing (or Airbus) any longer, McDonnell Douglas merged with Boeing.

McDonnell Douglas had a very successful run.  But the Boeing 747 went on to dominate the wide-body market.  And one wide-body paid a lot more bills than a bunch of narrow-bodies.  Commercial planes have only gotten bigger.  The Airbus 380 is a double decker that can carry over 800 passengers.  And is giving the Boeing 747 a run for its money.  Who knows what might have happened if not for these high-profile accidents.  McDonnell Douglas had even floated the idea of a double decker airplane.  By that time, though, it was too late.

Competition between Boeing and Douglas introduced the jet age.  Their continued competition gave us wide-body jetliners.  Average people could fly anywhere in the world.  And air travel got safer through the years.  Government regulation didn’t make this happen.  Yes, the government made some planes safer.  But not until after a crash.  And they were few and far between.  The vast majority of commercial aviation flew safely.  Because manufacturers and airlines have a vested interest in being safe.  For a very good reason.  Plane crashes don’t help you sell planes.  Or tickets.  But they can put you out of business.  Even if they aren’t your fault.  Something McDonnell Douglas knows only too well.

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