“ILS” stands for “Instrument Landing System,” a modern, reliable, and essential radio-navigation aid installed at every major commercial airport in the world today. Where landings in fog, rain, or blowing snow once caused anxiety and concern for pilots and passengers alike, the ILS solves the problem of landing in bad weather by providing flight crews with an electronic precision guidance pathway down to a safe landing in any type of weather. Top and side views of the approach as well as radio frequencies and other vital criteria are shown on an “approach plate” used by the pilots, either in printed form or on digital screens in the cockpit.

The three major components of the ILS are the “localizer” which indicates lateral alignment of the airplane with the runway centerline, the “glide slope” which tracks the angle and rate of descent, and three markers that signal distance to the runway threshold via an audio tone and indicator lights on the aircraft’s instrument panel. The outer marker is located approximately four miles from the runway threshold; the middle marker is approx. 3,500 feet; and the inner marker approx. 1,000 ft.

In the past, if pilots could not see either the approach lights or runway after descending to a certain height called the MDA, or “Minimum Descent Altitude,” they had to execute a missed approach by adding power, climbing back to a higher altitude and trying the approach again, or diverting to an alternate airport. With today’s advanced Category III or “Cat III” landing parameters, aircraft can be automatically guided all the way down to a safe landing on the runway in “zero-zero” conditions (no ceiling/no visibility) without the pilots ever seeing the ground.

With the recent spate of news stories about inflight incidents involving unruly passengers, you may wonder if it is indeed possible to open the door of an airliner in flight. Well rest assured, not only is that feat physically impossible, but in today’s highly aware passenger cabin environment, a troubled individual would have a very hard time even getting to the door handle without being seriously challenged by other passengers on board.
Technically, an airliner’s door cannot be opened in flight for two reasons. First, cabin altitude during cruise is approximately 8,000 feet above mean sea level, with the cabin pressurized to about 10 pounds-per-square-inch. (Air pressure at sea level is 14.7 psi.) This air pressure pushes firmly against the door, sealing it to the fuselage frame. Second, the door itself is purposely designed as a wedge-shaped plug so that it can’t be blown out of its opening by accident. The higher the cabin pressure, the more tightly the door is sealed. Hence, it is patently impossible to open the door (or overwing emergency exit) of an airliner inflight!

Have you ever noticed large panels on the upper surface of the wing just forward of the flaps that open partially during the approach, and then open all-the-way once the airplane has touched down on landing? These panels are called spoilers, and unlike lift-enhancing devices such as trailing-edge flaps or leading-edge slats, they are used to actually decrease lift. They also deploy asymmetrically to augment the ailerons on each wing for banking the aircraft, but are deployed together during the approach and landing.

Spoilers serve several very important functions. In flight, they effectively increase the aircraft’s rate-of-descent without causing an accompanying increase in airspeed. Once the main landing gear is firmly on the runway, the pilot deploys the spoilers to their fully extended position, creating the maximum amount of drag to slow the aircraft from its touchdown speed of approximately 120-to-140 mph to a more proper braking speed of 60-80 mph. Once the aircraft has decelerated to taxi speed, spoilers are stowed.

We’ve all seen pictures of airline pilots in the cockpit holding either a large yoke or side-stick controller that enables them to fly their aircraft safely and with great accuracy. However, when on the ground, how does the pilot steer the nose wheel to accurately guide the aircraft while navigating the maze of taxiways leading to and from the runway? Although pilots of smaller aircraft use rudder pedals for steering the nose wheel, airliners have a ground steering system that is quite a bit more unique.

Located on the console directly to the left of the captain’s seat is a small wheel or triangular-shaped device called the “tiller” used exclusively for steering when the airplane on the ground. Normally, pilots follow yellow centerline stripes painted on ramps and taxiways, but very large airliners require special techniques to maintain correct position on these stripes while turning, since their nose wheels can be located well behind and far below the cockpit. On takeoff, ground steering is used until the airliner’s aerodynamic controls become effective at speeds of about 60-to-80 mph, while the reverse procedure is used during landing.

From the first commercial airliners, aerial turbulence (once called “air pockets!”) has been a constant source of discomfort for passengers. Once airliners became pressurized, they flew higher above weather systems, but inflight turbulence was still a problem. Jet airliners flew even higher with even smoother rides, but still experienced “vertically active” air masses in flight that caused passengers’ seat belt signs to be constantly illuminated.
With the advent of digital flight control systems in the 1990s, computerized accelerometers were used to detect disturbances in the air and “signaled” the ailerons within milliseconds to mitigate vertical motion. This reduced the severity of the bumps to barely detectible levels. Latest-generation airliners like the Boeing 787 have active gust alleviation in all three axis, meaning the ailerons, elevators, and rudder all modulate up to 50 times-per-second to dampen any trace of turbulence, giving the aircraft an unbelievably smooth, almost “magic carpet” ride.

Compared to early prop airliners that had only a single red rotating beacon to signal their presence at night, the modern jetliner is lit up like a video game. Strobes, LEDs, and Logo Lights all illuminate various parts of a jet airliner, with every light serving a specific purpose. Traditional navigation lights, or “nav lights,” stem from the boating world with green on the right wingtip, red on the left, and white on the tail. The relative position of these three lights can instantly indicate the position and direction of an aircraft in flight.

High-intensity white strobe lights are mounted on an aircraft’s wingtips, and flash in simultaneous or alternating patterns to identify an aircraft from large distances. Fixed landing lights are located in the wing root or on landing gear struts, while retractable landing lights extend from the lower surface of the wing or flap track fairings. Red high-intensity LEDs (light-emitting diodes) replace the old rotating beacons and are mounted above and below the fuselage. Other lights illuminate the wing leading and trailing edges.

While passengers sit comfortably in an airliner’s cabin enjoying their inflight entertainment systems and meals served onboard (at least on international flights), myriad other activities are going on below the floor under their feet. Starting at the nose of the aircraft, a maze of computers and electronic equipment is housed in the avionics bays located just below the cockpit. Immediately aft of that is the wheel well that holds the retracted nose landing gear inflight, and then the cabin pressurization equipment.

Multiple air-conditioning “packs” are ahead of the forward baggage bay filled with luggage and cargo which is packed in large bins on wide-body aircraft. In the center of the lower fuselage is the wing spar carry-through, sometimes fitted with a center fuel tank on long-range aircraft. Behind the wing box are the main landing gear wells, and then the aft baggage bays. Lavatory service equipment is located in both the forward and aft lower fuselage, and finally the APU or Auxiliary Power Unit is mounted in the tail.

With large super-jumbo airliners like the Airbus A380 and Boeing 747-8 weighing nearly one million pounds at takeoff, their landing gear has to be designed not only to take all that weight while on the ground, but distribute it evenly so the aircraft does not crush the concrete runway or asphalt taxiway below. Even smaller aircraft are equipped with landing gear designed to distribute their weight evenly for runways, taxiways, and ramps at the smaller airports they serve around the world.

To solve this weight-distribution problem, aircraft designers initially came up with two and four-wheel “bogies” mounted on two main landing gear struts that effectively spread the airliner’s footprint weight very evenly on the ground. With the advent of wide-body jumbo jets, additional landing gear struts and wheels were added either under the center fuselage, or just inboard of the wing root. The Boeing 777 features a unique landing gear design with six main wheels located in tandem on each main strut.

To paraphrase the old saying, “If it works well, don’t change it.” Although the British de Havilland Comet was the world’s first jet-powered airliner in 1952, it was the Boeing 707 in 1958 that set the standard for all airliner designs to follow. Using four jet engines housed in streamlined nacelles mounted on pylons below the wing, the 150-passenger 707 established a design principal that is still used on nearly every jet airliner flying today.

With its swept-back wings and tailplanes, the 707 represented the epitome of “Jet Age” design. In 1969, Boeing’s jumbo 747 used the same configuration, only scaled-up in size to carry 350 passengers. As turbine powerplants became more powerful and reliable, only two were needed instead of four, as first shown on the Airbus A300. Today, nearly every modern jetliner from the Boeing 737, 767, and 777 to the Airbus A320, A330, and A380 use the standard configuration of podded engines mounted under swept wings with conventional swept tails.