The Flight: Pushback, Engine Start, and Taxi: Airplane and Airline FAQ

In turn, the gate agent receives from the captain a signed flight dispatch release verifying he has received all the FAA-required documents and that his physical and mental health will allow him to successfully complete the flight.

CLOSING THE DOORS

Each door on the plane, including the cargo doors, contains a tiny micro switch which sends a signal to the cockpit indicating whether the door is open or closed and locked. If for some reason they are not closed and locked, the captain will ask the flight attendants to verify their position. If the cabin crew says the doors are closed and locked, but the cockpit indicator reads otherwise, a mechanic must be called.

ACTIVATING THE HYDRAULICS

Immediately prior to pushback the crew will turn on the hydraulic pumps and pressurize the hydraulic system to 3000 pounds per square inch (psi). This ensures the brakes are pressurized for pushback. As mentioned earlier, there are at least three hydraulic systems on every aircraft, and each system has multiple pumps located near the wheel well area. When operating, they create a noise that passengers often hear, an on and off whirl, as they cycle to maintain 3000 psi. This noise is continuous, however other noises act to mute the sound in flight.

SIGNALING THE, GROUND CREW

In the moments before pushback the rotating beacons are turned on. Located on the top and bottom of the fuselage, these flashing red lights serve as anti-collision beacons in flight. On the ground they are a signal to the ground crew personnel to stand clear, the aircraft is set to push or power back.

BLOCK TO BLOCK TIME

The average flight time from point A to point B can accurately be predicted by using the seasonally adjusted average headwinds or tailwinds and the cruising speed of the plane scheduled for the route. Flight time, however, does not completely reflect the total time, called the block to block time.

Though it may only take ten minutes to pushback and taxi to the active runway, the experienced traveler knows that during the busy travel hours, a line for departure is most probable. On the arrival end, it may only be a short taxi from the active runway to your gate, but ramp congestion may block immediate access. The airlines know all this and make allowances.

Average taxi times are computed—some up to 30 minutes at busy airports during busy times—and added to the flight time, which gives block to block time. This is what’s reflected in timetables. Looking at a timetable, you may notice a midnight departure from New York to Chicago takes less time than the dinner flight. The difference is the built-in ground delay time. Sitting on a taxiway waiting for your turn for departure doesn’t necessarily mean a late arrival.

COCKPIT ANNOUNCEMENTS

Before we talk about specific delays, let’s mention why delay announcements from the cockpit sometimes seem late in coming. Early in a pilot’s career, you learn to always tell the passengers the truth. But until specific information is known, pilots tend not to commit to an announcement that is less than accurate. Air Traffic Control, which has the ultimate say on departure, regardless of your schedule, is sometimes noncommittal when the weather is causing numerous flight delays.

CONNECTING FLIGHTS

Most airlines use a hub-and-spoke route structure, allowing more cities to receive more frequent service to more destinations. The downside is that more and more flights require a plane change to a connecting flight. With connections come some possible misconnections. Dispatch and the supervisors in charge of the release or holding of a flight for connecting passengers are connected via computer to the reservations system and are well aware of who is running late. In fact, a specific list of any late connecting passengers is sent to all the applicable departure gates. Factored into a difficult judgment call is how long the flight must be held, for how many people, and if this is the last flight of the day to your destination. It also must be determined if a wait for a few late connections will jeopardize all the downline connections of the passengers already on board.

OVERBOOKING AND NO-SHOWS

Overbooking is another reason for delays. On any given day, 10 percent of the people holding reservations don’t show up. Around peak travel periods it is more. If a plane is only booked to capacity, every flight will depart with empty seats, even the busiest. If a flight is overbooked and the computer projection of no-shows is wrong, volunteers will be asked to get off in return for some form of compensation.

No-shows also affect the weight and balance planning for the flight. All airlines carry cargo in the same compartments used for luggage. If a plane is booked full space, weight limitations may dictate leaving some of the cargo for the next flight. However, if 45 passengers— 15 percent of the capacity of a L-101 1 or DC-10—do not show up, almost 9000 pounds of additional cargo can be carried. Again, last-minute work.

FINAL PAPERWORK

One of the most frustrating delays is waiting for all the required FAA paperwork from the gate agent, who wants no more than to release the plane on time. The same ground-based computers that are absolutely invaluable in running an airline play havoc with the system when they “go down.” Most of the time the computer outages are only a matter of seconds until the backups come on line.

FUELING DELAYS

Fueling delays are sometimes encountered. Suppose the last-minute forecast for your destination is worse than originally anticipated. As discussed in the dispatch section, airlines always carry lots of “unanticipated delay” fuel plus some. If an in-flight delay becomes anticipated before departure, it is prudent to add fuel to cover that delay so as to not reduce your unanticipated quantity.

PASSENGERS SEATED

One of the simpler reasons for a delay in pushback is that not every passenger is seated with all their carry-on luggage properly stowed. The captain is subject to a $10,000 fine by the FAA if the aircraft is moved on the ground with people standing. This explains some of those rather direct announcements.

RAMP TOWER

Airlines, especially at the larger airports, exercise control over their own ramp areas or share the responsibility with the adjacent airline. At smaller airports the sequencing of aircraft is done by the ATC ground-control personnel in the control tower. If multiple airplanes wish to push back onto the same ramp area—or taxiway—at the same time, somebody has to wait. Sequencing of airplanes is beyond the pilot’s control.

ANTI-ICE TRUCKS

As mentioned in Section 4, in cold, wintry weather, Type I de-icing fluid is good for 15 minutes prior to departure, and Type 2 is good for double that. Obviously, with a limited amount of time, the goal is to de-ice the planes as close to departure time as possible.

However, at some airports, where there are numerous flights scheduled within a short period of time, there may only be a handful of anti-ice trucks to service everyone. Having more is too costly, particularly in regions of the country with only occasional wintry weather. The car wash—like procedure now being used should reduce this problem.

FLOW CONTROL RESTRICTIONS

If Air Traffic Control anticipates any delays, mainly due to weather, that begin to approach 30 minutes, they will hold the aircraft at the gate (“gate-holds”) to avoid the extra congestion in the air. It used to be that ATC exercised less scheduling control and allowed more aircraft to hold in the air. The new system of taking the delay on the ground rather than in the air does not affect your arrival time slot at your destination.

Flow control delays caused by the departure airport weather require the spacing between the departing aircraft to be increased. This decreases the number of departures per hour. Flow control restrictions caused by en route weather can be isolated over a particular area and may require only a change in flight plan route. Flow control restrictions caused by destination weather can be of the most concern for someone catching a connecting flight at that airport. However, any weather that affects your flight will most probably have affected the inbound leg of your connecting flight also.

PUSHBACK VERSUS POWERBACK

Some airlines push back their airplanes, some power back. The primary disadvantage of a pushback is the cost of investing in a tug, those powerful-looking, compact vehicles that tow airplanes in and out of the gates. These vehicles, weighing between 60,000 and 80,000 pounds, can generate 480 horsepower in first gear, giving them lots of torque or pushing power.

The major advantage to using these tugs is that the driver is an FAA-licensed mechanic who maintains contact with the flight crew, through headphones, until the engines are started and the engine generators and hydraulics are brought on line. If there are any last-minute problems, he can immediately begin discussing solutions.

However, because of the high cost of these tugs—upward of $75,000—many airlines have made it policy to power back their airplanes, which means actually putting the aircraft in reverse and backing away from the gate.

The disadvantage to this is having to start all the engines at the gate. High power is necessary, which requires extra caution in the gate area. Most aircraft are capable of sucking debris off the tarmac up to 16 feet in front of the engine. Debris sucked into an aircraft engine can cause damage.

ENGINE START

First you must have clearance. As just mentioned, turbo fan engines have a danger zone of up to 16 feet in front, as well as hundreds of feet behind a running engine. Caution must be exercised.

All engines must be rotated or turned in order to start. A lawn mower uses a pull rope to start its engine. A car’s battery provides the energy for the initial rotation. On an aircraft with jet engines, air is used.

On the ground, the APU is the primary source of compressed air, though an external air cart can be used as a substitute. By opening the start valves, APU air turns the starter, which begins the engine spinning. As the engine blades accelerate, they begin to draw the necessary ambient air into the combustion section of the engine for the start.

At approximately 25 percent of maximum engine RPM there is enough air in the multiple burner cans for a start. One of the two ignition systems is energized—to provide the heat—and the fuel valves are opened to complete the start.

As the engine accelerates to its ground idle RPM’s of 40 to 60 percent, the starter is disengaged and the ignition is turned off. A jet engine is similar to your gas stove. You need the pilot light to initially ignite the gas flame on the burner, but once the flame is lit, the pilot light serves no purpose, except if a relight is necessary.

After the fuel and air are mixed and burned, their exhaust is routed across a two-stage exhaust turbine, causing it to turn. The exhaust turbine is mechanically connected back to the compressors in the front of the engine, causing them to turn, drawing more air into the engine for combustion, and the cycle continues. One reason aircraft engines are extremely reliable is that they have very few moving parts relative to the power they generate, and they only turn in one direction even during the use of reserve thrust on landing.

From your seat in the cabin a few things can be noticed during engine start. First, the APU only provides enough air for air-conditioning or engine start, and then just one engine at a time. All except the individual gasper fan above each seat must be turned off during the complete engine start cycle. First you will hear the air-conditioning being turned off, then, particularly during the summer, the cabin will become stuffier and warmer. Finally, the air-conditioning comes back on with more noise and more cooling flow than before the engine start.

Turbofan engines also have a unique rumble and whine as they start. With the starter turning the engine, the sound is hardly noticeable. However, when the fuel valves are opened and the engine accelerates toward idle RPM, a pitched rumble is quite distinct. Then, as the rotating parts of the engine stabilize, the rumble gives way to the familiar engine whine. On takeoff this same roar can be heard as the engines accelerate from idle to full takeoff power.

AFTER START CHECKLIST

In the cockpit there are multiple columns of engine gauges. One column for each engine. Each engine has two RPM gauges that indicate the rotation speed of the engine at two different locations. RPM’s are given as percent of maximum, making it easier to read than your car’s tachometer. An exhaust gas temperature gauge reads the temperature as it exits the “hot” section of the engine. Temperature limits power as well as RPM. Fuel flow meters measure the fuel consumption rate. Fuel flows can reach 18,000 pounds (2700 gallons per hour) per engine on fully loaded jumbo jets. Fortunately, those high power settings are only needed for takeoff and initial climb. Oil quantity, oil pressure, and oil temperature are familiar gauges. An engine pressure ratio (EPR) gauge is unique to aircraft. EPR is the ratio of the pressure exiting the engine compared to the initial pressure. Like a balloon that’s not tied and let loose, the greater the pressure exiting, the more potential velocity. Engine vibration meters sense the smoothness of the engine at multiple locations. Like a tire, an engine out of balance needs repair. If there is a problem indicated by non-normal engine readings, the mechanic is available to inspect the engine to determine if it is a faulty gauge or something more substantial.

TAXI CLEARANCE

At large airports, ramp control clearance must be obtained to begin taxiing. When leaving the ramp area, ATC ground control has jurisdiction.

At smaller airports the ground controller located in the airport tower has the sole responsibility for airplane taxi flow. Larger airports have a ground radar system so they can “see” airplanes even in the fog.

ASSIGNING A RUNWAY

At airports with multiple runways, Air Traffic Control uses the general direction of your destination to decide which runway to assign you for take off.

At Miami, for example, there are two parallel runways aligned east and west, one north of the terminal and one south. Planes traveling to South America will be assigned the south runway. For aircraft destined for Chicago or Boston, ATC will assign the north complex. This sequencing on the ground helps the separation in the air by limiting cross traffic.

However, the south runway in Miami is longer than the north one. Since heavier planes need longer runways, northbound flights may request the south departure runway even if it means a delay. Some runways are smoother than others, causing less wear and tear, particularly on the tires, and it might be requested. Pilots can request the longest runway for any operational need, and at no time has ATC ever turned down the request.

TAXI OUT—FINDING YOUR WAY

All commercial airports have airport charts, which are essentially road maps illustrating the various runways and taxiways. The taxiways are labeled with letter identifiers, like Alpha, Bravo, Charlie, Delta, and Echo. After leaving the ramp area, the pilot calls ground control and says, “ABC flight number twelve,” and ground control says, “Taxi to runway so and so,” via “Taxiway Alpha to taxiway Bravo.” If the airport is unfamiliar, pilots can then look on their maps to find their way.

During daytime the thick yellow taxiway center line is visible. At night the taxiways are illuminated with blue lights on the edge and green lights down the center. To prevent taxiing onto an active runway, one for which you’ve not been cleared, there are hold lines painted on the pavement. These cannot be crossed without obtaining clearance. Since these are difficult to see at night, these junctures also feature five amber lights running perpendicular to the taxiways. They indicate there is an active runway in front and permission is needed before taxiing onto or crossing it.

ACTUAL TAXI OUT

The taxi to the runway can occasionally feel a touch bumpy. Aircraft average 45,000 to 50,000 pounds of weight per each landing-gear tire, and though the concrete is almost two feet thick, this heavy tire footprint makes constant airport repairs necessary.

During taxi there seems to be constant acceleration and deceleration. But there’s a reason for this. Aircraft engines have fairly high idles. A lightly loaded plane in particular requires very little additional power to start it rolling. And, unlike a car, when you take your foot off the gas

—hand throttles, in the case of a plane—the aircraft may not slow right away. If you were to ride the brakes, they could become hot and less effective in case of an emergency stop.

The proper technique for taxiing a plane is to apply the brakes until the aircraft is traveling very slowly, release the brakes, let the plane reaccelerate, and repeat the procedure.

This maneuver is the most effective technique to keep your brakes their coolest.

The taxiways themselves are small compared to the size of a jumbo jet. Looking from a window seat, you can see the wings hang over the edge of a normal 75-foot-wide taxiway. And because the wheel track of the larger planes, like the 747, is up to 36 feet wide, a great deal of attention is required to keep the aircraft directly on the center line. Since the wheelbase—the distance from the nose wheel to the main landing gear—is up to 84 feet, particular skill is required to keep the plane on the center line throughout a sharp turn, which is one reason all passengers have to stay seated for taxi.

WHY FLIGHT ATTENDANTS TAKE THEIR SEATS DURING TAXI

After flight attendants have completed their safety- related duties, FAA regulations stipulate they remain in their seats throughout the duration of the taxi. Their primary responsibility after arming their emergency exit, and cross-checking across the aisle for the same, is to ensure that at least one flight attendant is available near every exit.

COCKPIT TAXI CHECKS

The captain taxis the airplane, using the taxi steering wheel located to his left, and separate from the flight controls. This steering mechanism, which on most aircraft is shaped like a tiller rather than a wheel, can hydraulically turn the nose wheel up to approximately 75 degrees left or right. Both the captain and co-pilot can taxi the plane straight, using the lower half of the rudder foot pedals. These pedals can turn the nose wheel up to 6 degrees left or right, sufficient for the takeoff and landing roll to be accomplished by either pilot. (As an option, a taxi tiller can be fitted on the co-pilot’s side also.) The upper half of the same foot pedals controls the main gear brakes. Unlike a car, the left or right brakes can be applied independently to aid in making a tight turn.

While the captain is taxiing, the co-pilot is responsible for ramp tower, ground control, and tower communications, as well as completing—with the flight engineer on a three-person crew—the taxi procedures. When the captain is free of taxi distractions, the applicable written checklists are jointly used to verify that all the taxi checks have been accomplished.

All departure performance figures computed by central load control and dispatch are reviewed. Interestingly, all the computations are based on the unpleasant assumption that one engine will fail at the most inopportune time, just at liftoff. Every piece of data used by the flight crew, whether or not the runway is long enough for takeoff, whether or not the airplane can clear any and all obstacles at the end of the runway or in the airport vicinity, whether the plane can stop on the remaining runway during a takeoff abort, is all based on engine failure figures.

As we taxi out, the cockpit crew is double-checking the runway length, the aircraft’s weight, and any local weather that could affect the takeoff.

Different weight limits are compared, and the most restrictive always apply. No matter how long the runway, each and every plane has a maximum weight it can safely lift and still meet all the emergency parameters. This maximum certified structural takeoff weight as defined by the manufacturers will never be exceeded. Most flights take off well below the maximum gross weight.

Airplanes closer to their maximum weight will require more runway than when the plane is lighter. It is inaccurate to assume a big plane like a 767 takes more runway than a smaller DC-9. A 767 is indeed bigger and heavier, but the engines are correspondingly more powerful; therefore, the thrust-to-weight ratio remains similar.

Takeoff limits are subdivided into runway and climb limits. Runway limits define the maximum this particular plane can weigh and still be accelerated to takeoff speed, an emergency abort made, and the plane safely stopped.

Climb limits define the maximum this plane can weigh, be accelerated to takeoff speed, have a catastrophic engine failure, and still climb and clear all obstacles.

Hot humid days and departures from high-elevation airports require longer takeoff runs. Headwinds are advantageous, increasing the runway allowable weights approximately 700 pounds for every knot of headwind. Tailwinds require penalties, up to 3500 pounds for every knot up to the maximum legal tailwind limit of 10 knots. Crosswinds are allowed, but the 90-degree crosswind component can not exceed the crosswind limit of the airplane, which by FAA certification standards must be no less than 25 knots. Runways that are wet can be slippery; a 15 percent margin of safety must be added.

FLAP SETTINGS

Once the various limits are double-checked, the proper flap setting must be determined. Most airliners have a choice between two settings, 5 and 15 degrees.

“Flaps 15,” the normal takeoff position on most aircraft, provides a great deal of additional lift. This flap setting provides the quickest and shortest ground run.

“Flaps 5,” requires a longer takeoff roll, because the lesser flap setting means a greater takeoff speed is required; however, the additional speed offers a greater initial climb gradient.

Full power or a lesser alternate power takeoff may be made. If weather, runway length, climb gradient, or any MEL (Minimum Equipment List) restrictions were a factor, full power would always be used. Alternate power is only considered when the aircraft’s actual weight is well below any legal limits. Adhering to very strict guidelines, an alternate power setting can be used primarily for noise abatement and reducing engine wear.

Any alternate power setting must still meet all the safety criteria governing engine failure at the most critical time, and be able to do so without increasing the power, though that option remains.

TAKEOFF SPEEDS

Takeoff speeds are directly proportional to the aircraft’s weight, adjusted for factors such as power setting, flap setting, runway condition, temperature, and departure airport elevation. The three critical speeds are V-1, V-R, and V-2.

The V- 1 (velocity- 1) speed, calculated from tables on board the plane, is the maximum accelerate-stop speed. A decision to abort a takeoff must be made before the V-1 speed. An abort above the V-1 speed does not guarantee enough time to decelerate to a complete stop on the remaining runway, therefore the aircraft is committed to takeoff. V-R is the initial rotation speed, the speed at which the airplane begins its actual takeoff. Quick calculations will show you that on exceptionally long runways, V-l can sometimes be a higher speed than the V-R speed. Since an aborted takeoff would never be attempted after the nose has been raised off the ground, V.1 and V-R in this case would be the same.

The V-2 speed is the single-engine climb speed. If a critical engine should fail at or above the V-1 speed, the V-2 speed will provide the best rate of climb.

FLIGHT CONTROL CHECKS

During taxi it is vital to check each and every flight control, making certain they are operating through their complete range. Since the crew has very limited, if any, view of the wings, and no view of the tail, flight control position gauges are installed. When passengers see the spoiler panels and ailerons move up and down during taxi, what they are seeing is the pilot checking the controls.

CABIN VISITS

If the pilots—mindful of the 15- to 30-minute time span crucial during de-icing—suspect any ice buildup, one flight crew member goes to the cabin windows as nonchalantly as possible, and verifies that the wings are completely free of frost and wet snow. If they are not, further de-icing is necessary. The term wet snow is important here. On very cold days dry snow can accumulate on the wings, but the frigid cold prevents any of it from sticking, and it will easily blow off on the initial takeoff run. Any doubts about wet or dry and it’s back to the de-ice trucks.

TAKEOFF SIGNALS

There are two ways the pilots can notify the flight attendants that it is close to departure time. One, if time permits, is a public address announcement ending with something like, “Will the flight attendants please prepare the cabin for departure.” If the cockpit crew is busy with a checklist, or talking with ATC, then communications between the cockpit and cabin is done with chimes. Two bells indicates that only a few minutes remain before departure. The flight attendants will double-check, but it is the passengers’ responsibility to ensure that their seat belts are securely fastened, their seatbacks and tray tables in their full upright position, and all carry-on luggage properly stowed under the seat in front of them or in a closed overhead bin.

CONTROL TOWER

With the engines started, the flight attendants seated, and all the checklists complete, all that is left is to wait for our turn for departure. The radios are switched from the ground control frequency to the tower, which is responsible for controlling all takeoff and landing clearances. On a runway used primarily for takeoffs, departures can average nearly one a minute. On a runway used for both arrivals and departures, no one will quarrel with the fact that inbound flights have priority.

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