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An Airbus A340 in cruise

The cruise (Engl. Cruise ) is in the aviation , the flight phase , which starts after reaching the planned cruising altitude and to the descent lasts. If the altitude or the flight level is changed during the cruise , this is referred to as a " cruise climb " or " cruise descent ".


With the exception of sightseeing flights, where the flight ends at the departure airport, and very short flights, most of the flight route is covered in cruising flight. Aircraft manufacturers therefore optimize the aerodynamics and engines of commercial aircraft for operation at the usual cruising altitudes . Higher cruising altitudes have the advantage that you can fly faster without the frictional resistance of the air and thus the fuel consumption increasing excessively. In addition, thunderclouds and mountains can be flown over, jet streams can be used and air traffic over commercial airports can be avoided.

Before the start of each flight, an altitude, more precisely a flight level, is determined during flight planning, which is classified as most appropriate for this flight , taking into account the maximum cruising altitude and the airspace structure . The planned cruising altitude is not binding in the flight schedule indicating which of the controllers of the air traffic control ( ATC is passed).

Due to the different performance of the different drives used in the aviation industry, the planned flight altitudes between about 9,000 vary feet above sea level (engl. Mean sea level , MSL, roughly 2,740 meters) and 43,000 feet MSL, converted 13105.97 m. The efficiency of piston engine-powered sports aircraft decreases sharply with increasing altitude , while a modern turbofan engine still works very effectively above 30,000 feet MSL = 9,143.70 m.

To calculate the maximum cruising altitude of a commercial aircraft, in this case referred to as the service ceiling , three main characteristics of the respective aircraft type are used:

  • The maximum structural altitude. It is determined by the aircraft manufacturer during approval and is usually due to the strength of the fuselage against stresses from the pressurized cabin.
  • The maximum flight altitude limited by the engine power. At this altitude, there must be enough engine power to perform a climb at a specified rate of climb.
  • The flight altitude limited by the maneuverability. During the flight, bank angles of at least 33 degrees ( FAA ) or 40 degrees ( JAA , EASA ) must be possible without the occurrence of stall.

In the flight planning cruising altitude as close to the optimum altitude (Engl. Is striven Optimum Altitude ) to lay. In most cases it is about 2,000 feet below the maximum altitude and is based solely on economic factors. The fuel costs are compared with the flight time-dependent costs of the flight. Flight-time-dependent costs are maintenance costs and personnel costs that arise from flight-time-dependent maintenance intervals and working time regulation (e.g. overtime regulation) of the flying personnel. A high cruising speed causes high fuel costs, but reduces flight time-dependent costs by shortening the flight time. The coefficient of fuel costs and flight time-dependent costs is known as the " cost index " and can be calculated individually for each flight.

Special circumstances while cruising

cruise climb, step climb

continuous climb - cruise climb
Step climb - due to fixed altitudes by air traffic control - 2000 ft "steps"

An aircraft's minimum fuel consumption per unit of distance is almost independent of altitude, but the speed at which it is achieved is proportional to the mass of the aircraft and inversely proportional to the density of the air. In other words, the heavier an airplane is and the higher it flies, the faster it has to be.

In order to get their customers to their destination as quickly as possible and to make the best possible use of the expensive aircraft, airlines strive to fly as high as possible and therefore as quickly as possible. However, there is a limit to the altitude and thus the speed, namely the speed of sound . If it is reached, an aircraft that is not designed for it can crash due to inability to steer. This means that the best flight altitude is the highest altitude at which the optimal speed is a little below the speed of sound. However, the speed of sound decreases with increasing flight altitude due to the falling temperature of the ambient air to about 295 m / s (approx. 1060 km / h) in the tropopause at an altitude of approx. 11 km.

As fuel consumption makes the aircraft lighter, it can climb a little higher before its optimum speed approaches the speed of sound.

In theory, the most economical variant would be a cruise climb , in which the aircraft is in a very slight climb during the entire flight. For reasons of flight safety, however, this is not possible, since an aircraft generally only receives clearance for a certain flight altitude.

Only the De Havilland DH.106 Comet in 1952, as the world's first jet airliner, still had sufficient scope for a continuous cruise climb in what was then still relatively empty airspace. After take-off, the first cruising altitude was 35,000 ft. The Comet then rose steadily to 40,000 ft with decreasing weight. The other commercial aircraft of this time were propeller planes, they did not fly at this altitude. Propeller planes had to expect no performance improvement from higher altitudes, but on the contrary a performance deterioration.

This freedom was repeated with the Concorde , which entered service in 1976. Due to its small number, it was able to increase continuously from FL 500 to FL 600 (50,000 ft to 60,000 ft, approx. 15 km to approx. 18 km flight altitude) during its transatlantic flights. For this purpose, Concorde received block clearances from air traffic control for an altitude range of 10,000 ft . That was not a problem, since the Concorde flew higher than the rest of the air traffic. With this cruise climb , she reached the maximum flight altitude directly before the start of the landing approach.

For normal air traffic, however, this climb takes place gradually over the course of the flight, since the pilot is bound to the flight levels prescribed by the air traffic controller . The cruise climb is carried out as a step climb . Of course, the step climb also creates coordination problems and additional work for air traffic control and is not always possible in overcrowded European airspace.


Thundercloud from an altitude of 8,000 meters

In the Earth's atmosphere, most weather phenomena, especially clouds, are possible within the troposphere . The tropopause , the beginning of the weatherless area of ​​the atmosphere , which is by far the larger, is directly adjacent to the troposphere . At the border of the tropopause a characteristic, locally limited wind development, the jet streams, forms . These are wind roads up to 100 kilometers wide and three kilometers high with the highest wind speed averaging around 200 kilometers per hour in their center.

Jet streams are used when planning long-haul flight routes. Sometimes a detour is accepted in order to get to the center of a jet stream with a tailwind. The increase in speed compared to the ground ensures that the flight destination is reached with less fuel consumption. It can also make sense to avoid a jet stream pointing in the opposite direction to the flight direction. The position and strength of jet streams are part of general weather conditions. Similar to other weather phenomena, they follow certain rules on average. In detail, however, they can only be predicted for a few days.

A byproduct of jet streams are Clear Air Turbulences (CAT). Under certain circumstances, turbulence forms at the edge of a jet stream, similar to the water eddies in a river between the middle and the river bank. If these turbulences are known to the meteorologists, they are marked as CAT areas on the weather maps for aviation. Pilots can either fly into or avoid these areas depending on their assessment of the severity of the turbulence. The name clear air comes from the fact that the turbulence suddenly appears in clear air without a visible warning in the form of a cloud. It is usually calm in the center of a jet stream.

Thunderstorms also regularly ensure that pilots have to deviate from the planned flight route in order not to be confronted with dangerous turbulence , hail and lightning strikes in a thunderstorm. Thunderstorms vary in height from region to region and season. Thunderstorms are highest in the tropical convergence zone around the equator, where they can reach a height of up to 16 kilometers and cannot be overflown by ordinary airliners. In addition to the risk of a lightning strike , if a thundercloud is directly traversed , the aircraft could suffer structural damage from the strong turbulence inside the thundercloud. For this reason, pilots are required to keep a safe distance from thunderclouds.

Crew duties

Cockpit crew

As a rule, the cockpit crew of a commercial aircraft consists of two pilots who appropriately divide the tasks in each flight phase. While the pilot not flying (PNF) carries out paperwork, radio communications and other administrative activities, the pilot flying (PF) is only responsible for handling the aircraft, in particular its control. This also includes the monitoring and operation of the autopilot, which is normally activated during the entire cruise. Both pilots are responsible for the uninterrupted observation of the airspace in order to avoid a collision. The routine also includes regular inquiries for weather information regarding possible emergency landing or alternate airports and the destination airport, as well as monitoring the on-board weather radar to find and avoid thunderstorm cells. Several communication systems are available for this: ATIS , VOLMET radio service of the weather stations and ACARS .

Furthermore, the fuel reserves are checked at fixed time intervals, as a so-called fuel check . The preparations for the landing approach are also done during the cruise, in particular the programming of the on-board computer, the settings on the navigation receivers and the approach briefing , in which the sequence of the landing approach and a possible go - around maneuver must be discussed point by point .

Cabin crew

The cabin crew usually begins the service before reaching cruising altitude, namely after the pilot switches off the seatbelt sign during the climb. In addition to the provision of food and drink, the cabin crew monitors the health of the passengers. Frequently, unforeseen medical emergencies occur during cruise that require first aid, calling a doctor and coordinating with the pilots regarding an emergency landing for medical reasons.

Economic activities such as the sale of duty-free goods and other goods are added when time permits. The cabin crew is usually informed by the pilot switching on the seat belt signal that the cruise has ended and the descent has been initiated. This is important information for the work allocation of the cabin.

Flight attendants on aircraft for the transport of goods or live animals are primarily responsible for the care of the cockpit crew during the flight, but also for the care of the animal keepers accompanying the animal transport.

Dangers in cruise

Health risks

The technology of the pressurized cabin of modern commercial aircraft leads to the air breathing in the aircraft cabin being dried out . The body loses fluids by exhaling moist air. This circumstance can lead to health problems for the crew as well as for the passengers, ranging from concentration disorders due to dehydration to the formation of blood clots ( travel thrombosis ).

Most turbulence injuries also occur while cruising. This applies above all to the flight attendants, who can hardly protect themselves against injuries caused by sudden turbulence during the serving process.

For passengers, cardiovascular disorders are the most common cause of medical emergencies during the cruise, whereby the degree of previous illness or previous damage plays an important role. This means that only a small proportion of these emergencies are causally related to the situation during cruise, in most cases the passenger brings the illness into the cabin without being adequately prepared for the flight.

Little research has been done into the effects of cosmic radiation , as it has an increased impact on crew and passengers at high altitudes and near the polar ice caps . It is possible to calculate the radiation dose that is absorbed during a certain flight. The value depends on the latitude, altitude and duration of the flight, and the current sunspot activity and, on intercontinental flights over the poles (e.g. from New York to Bombay), reaches about 100 μSv per flight segment, about one tenth of the ICRP annual limit value for the general population. The population dose for all flight passengers together was estimated at approx. 40 kSv / a (2000–2013). Airlines are increasingly required by law to record the radiation exposure of their employees, provided this is over one millisievert (mSv) per year, and to include this factor in the design of the duty roster.

Medical emergencies during the cruise cause the cabin crew to call a doctor. In consultation with the doctor, the cabin crew and the passenger, the pilot in charge must decide whether an unscheduled stopover is to be made or the flight to the destination continues. According to a study of over a thousand in-flight medical emergencies ( IME), the main causes of unplanned stopovers in this context were heart problems (28 percent), neurological disorders (20 percent) and food poisoning (20 percent).

To provide first aid, the crew has an on-board pharmacy available, which also includes medical equipment such as a blood pressure monitor and defibrillator , and the completeness of which must be checked and documented before each flight.

Aviation related risks

The risk of a collision between two aircraft while cruising is relatively low, but there have been isolated collisions in the past. The reasons for this depend heavily on the individual circumstances. It takes a chain of mistakes to make this event happen. In principle, the air traffic controllers are responsible for the spatial separation of the aircraft they are monitoring, but the pilots are also obliged to constantly monitor the airspace and avoid collisions. Technical support for this is provided by secondary radar and traffic notification and collision avoidance systems .

When flying at high altitudes, there is a risk of sudden or gradual loss of pressure , which requires an emergency descent. The cause can be an aircraft skin damaged by material fatigue or a defect in the compressed air supply through the compressors of the engines or their regulation ( pressure cabin ). The crew and passengers are briefly insufficiently supplied with oxygen , which can lead to problems due to hypoxia . According to the regulations of the aviation authorities, aircraft cabins must be equipped with oxygen masks , which are automatically released from the holder above the passenger seats in the event of a pressure loss in the cabin and release oxygen for a certain time. The cockpit crew has their own oxygen tanks and masks, which are designed as full face masks and can be quickly pulled over the nose and mouth with one hand ( Quick Doning Mask ).

As the air becomes colder and colder with increasing flight altitude and the associated decrease in the speed of sound, commercial aircraft during cruise reach their maximum permissible speed, which is specified by aircraft manufacturers using the M MO ( Mach Maximum Operating Number ) in the form of a dimensionless Mach number . It is slightly below the critical Mach number and thus prevents boundary layer detachments on the wings as a cause of flow breaks and the associated risk of falling, as well as sudden extreme mechanical loads on the aircraft structure. Mach 1 is the speed of sound. Most commercial aircraft have a maximum speed limit of Mach 0.7 to Mach 0.9. The Mach number cannot be read from the barometric airspeed indicator , but has to be determined through a computational process using the on-board computer and is displayed by the Machmeter . Commercial aircraft are usually equipped with this instrumentation.

Image 1: Coffin Corner - "Sarge corner" (red marked angle)

A special aerodynamic characteristic of flights at high altitude is the so-called Coffin Corner , which translates directly into: coffin corner. Thus an aerodynamic situation is described in which a speed increase a Mach number exceeding meant during a reduction in speed to the same moment stall would result. This situation can be represented graphically by the intersection of two curves, the intersection forms a corner, hence the name coffin corner. Both exceeding the Mach number ( high speed stall ) and stalling ( low speed stall ) lead to a loss of buoyancy and, in the worst case, to a fall. A tangible preliminary stage of the high speed stall is the high speed buffet , a low-frequency shaking that is generated in the transition from the subsonic to the supersonic area on the wings by small successive compressions. If the aircraft is not supplied with energy in this phase, it brakes automatically due to the high air resistance generated by the compression shock and returns to subsonic flight.

Thanks to the use of autopilot and autothrottle (automatically controlled power levers) in conjunction with the on-board computer, incidents in connection with the coffin corner are extremely rare today.

Fatigue of the pilots during the cruise occasionally leads to a lack of concentration, which in itself does not trigger a catastrophe, but represents a disruption to flight operations. This includes ignoring calls by air traffic control, switching the radio frequency to the next air traffic control sector. If the pilots do not report to the air traffic control unit whose area they are overflying, the flight is considered to be “ unidentified traffic ”. This can entail an interception operation by the country's air forces, in Germany there are air force alarm groups available around the clock, ready for a so-called "Quick Reaction Alert" (QRA, analogously to German "quickly reacting alarm operation"). The identification is primarily used to collect route charges and to check the overflight permit.

According to the publication of a leading aircraft manufacturer, the statistical risk of a crash while cruising is relatively low. However, the death rate is higher than any other phase of flight as it is unlikely to survive a height crash.

Impact on the environment

Contrails running into one another

The fuel consumption of modern commercial aircraft during cruise is between 500 and 3,000 kilograms of kerosene per hour and engine. A twin-engine aircraft with seating capacity for around 200 passengers consumes around 2500 kilograms of fuel per hour of cruise. The carbon dioxide emissions from this process pollute the carbon dioxide balance of the atmosphere to a certain extent. In addition, soot particles and other combustion residues are released into the atmosphere.

The formation of contrails by airplanes also contributes to a not inconsiderable part of global warming . (See Contrails - Effects on Climate ). Most of the time, contrails form at altitudes over 8 km, which corresponds to the cruising altitudes of almost all commercial aircraft. The engine exhaust gases, which contain water vapor and soot particles and are emitted by the jet engines , experience a strong cooling on the one hand due to the sudden expansion and on the other hand due to the ambient temperature of approx. −40 ° C. When the temperature has reached the dew point of the ambient air , the ice crystals form around the soot particles as crystallization nuclei , which are perceived in their quantity as contrails. The extent of contrail formation by commercial aircraft at altitudes below 8 km depends on the relative humidity of the ambient air . If this is close to 100%, contrails also form here, which, however, consist less of ice crystals and more of water droplets.

The visible water vapor itself does not endanger the environment. In the case of heavy air traffic over certain areas of land, over which several airways cross, one can observe that the contrails of the various aircraft merge into a high cloud cloud and thus measurably lead to a reduced sunlight irradiation of these areas of land.

Individual evidence

  1. Flight Crew Training Manual, Boeing 737-New Generation, doc. FCT 737 NG (TM), April 1, 1999. p. 4.5
  2. Flight Crew Training Manual, Boeing 737-New Generation, doc. FCT 737 NG (TM), April 1, 1999. p. 4.7
  3. ^ Thomas Schwarz, Sebastian M. Schellong, Stefan Schwarz: Thrombosis risk after long-haul flights. In: Dtsch Arztebl. 102 (25), 2005, pp. A-1813 / B-1531 / C-1446.
  4. ^ European Program Package for the Calculation of Aviation Route Doses
  5. ^ LE Alvarez, SD Eastham, SR Barrett: Radiation dose to the global flying population. In: J. Radiol. Prot. 36, 2016, p. 93-103. PMID 26769857 doi: 10.1088 / 0952-4746 / 36/1/93
  6. Joint Aviation Requirements, Amendment 13. S. 1-D-14.
  7. Tony Goodwin: In-flight medical emergencies: an overview. In: BMJ. 321 (7272), Nov. 25, 2000, pp. 1338-1341. PMC 1119072 (free full text)
  8. Do buffet boundaries. In: Airplane Flying Handbook: FAA-H-8083-3A. Pp. 15-8. ( ( Memento from September 22, 2008 in the Internet Archive ))
  9.  ( page no longer available , search in web archives )@1@ 2Template: Toter Link /
  10. , p. 21.