Table of Contents
5.1.6 Turbulence
In the following, turbulence/air turbulence is understood to be air currents in the atmosphere with a pronounced vertical component upwards or downwards. Heavy turbulence can occur under different weather conditions and at various times of day (Example "Injuries of persons due to turbulences").
Air turbulence can accelerate aircraft that pass through it sideways and lengthways in the direction of flight, but especially in vertical directions. The horizontal accelerations/decelerations are felt as hard shocks. A single impulse will at least endanger the passengers. When flying through turbulent waves periodic impulses occur, the frequency of which depends on the flight speed. If this frequency corresponds with the natural frequency of some of the aircraft parts (wings, etc.), resonance can overstress the aircraft. There have been a few recorded extreme cases in which aircraft were destroyed. Examples 5.1.6-2 and 5.1.6-3 describe the loss of an engine due to heavy turbulence.
Atmospheric turbulence can occur in several very different ways (Ref. 5.1.6-1):
- Thermally-induced turbulence requires an unstable layer of the atmosphere. Individual pockets of hot air and turbulence occur. These are caused by the lower density of the warmer air, which rises upwards. Heavy thermal turbulence occurs in thunderstorms, but is rare otherwise.
- Friction-induced turbulence occurs through deceleration of the airflow by rough areas. Rough areas might include water waves, buildings, or trees. The turbulence created at the rough surface can rise up to an altitude of 1000 meters. The strength of the turbulence is influenced by the thermal layering of the air. Unstable layers with cold air increase the turbulence, while stable layers of warm air decrease it.
- Orthographically-induced turbulence is related to friction turbulence. It is created by the relief of the earth`s surface, i.e. around hills or mountains. If it overlays with thermal turbulence (foehn) this effect can increase considerably (Fig. "Heavy turbulence").
- Dynamic turbulence is to be expected in relatively high altitudes such as the upper troposphere and the lower stratosphere. It is related to sudden, large vertical temperature gradients and jet streams. When warm air lies over colder air, it is called an inversion. At this boundary wind (wind shear) can cause rhythmic vibrations in the air that increase with the size of the temperature difference. Typical wave lengths are about 500 meters, and the amplitudes vary from a few meters to several hundred meters. Especially pronounced wind shear can be expected around jet streams. Vertical gradations of the wind speed can reach up to 40 m/s at an altitude difference of 1000 m. Turbulence in jet streams can be heavily influenced by mountain ranges. For example, lee wave turbulence can occur up to 200 kilometers behind the ridge of a mountain range.Turbulence zones were observed over a time span of a few hours. Their thickness is at most 1-2 kilometers on a stretch of up to several hundred kilometers.
Modern commercial aircraft fly at high speeds and high altitudes. Experience has shown (Example "Injuries of persons due to turbulences") that there are relatively frequent occurrences of powerful vertical accelerations in clear air (Clear Air Turbulence = CAT) near turbulences and jet streams. The term CAT is often used to refer to all turbulences in the atmosphere.
Figure "Turbulence measures": At the typical flight speeds and sizes of modern commercial aircraft, turbulence with a spread of 50 to 300 meters has a strong influence. Smaller abnormalities are passed through without any noticeable reaction, but the aircraft is carried by larger turbulences. Flying through turbulence has the following effects (Ref. 5.1.6-6):
- Impairment of the steerability of the aircraft - Decreased flight comfort
- Altitude loss
- Vibrations; vibrations at resonant frequencies can overstress the aircraft structure
The strength of the effects on the aircraft are given in the table (Fig. "Turbulence measures").
Examples 5.1.6-1:
Excerpt (Ref. 5.1.6-2.1): “About 10 minutes before arrival the aircraft encountered Turbulence….'as a single tremendous jolt'. …The aircraft was descending through 10 000 ft. One Passenger and one flight attendant were seriously injured…..”
Excerpt (Ref. 5.1.6-2.2): “While cruising at night in clear air (well above clouds), …when the turbulence increases ….”
Excerpt (Ref. 5.1.6-2.3): “The aircraft cruised in at….when it encountered clear air turbulence.Several passengers and flight attendants where thrown about the cabin of the aircraft….The flight crew said they were operating in cirrostratus clouds at the time of the occurrence and no echoes were being shown on the airborne radar.”
Excerpt (Ref. 5.1.6-2.4): “The aircraft encountered moderate to severe turbulence during the nighttime flight when it passed through clouds..”
Excerpt (Ref. 5.1.6-2.5): “Flight attendant and a passenger received serious …injuries…during severe turbulence during decent…..Turbulence was forecast to be in the area…“
Excerpt (Ref. 5.1.6-2.6): ”…The aircraft encountered `severe chop with severe turbulence, in clear air'.One flight attendant and two passengers were injured …“
Comments: As the examples show, heavy turbulence can occur at any time of day and in various weather conditions. The cases obviously have been documented by the US authority NTSB because injuries occurred and so they had to be classified as aircraft accidents. The injury of a flight attendant may be caused by activities which rule out the security by a safety belt. That frequently also a single passenger was injured, may be traced back to a collision with the flight attendant.
Concerned have been the most different airplane types. The especial sensitivity of an aircraft type can not be identified.
Example "Break away of the pylon" (Ref. 5.1.6-3):
Excerpt: „Shortly after takeoff from Anchorage, the airplane flew into an area of severe turbulence, while climbing through the altitude of about 2000 feet. The number 2 engine and engine pylon separated from the airplane. The flightcrew declared an emergency and the flight returned to Anchorage, where an uneventful landing was accomplished. The investigation revealed that a strong easterly wind interacted with mountains east of Anchorage (Fig. "Heavy turbulence"), which produced mountain wave activity. The aircraft encountered severe or possibly extreme turbulence. There was evidence that this resulted in dynamic multi-axis lateral loadings that exceeded the ultimate lateral load-carrying capability of the number 2 engine pylon, which had already been reduced by the presence of a fatigue crack near the forward end of the pylon's forward firewall web.”
Comment: That a near the fuselage positioned aeroengine is concerned is obviously typical for this aircraft type (example 10-4, example 10-10.1 and example 10-11). Here additionally an especially weakening of the mounting bolts by corrosion and possibly a load by gyroscopic loads can be supposed.
Figure "Heavy turbulence" : The top diagram shows the turbulence near a mountain range (see Ref. 5.1.6-1). This foehn situation produces rotor clouds on the lee side of the mountains which can repeatedly vertically accelerate commercial aircraft.
In extreme cases this can evidently lead to sudden failure of the entire aircraft structure (Ref. 5.1.6-6).
The lower diagram depicts the situation described in Example "Break away of the pylon".
Ref. 5.1.6-8 describes horizontal vortex tubes (HVTs) at high altitudes that occur through complex, not yet fully understood processes inside jet streams. Inside one of these HVTs, an older four-jet cargo aircraft lost an engine and about 6 meters of wing in 1992. Several cases with personnel damage through extreme acceleration of commercial aircraft have been linked to HVTs. These horizontal turbulences are similar to the roller-shaped turbulence found on the lee side of mountain ranges, but usually occur at high altitudes (usually above 20km), where is no identifiable connection to turbulence on the ground. This means that future aircraft generations that fly at high altitudes are especially threatened by this type of turbulence.
Example "Engine separation" (Ref. 5.1.6-4):
Excerpt: …”(a cargo) freighter overran at Belgium's Ostend airport… during an emergency landing after one of its four engines separated during flight…. The incident began about 20min after take-off, during the early hours of 14 November, when the 707 experienced severe turbulence, causing its number 3…engine to separate.
Although the crew reacted to the engine failure by running through the shut-down procedure, they did not realise that the engine had fallen off (Figure "Heavy turbulence"). With the aircraft loosing hydraulic pressure, the crew decided to divert back….
A similar incident involving a…707-320 occurred in March 1992, when both engines on the starboard wing separated during severe turbulence while it was on route from Luxembourg…The aircraft made a successful emergency-landing…“
Comments: It is evidently suspected that maintenance errors at the very least promoted the separation of the engines in the first case. Evidently the aircraft had attracted attention two weeks before the damage because of its all-around poor condition. Ref. 5.1.6-7 includes the following:
„Delayed C-check maintenance….is being linked to the loss of its No 3 engine…“
This corresponds with Example "Break away of the pylon", which describes verified damage to the engine pylon.
Both examples indicate that, in case of engine separation, special attention should be given to the possibility that the engine suspension was weakened. The mounting of the aeroengine/pylon of some aircraft types is obviously especially prone for influences like corrosion and wear (example 10-10.1).
It is interesting, that also in central Europe it must be reckoned with intense turbulences.
Figure "Stall tendency": The vertical air currents in areas of heavy turbulence such as thunderstorms can change the inlet flow into the engine so much that it causes a compressor surge (Ill. 11.2.1.1-13, Ref. 5.1.6-5).
5.1.6.1 Measures against damage from turbulence
- Sufficient maintenance and inspection of the engine suspensions; this is especially for older engine and aircraft types, which are evidently more likely to be used by the operators for tasks (freight transport, etc.) for which maintenance tends to be less thorough.
- Proper inspections (in accordance with any available guidelines from the fuselage and engine manufacturers) must be conducted after the aircraft experiences dangerously high accelerations in turbulence.
References
5.1.6-1 J. England, H. Ulbricht, “Fluameteorologie”, Transpress, VEB-Verlag für Verkehrswesen, Berlin, page 242-254.
5.1.6-2.1 NTSB Identification ANC87FA021, Microfiche number 31976A, Index for Dec 1986.
5.1.6-2.2 NTSB Identification CHI87FA106, Microfiche number 36768A, Index for Mar 1987.
5.1.6-2.3 NTSB Identification NYC87FA264, Microfiche number 40010A, Index for Sept 1987.
5.1.6-2.4 NTSB Identification ATL88LA034, Microfiche number 35523A, Index for Nov 1987.
5.1.6-2.5 NTSB Identification CHI88FA048, Microfiche number 37766A, Index for Jan 1988.
5.1.6-2.6 NTSB Identification CHI88IA081, Microfiche number 37975A, Index for Mar 1988.
5.1.6-3 NTSB Identification DCA93MA033, Microfiche number 48294A, Index for Mar 1993.
5.1.6-4 “707 suffers in flight engine separation”, periodical “Flight International” 25 November-1 December 1998, page 16.
5.1.6-5 N.E. Borden, Jr. “Jet-Engine Fundamentals”, Hayden Series in Aeronautical Technology, Hayden Book Company, Inc. New York.
5.1.6-6 W. Eichenberger, “Flugwetterkunde”, Schweizer Verlagshaus AG, Zurich
5.1.6-7 H.D.Wulf, “Delayed maintenance blamed for Nigerian 707 engine loss”, periodical “Flight International”, 9-15 December, 1998, page 17.
5.1.6-8 W.B. Scott, “NCAR Finds Horizontal Vortices That Can Imperil Aircraft”, periodical “Aviation Week & Space Technology” August 14, 2000, pages 53 -55.