Hi,
Originally Posted by HazelNuts39
The engines don't normally stall when the airplane stalls. The engines may stall or surge when the intake is exposed to extreme angles of attack or sideslip. The airplane in cruise configuration stalls at an angle of attack that varies between 14 - 15 degrees at low altitude, and 6 - 7 degrees at high altitude. I wouldn't expect the engines to stall at angles below 30 - 35 degrees, depending somewhat on power setting.
which is someway conforting my point.
Stall is a function of AOA, altitude and speed. She may have gently stalled at cruise level but should have recovered "naturally" while losing altitude and speed... unless her AOA would have also increased all her way down without ever exceeding the limits where the power plants would have also experienced any trouble with that.
Which process in itself is fairly unlikely without a very large AOA exceedance, in range of some "deep stall" attitude (50 degrees?), which will imply troubles for her power plants airfoil all her way down.
Next come the distance covered: four full minutes elapsed between last position (02:10:30) and end of ACARS (02:14:30) while less than two is needed to bring her down (at an average rate of about 300 ft/second).
This would mean two more minutes of level cruise flight before entering such a stall (02:12:30), then more than 16 Nautic Miles covered: consequently a crash site somewhere about 20+ NM from LKP.
Originally Posted by HazelNuts39
And if they stall, they don't usually flame out immediately, more likely they will surge and overtemperature.
Not systematically, and certainly not when external/environmental factors like ingestion of ice, rain or foreign objects are concerned, not only standard "airfoil" related troubles. This is something about what I already posted several times in this thread or the previous ones.
You can look, for example, at this study published in Boeing's AERO Quaterly 4.07
AERO - Engine Power Loss in Ice Crystal Conditions
by Jeanne Mason,
Senior Specialist Engineer,
Engine Performance and Operability,
Propulsion Systems Division
Originally Posted by Jeanne Mason
High-altitude ice crystals in convective weather are now recognized as a cause of engine damage and engine power loss that affects multiple models of commercial airplanes and engines. These events typically have occurred in conditions that appear benign to pilots, including an absence of airframe icing and only light turbulence. The engines in all events have recovered to normal thrust response quickly. Research is being conducted to further understand these events. Normal thunderstorm avoidance procedures may help pilots avoid regions of high ice crystal content.
I also posted, several pages back, an article about CFM-56 engines related problems with dual flameouts at cruise level in certain conditions (tropical weather, ice particles and no alert).
Lets have a look at this NASA picture showing what a convective storm really looks like above the standard 22,000 ft freezing level:
This NASA Tropical Rainfall Measurement Mission (TRMM) combined satellite radar image shows a vertical cross-section of a convective storm. The image shows the freezing level clearly by the "bright band" where ice particles become coated with melted water and are excellent reflectors of radar energy. Below the freezing level, liquid water is highly reflective. Above the freezing level, while the concentration of moisture may still be high, the cloud is mostly composed of frozen ice particles with radar reflectivity below 20dBZ (units of radar energy). Small ice crystals are irregular in shape and poor reflectors of radar energy. These small ice crystals are believed to be associated with engine power-loss events.
Strikingly, this may also answer the main question why AF 447 did not take another path accross the weather as its radar could have been blind at this flight level:
Originally Posted by Jeanne Mason
On-board weather radar can detect large particles such as hail, rain, and large ice crystal masses (snowflakes). Small particles, such as ice crystals in high concentrations near thunderstorms, are invisible to on-board weather radar, even though they may comprise the majority of the total mass of a cloud
Overall, most of the conditions described by the researchers were present during AF 447 flight:
Originally Posted by Jeanne Mason
Researchers have identified several conditions that are connected to engine ice crystal icing events. The most important factors are:
* High altitudes and cold temperatures. Commercial airplane power-loss events associated with ice crystals have occurred at altitudes of 9,000 to 39,000 feet, with a median of 26,800 feet, and at ambient temperatures of -5 to -55 degrees C with a median of -27 degrees C. The engine power-loss events generally occur on days when the ambient temperature is warmer than the standard atmosphere (see fig. 4).
* The presence of convective clouds. Convective weather of all sizes, from isolated cumulo*nimbus or thunderstorms to squall lines and tropical storms, can contain ice crystals. Convective clouds can contain deep updraft cores that can lift high concentrations of water thousands of feet into the atmosphere, during which water vapor is continually condensed and frozen as the temperature drops. In doing so, these updraft cores may produce localized regions of high ice water content which spread downwind. Researchers believe these clouds can contain up to 8 grams per cubic meter of ice water content; by contrast, the design standard for supercooled liquid water for engines is 2 grams per cubic meter.
* Areas of visible moisture above the altitudes typically associated with icing conditions. This is indicated by an absence of significant airframe icing and the ice detector (when installed) not detecting ice, due to its ability to detect only supercooled liquid, not ice crystals.
These additional conditions are also typically found during engine ice crystal power-loss events.
* No pilot reports of weather radar returns at the event location.
* Temperature significantly warmer than standard atmosphere.
* Light-to-moderate turbulence.
* Areas of heavy rain below the freezing level.
* The appearance of precipitation on heated windshield, often reported as rain, due to tiny ice crystals melting.
* Airplane total air temperature (TAT) anomaly-reading zero, or in error, due to ice crystal buildup at the sensing element (see case study on following page).
* Lack of observations of significant airframe icing.
Graph Legend:
--- graph tiret (above standard) = Standard Atmosphere +10C events
___ graph plain trait = Standard Atmosphere events
... graph dots (below standard) = Standard Atmosphere –10C events
S~
Olivier