question: When experiencing (heavy) turbulence it somtimes feels like the airplane suddenly drops a few feet. Does this actually happen or what causes this feeling if it doesn't?
comment: You feel that sudden drop because the plane is in fact moving slightly downwards. This happens for the same reason your feels like it's going downwards when you go down a steep slope. The only difference is that your car drives on pavement/ground, and a plane flies on top of high pressure air. Wings create a pressure difference, high pressure on the bottom and low pressure on the top, this is how they fly. Turbulence is caused by an uneven pressure difference when flying through certain areas. This creates a difference in lift which causes the plane to move up or down. The simplest way to put it is like driving down a rough road in a car. It's not dangerous, it's just unpleasant.
passages: Flight Data Recorder shows that during the Go Around, when the aircraft was climbing through with a pitch of 4–5 degrees nose up and at an indicated air speed of , there was an external influence on the aircraft at 12:56. This external influence resulted in the aircraft pitch changing to 7 degrees nose down within the next five seconds. Windshear warning came on at 12:56, pitch attitude steepened further to about 9–10 degrees nose down, and the speed increased to . As a consequence, the aircraft rapidly lost height.
   Because aircraft move so quickly, they can experience sudden unexpected accelerations or 'bumps' from turbulence, including CAT - as the aircraft rapidly cross invisible bodies of air which are moving vertically at many different speeds. Although the vast majority of cases of turbulence are harmless, in rare cases cabin crew and passengers on aircraft have been injured when tossed around inside an aircraft cabin during extreme turbulence (and in a small number of cases, killed, as on United Airlines Flight 826 on December 28, 1997). BOAC Flight 911 broke up in flight in 1966 after experiencing severe lee-wave turbulence just downwind of Mount Fuji, Japan.
   Investigators suspect that MCAS was triggered by erroneous angle of attack (AoA) inputs, as if the plane had pitched up excessively. On both flights, shortly after takeoff, MCAS repeatedly actuated the horizontal stabilizer trim motor to push down the airplane nose. Boeing emphasized in the service bulletin issued November 6, 2018, that this could happen when the plane is under manual control, along with a number of indications and effects. 
   The approach and landing were normal from the air traffic control (ATC) point of view, with no emergency declared according to ATC recordings at the time. The crew reported that they were going around, after which the tower instructed them to climb to 4,000 feet, which was acknowledged by the crew. Shortly after, the tower instructed the next flight to go around and alerted emergency services. Wind shear and an ambient temperature of were reported.
   At 21:35, Flight 3701 requested clearance to climb to , the maximum operating altitude of the Bombardier CRJ series. Clearance was granted by air traffic control (ATC) at 21:36, and the plane climbed to its new cruising altitude shortly thereafter. At 21:54, the pilots began to struggle with control of their plane. Both engines lost power and the plane's stick shaker warned the pilots of an impending stall. The pilots declared an emergency and descended, temporarily regaining control of their aircraft at .
   Low-g condition
  
  Low-"g" condition is a phase of aerodynamic flight where the airframe is temporarily unloaded. The pilot—and the airframe—feel temporarily "weightless" because the aircraft is in free-fall or decelerating vertically at the top of a climb. It may also occur during an excessively rapid entry into autorotation. This can have a disastrous effect on the aircraft, particularly in the case of helicopters, some of which need the rotor to constantly be under a non-zero amount of load.
  
  Section::::Effects.
  
  In smaller airplanes
   Shortly thereafter, the first officer stated: "... it looks worse to me over there." The crew then requested and received clearance from ATC for the 180-degree turn. The turn became extremely steep, with a bank of over 90 degrees and a nose pitch-down of 40 degrees. As they were trying to recover from the turn, the FDR indicated a peak acceleration of 4.3 "g", which the NTSB concluded caused a structural overstress and inflight breakup.
   BULLET::::- 7 January 2017 – a private Bombardier Challenger 604 rolled three times in midair and dropped after encountering wake turbulence when it passed under an Airbus A380 over the Arabian Sea. Several passengers were injured, one seriously. Due to the G-forces experienced, the plane was damaged beyond repair and was consequently written off.
   The NTSB also stated about another accident in 1997, that "a display of angle of attack on the flight deck would have maintained the flightcrew's awareness of the stall condition and it would have provided direct indication of the pitch attitudes required for recovery throughout the attempted stall recovery sequence." The NTSB also believed that the accident may have been prevented if a direct indication of AoA was presented to the flightcrew (NTSB, 1997)."
   The graphs obtained from the recorded parameters show that, when the stick was pulled back, the elevator was immediately set in pitch-up position, but that the airplane’s response time was very slow compared to the usual time, the nose lifting off late while the acceleration was continuing. Such a situation at the beginning of the climb-out indicates either a limitation in the elevator’s operation or an airplane with a center of gravity that is too far forward.
   An investigation from the "Bureau Permanent d'Enquêtes d'Accidents et Incidents d'Aviation" of the DRC Ministry of Transport and Channels of Communication found that "[t]he most probable cause of the accident was the aircraft's encounter with a severe Microburst like weather phenomenon at a very low altitude during the process of Go Around. The severe vertical gust/downdraft caused a significant and sudden pitch change to the aircraft which resulted in a considerable loss of height. Being at very low altitude, recovery from such a disturbance was not possible."
  
  Section::::See also.
  
  BULLET::::- List of accidents and incidents involving airliners by location
   This effect was first discovered when many accidents with Bell UH-1 and AH-1 helicopters occurred. These particular helicopters simply crashed without any obvious cause. Later, it was found that these accidents usually happened during low terrain flight after passing a ridge and initiating a dive from the previous climb.
  
  Articulated and rigid rotor systems do not lose controlling forces up to 0"g", but may encounter this depending on their flapping hinge offset from the mast. However, dangerous situations, as with a teetering rotor, may not occur.
  
  On fixed-wing aircraft
   After listening to the cockpit voice recorder the NTSB stated that "Crew communications consistent with a loss of control of the aircraft began approximately 18 seconds prior to the end of the recording". On March 12 the NTSB stated that the airplane "pitched nose down over the next 18 seconds to about 49° in response to column input." Later that same day the statement was changed to "...in response to nose-down elevator deflection".
  
  Section::::External links.
  
  BULLET::::- Diagram of Atlas Air 767-300BCF Specifications
  
  BULLET::::- Boeing serial no. 25865 photos at jetphotos.com
  
  BULLET::::- Flight information and details at FlightAware
   As the aircraft reached its V1 speed, the captain called "rotate". The pitch then increased from 0.2 to 5.3 degrees. According to the data from the control column, the crew at the time were still applying forward movement to the control column (nose down input), but somehow the nose raised upwards. Data from the aircraft indicated that the crew added more force to the control column, from 14.5 to 17.4 degrees. The aircraft reached V2 and began to lift off.
   The first officer said it was "just like we lost lift." As the captain attempted to level the wings, the crew used right rudder to manoeuvre the aircraft back toward the ground and avoid the water below. The accident report found that "the first officer said that they seemed to agree that the airplane was not going to fly and that their control inputs were in unison." Rachuba and Majure continued to try to hold the nose up to impact in a flat attitude, although Rachuba later stated that there were no "heavy control inputs." The final report further noted that "first officer stated that he did not touch the power levers." The first officer later told investigators that the flight crew's primary focus was to find a safe place to land.
   BULLET::::- PIA Flight 688 — An engine failure occurred in-flight. However, the pilots apparently lost their spatial awareness. The pilots became confused, and the aircraft then crashed in an inverted attitude. All aboard were killed
  
  BULLET::::- AirBlue Flight 202 — an Airbus A321 was on final approach to Islamabad's Benazir Bhutto International Airport when it crashed into Margalla Hills, killing all 152 people on board in Pakistan's deadliest plane crash. The Captain forgot to pull the heading indicator button, causing the plane to fly straight into the hill. The First Officer aware of this, but failed to inform the Captain.
   The National Transportation Safety Board concluded that the crash was caused by "a premature descent below a safe approach slope followed by a stall and loss of aircraft control." Although the reason for the premature descent could not be determined, investigators concluded it was probably the result of either "a deliberate descent below the published minimum descent altitude to establish reference with the approach lights and make the landing," "a visual impairment or optical illusion created by the runway/approach lighting systems," and/or "downdrafts near the approach end of the runway."
  
  Section::::Aftermath.
   According to the NTSB's report, two witnesses fishing nearby testified "that they saw a low-flying airplane in a steep right bank. According to these witnesses, as the right bank angle increased, the nose of the airplane dropped and continued downward. The airplane struck the ground in a nearly vertical attitude."
   In an initial deposition, the Embraer flight crew testified that they were cleared to flight level 370, approximately above mean sea level, by Brasília ATC, and were level at that assigned altitude when the collision occurred. They also asserted that they had lost contact with Brasília ATC at the time of the collision, and their anti-collision system did not alert them to any oncoming traffic.
   Just 2 seconds after taking off from Runway 34, the aircraft began to shake violently. While climbing through an approximate height of 50 ft and with an air speed of about 170 miles per hour, Flight 301 rolled severely to the left and then to the right, with a bank angle of 50 and 55 degree. The flight crew immediately applied ailerons and rudder input to correct the attitude of the aircraft. The autopilot cavalry chimed and the sink rate warning sounded.
   The head-down illusion involves a sudden linear deceleration (air braking, lowering flaps, decreasing engine power) during level flight where the pilot perceives the illusion that the nose of the aircraft is pitching down. The pilot's response to this illusion would be to pitch the nose of the aircraft up. If this illusion occurs during a low-speed final approach, the pilot could stall the aircraft.
  
  Section::::Visual.
  
  Visual illusions are familiar to most of us. Even under conditions of good visibility, one can experience visual illusions.
  
  Section::::Visual.:Linear perspective.
   You damned well shouldn't do.br
  
  I have smacked the tiny sparrow,br
  
  Bluebird, robin, and the rest.br
  
  Dragged vorticies through branchesbr
  
  Throwing eggs out of their nests.
  
  I've hurled through total darknessbr
  
  Just as blind as I could be,br
  
  And spent the night in terrorbr
  
  Of things I could not see.br
  
  I've turned my eyes to heavenbr
  
  Sweating bullets through the flight,br
  
  Reached out my hand and pressed-to-testbr
  
  —the Master Caution light.
  
  BULLET::::- C-130 Version
  
  "Low Flight" 
  
  Oh, I have slipped throughbr
  
  swirling clouds of dust, br
  
  A few feet from the dirt,br
  
  I've flown the C-130 low enough,br
   Flight 517 departed Trenton without incident and encountered no turbulence or unusual weather en route to Richmond. While on approach to Richmond International Airport, at an altitude of about MSL, the captain felt a brief "kick" or "bump" on the right rudder pedal. Around the same time, a flight attendant at the rear of the plane heard a thumping noise underneath her. As the plane continued to descend through , the captain suddenly experienced a loss of rudder control and the plane rolled sharply to the right.
   The Indian board of inquiry determined the probable cause of the crash to be "Deliberate reduction of engine power by the pilot 12 seconds prior to first impact due to altitude unawareness resulting in a high rate of descent, very heavy landing and the undershooting of the aircraft by 1300 feet."
   At 18:10:24 the ground proximity warning system (GPWS) began to sound a "sink rate" warning in the cockpit. The aircraft was descending at an angle of 7 degrees, when the angle of descent for most aircraft landing on that runway was 3 or 4 degrees. Both pilots ignored the warnings. At 18:10:44 the warning system in the cockpit began to sound. The captain responded to these warnings with "that's all right."
  