Mach tuck

Mach tuck is an aerodynamic effect, whereby the nose of an aircraft tends to pitch downwards as the airflow around the wing reaches supersonic speeds. Note that the aircraft is subsonic, and traveling significantly below Mach 1.0, when it experiences this effect. [cite book
title = Pilot’s Handbook of Aeronautical Knowledge
year = 2003
publisher = U.S. Federal Aviation Administration
location = U.S. Government Printing Office, Washington D.C.
id = FAA-8083-25
pages = pp. 3-37 to 3-38
url=http://www.faa.gov/library/manuals/aviation/pilot_handbook/e ]

Initially as airspeed is increased past the critical Mach number, the wing develops an increasing amount of lift, requiring a nose-down force or trim to maintain level flight. With increased speed, and the aft movement of the shock wave, the wing’s center of pressure also moves aft causing the start of a nose-down tendency or “tuck.” If allowed to progress unchecked, in an aircraft not designed for supersonic flight, Mach tuck may occur. Although Mach tuck develops gradually, if it is allowed to progress significantly, the center of pressure can move so far rearward that there is no longer enough elevator authority available to counteract it, and the airplane could enter a steep, sometimes unrecoverable dive. [cite book
title = Airplane Flying Handbook
year = 2004
publisher = U.S. Federal Aviation Administration
location = U.S. Government Printing Office, Washington D.C.
id = FAA-8083-3A
pages = pp. 15-7 to 15-8
url=http://www.faa.gov/library/manuals/aircraft/airplane_handbook/ ] In addition as the shockwave goes towards the rear, it can impinge upon the elevator control surfaces and this can greatly exacerbate the nose down tendency. Partly for this reason, supersonic and subsonic aircraft often have an all-moving tailplane (a stabilator) which lacks separate elevator control surfaces. [ [http://selair.selkirk.bc.ca/aerodynamics1/High-Speed/Page2e.html Transonic Aircraft Design ] ]

Historically, recovery from a mach tuck has not always been possible. In some cases as the aircraft descends the air density increases and the extra drag will slow the aircraft and control will return.

References

USGovernment|sourceURL= [http://www.faa.gov/library/manuals/aircraft/airplane_handbook/ Airplane Flying Handbook]
USGovernment|sourceURL= [http://www.faa.gov/library/manuals/aviation/ Pilot's Handbook of Aeronautical Knowledge]