How does a fighter “thump” an intercepted aircraft?

Question

In P&E Never Again: A Thump on Final, commercial pilot Mike Mercer describes an encounter with an F–16 shortly before rounding out to land.

Without warning, the Baron abruptly yawed 15 degrees right, and there was a thunderous noise on the right side of the aircraft. I was standing on the rudder pedals to maintain directional control and managed to keep things more or less aimed at the runway … On the rollout I got a look at the F–16 in full afterburner climbing back to altitude and turning to circle the field. I concluded immediately that we had been “thumped,” a procedure we used in the military that was designed to rattle and harass the pilot of an intercepted aircraft.

For context according to the linked account, this incident took place inside a VIP TFR that the author flew into unwittingly. His hand plotting and ADS-B display on his EFB led him to believe the TFRs were well north of his destination. Shortly after departure, the author attempted to request but was unable to obtain flight following.

What maneuver does an intercepting fighter perform to “thump” its target? Other than the severe yaw that the author mentioned, what are other typical aerodynamic effects?

Answer 1

'Thumping' is an (unofficial) term for maneuvering the aircraft so that the other (target) aircraft is caught up in its wake. As a practice, it seems quite widespread, atleast during cold war scenarios, and is explained elsewhere:

Aircrews call it “thumping” — the aviator’s version of a baseball hurler’s brushback pitch. A fighter jet zooms up behind a lumbering reconnaissance craft, then blows past with afterburners blazing, pulling up and away just in front of the larger plane so its jet wash disrupts the airflow around it.

The effect is the same as flying into other aircraft's wake turblence, though the effects may not linger as long as the aircraft passes quite fast (that being the idea- to harass, rather than to cause an accident).

That said, I doubt it was done deliberately in this case as it is quite dangerous to both the aircraft and intercept procedures are quite clearly defined.

Intercept procedure; from FAA media release

Note that it specifically states to be cautious of wake turbulence. Maybe the author got caught in the wake turbulence of the fighter accidentally and it brought back memories of his service days.

Answer 2

In a thump the jet positions itself slightly in front of, and under one wing. Works well on a prop aircraft, as the airflow is disrupted across the wing, making the plane fall, rather abruptly, 20 to 60 feet. Once it is clear of the wash, normal flight resumes. However, the jet can reposition itself again, for another thump.

There is little risk to the aggressor in this case.

Addendum #1

From an ABS news interview, http://abcnews.go.com/Nightline/story?id=128859&page=1

Q: Isn't there a certain amount of gamesmanship going on at least in your experience with the Russian pilots?

A: There was. Most times it was a very straight-forward operation. They would come join on us, see that we were the same airplane that was out there the day before probably, and then leave. Every once in awhile they would get a little closer, stay a little longer and on two occasions they did what we call a thumping maneuver — where they'd come up underneath us and then pop up in front of our nose and put us into their wing tip vertices and their jet wash and bounce the airplane. And one time they bounced so much that the auto pilot kicked off and we started to make a turn that we hadn't planned on and we had to quickly react to that.

Q: This thumping occurs when they are flying right underneath your aircraft and then pull up in front of you?

A: Yes. What they do is they come up right underneath your aircraft and then pull up right in front so that the wing tip vertices and the turbulence from their wings and their engines go right in front of the airplane and get the fuselage and wings of the P3.

From the book, The Chinese Navy: Expanding Capabilities, Evolving Roles edited by National Defense University (U S ), Institute for National Strategic Studies, Phillip Charles Saunders, Christopher D. Yung, Michael Swaine, Andrew Nien-Dzu Yang

“In a spiraling exacerbation of tension, the Chinese became increasingly reckless in their responses to American reconnaissance missions. PLA Navy Air Force (PLANAF) pilots began harassing American pilots, either as ordered from above or on their own volition, often employing alarming closure rates during intercepts and sometimes even pulling ahead of the larger aircraft and “thumping” it with engine afterburner exhaust. (*26) Finally, on that fateful April Fools' Day 2001, “hot-dogging” Lieutenant Commander Wang Wei captured the world's attention when he struck a U.S. Navy VQ-1 EP-3E on his botched attempt at a third “join-up.” … The pilot of the EP-3, young U.S. Navy Lieutenant Shane Osborn, recovered from a steep 8,000-foot uncontrolled dive and flew toward Lingshui on Hainan Island in his badly damaged aircraft (*27) Osborn made an engine-out, no-flap landing without an airspeed indicator at the PLA Navy airfield...”

And this from Congressional Research Services, Order Code RL30946, China-U.S. Aircraft Collision Incident of April 2001: Assessments and Policy Implications

On several occasions, PLA pilots had overtaken EP-3 aircraft from the stern at high speed, passing underneath and abruptly pulling up in front of the American aircraft at close range. The practice known in some pilot circles as "thumping" is essentially an aerial tweak, intended to cause consternation to the victim as he is suddenly confronted with the noise, jet wash, and the jet in unexpectedly close proximity.

Normally thumping does not make the media, but in 2001 an incident involving the EP-3 suffering significant airframe damage elevated this to public light. While journalists, media and other non-pilots may comment on thumping and simplify it to be wake-turbulence, or whatever, it should be clear that the thumping maneuver is intended to induce loss-of-control, even if only temporary, on the victim aircraft.

Furthermore, contrary to press reports, the maneuver is more effective when performed asymmetrically on the victim aircraft, as it can induce a unilateral stall-spin due to the loss of lift on one side of the wing, which increases the "effectiveness" of the maneuver.

A former USAF one-star indicates that more than once he had fleet airframe damage and had to remind pilots to avoid doing such maneuvers during training sessions. He also indicates that there is no SOP or written guidance on this procedure, as of his retirement in 2006.

In summary, thumping is not a normal wake turbulence maneuver. It is not a ICAO style intercept maneuver. Executed asymmetrically, this deliberate maneuver has a very significant aerodynamic effect in that it causes loss of lift on one (or more) wing, unexpectedly. While on low approach, it is extremely risky. The maneuver is not documented as a standard maneuver, nor is it officially sanctioned. There are numerous memos discouraging or prohibiting the practice of the procedure. A Vietnam era F-4 pilot recalls the maneuver being used in Vietnam, but does not know when it was first employed. He points out that the capability of the F-4 and other Vietnam-era fighters facilitate the maneuver.