Blue Angel crash, Air Safety Week Article

For your information.  I was particularly interested in the paragraph:

If the leader banks slightly further toward the rejoining #6 for
geographic positioning alignment, this can have a much magnified and
disproportionate effect on the team-member rejoining from well down on
the "low side". It can cause for the rejoining team-member an unnoticed
and inadvertent significant height loss

Darwin, O.F.

Fallen Angel
The Facts Behind The Blue Angels Air Show Crash
Air Safety Week 04/30/2007
Blue Angel #6 crashed about 4:00 p.m. on April 21, three miles from
Marine Corps Air Station Beaufort in South Carolina. The F/A18 clipped
the top of a tree during a formation rejoin behind the crowd, in
preparation for the team's grand finale flypast for a pitch and break
into the circuit for landing.

Understanding why these spectacular accidents happen increases the
aerodynamic body of knowledge for accident investigators in the
commercial aviation arena.

The April 21 aircraft wreckage was scattered along a rural roadway,
hitting a vehicle and several homes. Eight people on the ground were
injured and the pilot died. This was the 26th death in the aerobatic
team's history and the third fatal Blue Angels crash in 10 years.

In 1999, two pilots of the Blue Angels were killed when an F/A-18
crashed into a stand of pine trees in Georgia as the team practiced for
a show. The rejoin join-up comes after the downward bomb-burst known as
the fleur-de-lis (or Delta Vertical Break) scatters the six team-members
to the four points of the compass.

Conjecture about the cause of these crashes has centered on the fact
that the team doesn't wear lower body G suits because the sudden
inflation of the suit under g onset can affect the delicate stick-grip
required for close formation flying. The inference is that G-LOC (or G
induced loss of consciousness) may have been to blame.

Others have hypothesized about a birdstrike penetrating the canopy and
disabling the pilot. However, there is another possible explanation,
based on eyewitness accounts of the aircraft's final maneuvering and a
snatch of video footage.

During the low-level rejoin, the classic threat is that of the
concentration and focused gaze of the rejoining pilot on the low
(inside) side of the leader's turn towards him being upward. (i.e., he
is looking up skywards at the lead aircraft and also taking spacing on
the aircraft that he is supposed to follow in the rejoin).

If the leader banks slightly further toward the rejoining #6 for
geographic positioning alignment, this can have a much magnified and
disproportionate effect on the team-member rejoining from well down on
the "low side". It can cause for the rejoining team-member an unnoticed
and inadvertent significant height loss

This consequence is caused by the distant rejoining team-member rolling
and pushing in concert with the leader's roll to a higher bank angle, to
maintain his correct extended echelon rejoin "line" of relativity (and
to keep visual contact).

This is a well-known cause of accidents in military rejoins,
particularly at night or over water, where inadequate peripheral vision
of the proximity of terrain/water can fail the "low-man" formation
rejoiner, simply because of where he is looking.

The dynamics of the Blue Angels crash are difficult to visualize in four
dimensions. However #6, joining in turn as the last rejoiner, had to
resolve his early rejoin geometry and go from high 8:30 (relative to
Lead) with a large heading differential to low on the inside of the
leader's left turn (as the formation re-formed up behind the crowd at
500ft AGL).

In the video of the mishap, the aircraft is seen in a steep left bank
with a high descent rate. He gets through about 90 degrees of turn,
rolling left and descending, still looking towards Lead for his echelon
rejoin line and to be co-planar with Lead's bank-angle. He is about
1,500 ft behind when he vanishes into the treetops.

To visualize an "echelon rejoin line", think of a line between the
leader's eyes and his own port wingtip, and extend it. That projected
line changes both as Lead turns (laterally) and whenever he rolls
(vertically). Lead can therefore unintentionally influence a large
height change on a distant rejoiner looking to straddle that line,
simply by changing his bank angle.

Why does the rejoiner need that line? Rejoin geometry says that for a
brisk but controlled rejoin, he must rejoin along that echelon line (or
just below it) or risk latterly losing sight of the leader (or the whole
formation) below his RH canopy sill, and that would constitute a severe
collision hazard.

The dilemma is that his concentration and focus must necessarily be on
the formation (to his right and above) and the required geometry of his
rejoin, and so he can fatally forget momentarily about the ground on his
lower left-hand side. A moment is all that it takes.

Overcooked rejoins have always been one of the greater formation flying
threats, particularly at low-level and with the added pressure of time
when performing in front of a crowd.

Joe Farrell, who had a plane on display at Saturday's air show, says
that the jet largely appeared in control.

"It looked like it was in absolute control all the way into the ground,"
Farrell says. "We watched the guys try to reform. He made the turn and
slid right into the ground."

Saturday's airshow was the seventh display from the commencement of the
team's flight season, which began last month.

The Navy's Blue Angels are stationed in Pensacola and scheduled to fly
in another 66 air shows this year. The F/A18's flight recorder was found
in a pond next to the crash site. Military investigators say it is in
good shape, and it has been sent away for analysis.