TF/A-18 Hornet Dynamic Departure
TF/A-18 HORNET DYNAMIC DEPARTURE
Courtesy Ron Haack
The attached video records the loss of control of a brand new two-seat Hornet during a flight test event at NAS Patuxent River, Maryland, USA on April 2, 1985. Lt Col Tom Wagner tp, USMC was PIC and Sqn Ldr Ron Haack tp, RAAF was in the aft seat for observation.
TF/A-18 BuNo 161947 came off the McDonnell Douglas assembly line with an uncharacteristic roll divergence at 20° Angle of Attack (AOA - Alpha). McDonnell spent about 50 flight hours attempting to isolate the cause without success and the aircraft was transferred to the USN at a discount and assigned to the Strike Aircraft Test Directorate, NAS Patuxent River for further evaluation.
Coincidentally, the initial contingent of RAAF crews were also at Pax Rv with A21-101 and A21-102 conducting certification of RAAF unique modifications and analysis of system endurance in preparation for the Trans Pacific ferry to Aus.
As the only other two-seat Hornet at Pax was the very first prototype and not representative of production A/C, the USN sought a handling qualities comparison between 161947 and one of the RAAF two-seaters. I was also interested in the anomaly and traded a flight in one of ours for a flight in 161947.
As with all flight test, handling qualities evaluation progresses incrementally from benign to worst case condition and the video records the last test point in the last mission of the planned USN evaluation.
The two-seat Hornet exhibits weakened, inherent directional stability simply due to the large keel surface forward of the Center of Gravity (CG) formed by the extended nose and canopy.
For the last test flight, 161947’s directional stability was deliberately weakened further by a centreline fuel tank, wing tip missiles and maximum lateral mass asymmetry; two VER (Vertical Ejector Rack) mounter 1,000lb GP bombs on the left outboard wing station yielding 22,000ftlbs of lateral mass asymmetry.
Planned manoeuvres involved a series of Wind Up Turns at Mach 0.7, 0.8, 0.9. (WUT – a manoeuvre that uses bank and dive/climb pitch attitude to maintain constant Mach/IAS at the required “g” or AOA condition.). WUT’s were conducted both to the left and right and once stable at the test condition (speed & AOA), full deflection roll control was step applied either into or out of the turn direction, e.g. right WUT, stable, full left stick, restabilise then full right stick.
The departure occurred at the last test point of the series in a right WUT at Mach 0.9, full A/B to sustain, and full left stick at 20° Alpha (approx. 6.5g). The A/C initially rolled left about 30° then sliced violently to the right like a Frisbee. Once initial energy was dissipated, the A/C settled into an inverted, rather smooth yaw rotation to the right. Both engines flamed out and relight automatically once the airflow streamlined and as can be seen, aerodynamics ultimately took over, pitched the A/C nose high then into a dive from which recovery was affected.
Once the A/C settled into the steady yaw rotation, I had the impression the fins had separated, and we were going to continue rotating into Chesapeake Bay so I reached for the
seat pan handle. Primary flight displays were in spin mode so basic flight parameters were not displayed on the MFDs but the STBY instruments on the center pedestal behind the stick indicated 18,000ft so I decided I could wait a while longer before ejecting. At that point the A/C rolled upright, pitched nose high then into a dive like a dart and we were able to recover.
We entered the WUT at 30,000ft and recovered to level flight at about 12,000ft. In the heart of the departure, the A/C lost 10,000ft in about 6 seconds so a fairly dramatic rate of descent. Surprisingly, there was no evidence of physical damage, all external stores remained attached and all systems operated normally.
Some time later the USN determined that slight eccentricity of the Radome was generating a small side force sufficient to cause the roll off with weakened direction stability at medium angles of attack. A boundary layer treatment was applied to the lower surface of the radome and that eliminated the boundary layer asymmetry and thus the side force.
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