Page 20 - August18T

Jet Journal
Stability, Steep Turns
& Spirals
by Thomas P. Turner
The recent, fatal crash of a vintage military trainer provides valuable lessons for pilots of twin and turbine airplanes as well. The NTSB reports:
...a North American SNJ-5 airplane impacted terrain following a loss of control during initial climb after takeoff from runway 13R at Kingsville Naval Air Station (NQI), Kingsville, Texas. The pilot and pilot rated passenger were fatally injured and the airplane was destroyed. Visual meteorological conditions prevailed. Witnesses reported that the airplane took off on runway 13R and had requested a right-hand teardrop turn for a departure toward the north. The witnesses reported seeing the airplane in a steep right bank with some reporting that the bank angle exceeded 90 degrees of bank. The airplane descended nose low and the right bank angle lessened before the airplane struck the ground.
Stalls and spins get the lion’s share of coverage in instruction and in article and videos concerning Loss of Control – In- flight (LOC-I). The record shows that LOC-I events are the most common fatal accident scenario, and most LOC-I events appear to be stalls that often develop into a spin before impact.
There is another LOC-I sequence that is neither a stall nor a spin. It is a natural outcome of aircraft stability, and a charac- teristic of all longitudinally (pitch) stable airplanes. Yet it is not mentioned by name, trained or evaluated in Practical Tests for pilot certificates or ratings. The sequence is a spiral dive, and it is what witnesses of the SNJ crash seem to describe.
Here’s how the FAA’s Airplane Flying Handbook (AFH) explains a spiral, with my emphasis added in bold font:
A spiral dive, a nose low upset, is a descending turn during which airspeed and G-load can increase rapidly and often results from a botched turn. In a spiral dive, the airplane is flying very tight circles, in a nearly vertical attitude and will be accelerating because it is no longer stalled. Pilots typically get into a spiral dive during an inadvertent IMC encounter, most often when the pilot relies on kinesthetic sensations rather than on the f light instruments. A pilot distracted by other sensations can easily enter a slightly nose low, wing low, descending turn and, at least initially, fail to recognize this error. Especially in IMC, it may be only the sound of increasing speed that makes the pilot aware of the rapidly developing situation. Upon recognizing the steep nose down attitude and steep bank, the startled pilot may react by pulling back rapidly on the yoke while simultaneously rolling to wings level. This response can create aerodynamic loads capable of causing airframe structural damage and /or failure.
1. 2. 3. 4. 5.
The AFH recommends this spiral dive recovery technique: Reducepowertoidle Applyforwardelevator(“unloadthewing,”i.e.,reducetheGload) Rollwingslevel
Gentlyraisethenosetolevelflight
Increasepowertoclimbpower
Thisexcerptdoesn’texplainwhyanairplanewillnaturallyenteraspiralorhowsuchspiralsdevelop. Thislackof emphasis in training syllabi and complete absence in Practical Test evaluation means many, perhaps most pilots may be unprepared to recognize and recover from a spiral. Let’s delve into why a spiral is a natural outcome of aircraft stability, how a pilot may enter a spiral (it’s not just an attempted visual flight into IMC phenomenon), and why knowing about spirals is important to VFR-only and instrument pilots alike.
18 • TWIN & TURBINE August 2018
PHOTO COURTESY OF PAUL BOWEN PHOTOGRPAHY