Resulting Safety Initiatives

Design Changes

Following certification of the 707 in late 1958, additional swept wing jet transports entered service and experience with this type of aircraft increased rapidly. This early operational experience indicated that these aircraft could easily exceed the airspeed operating limitations, and undesirable longitudinal pitch stability could occur under extreme conditions. This experience prompted several changes to the existing transport airplane certification regulations. Amendment 12 to Civil Air Regulations (CAR) 4b and Special Regulation 450, became effective May 3, 1962. The preamble to amendment 12 provides a comprehensive explanation of associated changes:

CAR 4b - Amendment 12 Preamble and complete set of regulatory changes

Airplane Stability

CAR 4b.151 Static longitudinal stability

CAR 4b.155 Stability during cruising

These changes introduced the concept of a boosted control system where the control surfaces may not be directly connected to the control device (such as a control yoke), and required that a control surface move in the correct sense, based on the control input, and added the requirement for specific "stick force" gradients of 1 pound per six knots of airspeed change. This rule change also adopted the nomenclature changes required by CAR 4b.191.

With the inception of 14 CFR 25, these rules transitioned into:

14 CFR 25.173 Static longitudinal stability, and 14 CFR 25.175. Demonstration of static longitudinal stability.

Airplane Handling Characteristics

CAR 4b.191 High speed characteristics.

This regulatory change introduced the requirement to evaluate airplane upset characteristics, and requirements for recovery capability following an inflight upset condition. This regulation also utilized new nomenclature as introduced in 4b.711 and SR-450.

With the inception of 14 CFR 25, this rule transitioned into:

14 CFR 25.253 High speed characteristics.

Following this accident, and other similar events, numerous Special Conditions were issued as certification criteria for a number of new designs.  These Special Conditions required evaluation of  airplane characteristics resulting from a specified degree of mistrim.  These Special Conditions were ultimately codified into a new rule:

14 CFR 25.255  Out of trim characteristics.

This rule also specified control force limits for inflight maneuvering (stick force per g), and that control force reversal could not occur prior to reaching VDF/MDF.

Airplane Operating Speeds

CAR 4b.711 Maximum operating limit speed VMO/MMO

This regulation defined the maximum operating speeds VMO/MMO, specified the type of upset maneuver to be evaluated during airplane certification, and in order to establish adequate margins from dive speeds during upset recovery maneuvers, required that not more than 1.5 G be used for recovery.

In addition, Special Regulation (SR) 450 was issued to explain the change to the maximum operating speed reference to VMO/MMO.

(SR-450) Airspeed Operation Limitation for Transport Category Airplanes

With the inception of 14 CFR 25, this rule transitioned into:

14 CFR 25.1505 Maximum operating limit speed

Airplane Instruments

CAR 4b.603 Flight and navigational instruments

This regulatory change required the incorporation of an aural overspeed warning device for any airplane with a VMO/MMO that was greater than .8 VD/MD, or .8VDF/MDF, and the required warning thresholds as a function of airspeed and Mach number.

With the inception of 14 CFR 25, this rule transitioned to:

14 CFR 25.1303 Flight and navigation instruments.

CAR 4b.612 Flight and navigational instruments

This regulation adopted the new terminology VMO/MMO and VFC/MFC as specified in CAR 4b.711

Airplane Flight Manual

CAR 4b.740-1 Preparation of Airplane Flight Manuals for aircraft certificated in the transport category (FAA policies which apply to CAR 4b.740)

  • Adopted the new terminology of VMO/MMO

CAR 4b.741 Operating limitations

  • Required inclusion of the maximum operating limit speeds in the flight manual, and a corresponding statement that these speeds could not be exceeded during normal operations.

Operational Changes

The Following Advisory Circulars pertinent to operations were issued:

AC 120-5 High Altitude Operations in Areas of Turbulence issued August 26, 1963 - Provided general guidance on appropriate turbulence penetration speeds, attitude control, and autopilot use. AC 90-12 Severe Weather Avoidance, issued April 15, 1964 - Provided general guidance on procedures and limitations of Air Traffic Control for handling air traffic in area of turbulence. AC 120- 13 Jet Transport Aircraft Attitude Instrument Systems issued June 26, 1964 - Provided information on suggested methods to improve the performance and enhance interpretation of attitude indicators. At the time of the accident, attitude indicators displayed pitch markings up to a range of +/- 20 degrees on a black background. To improve situational awareness, a light background above the level pitch line was suggested to provide quick recognition of relative pitch attitude. Additional pitch markings were added to provide relative pitch indications for extreme attitudes.

Flight Instruments

Photo of 707/720 Sperry HZ-4 artificial horizon.
Photo of redesigned 707/720 Sperry HZ-4 artificial horizon. Note that this specific instrument has been redesigned to better indicate sky/ground, and pitch attitude.

Operational procedures for jet transports in turbulence were reviewed as a result of the Northwest Flight 705 accident and other pitch upsets. Some of the significant operational changes follow:

Turbulence Penetration Speed: Prior procedure established the turbulence penetration speed mid-way between the stalling speed and the speed at which the design limit load would be reached if the design gust was encountered. A higher speed provides adequate structural margin while reducing control problems and provides a greater margin above stall. For simplicity, a constant IAS was adopted below 34,000 feet and a constant Mach number above. This change resulted in an approximate 25-30 knot increase in recommended turbulence penetration speed for most jet transports. Further, stall / buffet boundary diagrams for 1 and 1.5g loads were incorporated in flight manuals (Refer to Northwest Flight Standards Bulletin No 8-63 for examples.)

Attitude Flying: The attitude indicator should be the primary instrument in turbulence as other instruments may provide misleading information on pitch attitude. Altitude control and airspeed control become secondary to attitude control.

Autopilot Operation: The autopilot should be used with the altitude hold function off. Trim should be monitored since the autopilot may re-trim. Use of the autopilot will relieve the crew of basic flying and allow for better overall monitoring.

A summary of the industry efforts was presented in AIAA Paper No. 64-353 "Jet Transport Operation In Turbulence" by Paul A. Soderlind, 1964.

In 1998 a Federal Aviation Administration (FAA)/Industry work group co-chaired by Boeing, Airbus and the Flight Safety Foundation developed the 'Airplane Upset Recovery Training Aid' as training program guidance for upset recovery training for air carrier flight crews. (Upset Recovery Training Aid)

Back to top