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FAR § 25.203 Stall characteristics.

Sun Jul 14, 2019 10:18 am

Does anyone know what jet prompted this law?
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-84, 60 FR 30750, June 9, 1995]

It seems unlikely that it was the B737 since the first order for that jet was on Feb. 15, 1965.
Any other technical comments would be welcome. I am curious if the B737 MAX is the first design to require something like MCAS to keep in compliance with this regulation.

§ 25.203 Stall characteristics.
(a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls.
(b) For level wing stalls, the roll occurring between the stall and the completion of the recovery may not exceed approximately 20 degrees.
(c) For turning flight stalls, the action of the airplane after the stall may not be so violent or extreme as to make it difficult, with normal piloting skill, to effect a prompt recovery and to regain control of the airplane. The maximum bank angle that occurs during the recovery may not exceed -
(1) Approximately 60 degrees in the original direction of the turn, or 30 degrees in the opposite direction, for deceleration rates up to 1 knot per second; and
(2) Approximately 90 degrees in the original direction of the turn, or 60 degrees in the opposite direction, for deceleration rates in excess of 1 knot per second.
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Re: FAR § 25.203 Stall characteristics.

Sun Jul 14, 2019 11:16 am

Part 25 and its predecessor CAR 4B go back a long ways. Stall characteristics for non-FBW planes are pretty settled matter. There needs to be a speed stable without stick force lightning in the approach to stall, basically once in trim slowing requires a constant increase in back force. That’s the basic with some exceptions.

Planes have had various “fixes” for any lack of natural buffet or roll off for decades-shakers, pushers, auto trim inputs and the FBW planes have it all in the software.

It’s not a law, it is a CFR and the AMC Advisory Circular goes into more detail. It’s a not a cut and dried process
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Re: FAR § 25.203 Stall characteristics.

Mon Oct 07, 2019 2:04 am

GalaxyFlyer wrote:
It’s not a law, it is a CFR

Laws created by agencies are called “regulations.” Regulations usually must be authorized by a statute, and are subordinate to statutes. However, they have the same legal force as statutes.

Congress passes the laws that govern the United States, but Congress has also authorized federal agencies to help put those laws into effect by creating and enforcing regulations.
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Re: FAR § 25.203 Stall characteristics.

Mon Oct 07, 2019 2:10 am

There is difference between laws and regulations.
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Re: FAR § 25.203 Stall characteristics.

Mon Oct 07, 2019 2:57 pm

Probably early experiences with the first jets. Investigation findings and other suggestions often require some time to become regulations. I'm rather surprised that there wasn't an earlier version. Stall has been a known risk since the very first flights.

Research brought me to a crash of a DC-8 in 1964: ... Flight_304
2.2 Conclusions

Based on the limited information available to it, the Board concludes that, although the exact time of trim failure cannot be established, such failure did occur and either contributed to the introduction of a PIO in turbulence or was contributory to the failure to recover therefrom, and that the inoperative PTC (Pitch Trim Compensator) also contributed whether retracted or extended, and that there is a strong possibility that it was at least partially extended. The exploration of the histories of this and other DC-8 aircraft suggests also that there could have existed some degree of control system misrigging which could have been additive to any other control difficulties.

At this point it would be appropriate to summarize the many factors with which the pilot may have been required to contend on the night of the accident. It should be noted that none of these factors in itself constitutes a hazard or even a serious situation, however, several or all of them in combination could create conditions under which control of the aircraft could be lost, partially or completely.

a. Findings

Night, instrument conditions prevailed.
Moderate to severe turbulence was encountered.
The PTC was inoperative and may have been partially or fully extended.
The stabilizer drive system failed in the 2-degree AND position at some time during the flight.
The altitude indicator, which was small with a solid black background, was difficult to interpret at night.
The pitch indication of the altitude indicator was "geared-down" but not indexed as to degrees.
The aircraft exhibited marginal to non-existent speed stability and a stick force per g characteristic which test pilots have interpreted as unstable.

b. Probable Cause

The Board determines the probable cause of this accident was the degradation of aircraft stability characteristics in turbulence, because of abnormal longitudinal trim component positions.

The pilots had trimmed the horizontal stabilizer to the full nose-down position, to counter the excessive nose-up attitude that, in turn, was caused by a malfunctioning pitch trim compensator that had extended too far. Once the upset occurred, it was not possible to trim the horizontal stabilizer back to the nose-up position, because of the severe G-forces generated by the crew's pulling back on the yoke after the upset.

Problems with stabilizer trim appear to be much older than MCAS.

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