Stall Warning vs Stall Alerting - a Plea to the FAA

A couple of recent fatal traffic pattern stall accidents have stimulated me (and I need much more stimulation at my current age than I used to) to write this post.??In spite of my feeble attempt at humor, this is very important to me.??I hope someone in a leadership position in the FAA will read it.

The first stall accident was a Challenger 605 that crashed during a circling approach into Truckee-Tahoe airport in July 2021. The aircraft was in perfect condition, nothing broke, weather was day VMC, light wind, pilots were well-trained – but 6 fatalities.

Five months later, the second stall accident happened. It was a Lear 35A that crashed during a circling approach into Gillespie field in El Cajon. Again, the aircraft was in perfect condition, nothing broke, weather was marginal night VMC, light wind, pilots were well-trained – but 4 fatalities.

When the final reports are written, the primary causal factor in both of these accidents will undoubtedly be Pilot Error. Can’t argue. Would never have happened to me or you, right???

The Stall WARNING system in these aircraft did not help because it’s trigger points don’t give adequate time for corrective action.

But IMHO, if an effective low airspeed/stall ALERTING system had been installed in these aircraft, the pilots would have noticed their error early in the accident sequence and would have corrected their error. These 10 people would be alive today.

Common elements in these 2 accidents include:

Similar Scenario:

·??????Maneuvering in the traffic pattern

·??????Task saturation

·??????Head out of the cockpit

·??????Interrupted/delayed instrument scan

Same Result:

·??????Inadvertent/un-noticed loss of airspeed

·??????Accelerated stall

·??????Strong roll off that was uncontrollable

·??????Impact with terrain

Another common element is that both of these aircraft had the same type of FAA approved (actually mandated) Stall WARNING System installed in the form of a Stick Shaker and a Stick Pusher; systems that were specifically designed to mitigate against this type of scenario. In fact, this type of FAA approved Stall WARNING system is installed in 1000s of Commercial and Business/Corporate aircraft that are in daily operation as this is written.

It is my opinion that both the Challenger ‘Stall Protection System’ and the Learjet ‘Stall Warning System’ proved their?ineffectiveness?in these accidents. Does anybody disagree with me? Will these types of warning systems perform any better in other aircraft?

The question is: Why were they ineffective and what can we learn from these accidents to prevent a repeat in the future? I hope this post can make a contribution. I hope the FAA is listening. It’s not just what we learn from accidents like this or how many safety seminars that we attend, it’s about the ACTIONS that we take to prevent a recurrence.

Why should anybody read my Rant?

Like Sledge Hammer famously said many times: ‘Trust me, I know what I’m doing!’

My ‘linkedin’ connections out there know that my main area of technical expertise is in low-speed/high angle-of-attack aerodynamics, flight mechanics and flying qualities.??Most of my efforts over the years have been applied to the fighter design/flight test world with the F-5, F-16 and F-20.??But I started my career at Learjet working with Pete Reynolds trying to improve the stall characteristics of the Learjet 24 and 25. So I hope the people that take the time to read this will give it at least a little credibility.

Long introduction but here’s the ‘meat’.

Stall WARNING Fundamentals:

For a Business/Corporate aircraft to have success in the market, a low?Stall?speed can be as important as a high?Cruise speed.

An aircraft with a high Stall speed is much harder to sell. The higher speed leads to faster approach speeds which many pilots are not comfortable with. From a practical standpoint, it causes an increase in Balanced Field Length making fewer airports available for customers to operate from. Hell, brakes even wear out sooner.

As a result, engineers and designers work just as hard to achieve a low Stall Speed as a high Cruise Speed.??

But there’s more to it than just the Stall?Speed.??To be certified, clear and distinctive WARNING of impending stall must be provided to the pilot and the aircraft must remain CONTROLLABLE through the stall.

If the designers have done a really good job, this warning can be generated ‘naturally’ by careful tailoring of the aerodynamics of the wing and controllability can be maintained by assuring the control surfaces retain effectiveness in the stall.?

A ‘good stalling wing’ is one where the aerodynamic flow begins to separate from the inboard trailing edge at moderate angle-of-attack and then progresses forward/outboard and increases in intensity as angle-of-attack increases. This separation gives a good ‘tactile’ warning to the pilot in the form of airframe buffet but it needs to develop somewhat symmetrically so that any roll tendencies can be controlled by the pilot.

A ‘bad stalling wing’ is one where the aerodynamic flow breaks down suddenly and asymmetrically with no warning to the pilot. In a worst-case scenario, one complete wing panel stalls before the other and a violent roll off results that cannot be controlled. The original Learjet 20 series aircraft exhibited this type of stall behavior. I know it well.

If the ‘natural’ aerodynamic stall warning/stall behavior is found in flight test to not be adequate, the FAA requires an ‘artificial’ stall warning system be installed. In most cases, this takes the form of a?Stick Shaker and a?Stick Pusher.

To be useful, the Stick Shaker/Stick Pusher must be triggered at airspeeds that are higher than the natural stall speed.??The question is: how much higher? If the natural stall behavior is closer to the ‘good stalling wing’ described above, the trigger points can be relatively close to the aerodynamic stall. But if the natural stall behavior is closer to the ‘bad stalling wing’, the trigger points must be raised.

Because the Stick Pusher speed will now become the ‘official’ Stall speed for performance calculations, this sets up an inherent conflict between the OEM and the FAA. The manufacturer wants to set the trigger airspeed lower so that?Performance?will not be too adversely affected; the certifying agency wants to trigger it higher for increased?Safety.

In many cases, Safety loses. This is a reality, not a criticism.

In the Challenger 605 and the Learjet 35A, the Stall Warning Systems trigger the Shaker at 7% above stall and triggers the Pusher at 1% above stall. In a 1g, wings-level stall at 15K feet, this is plenty. In an accelerated (turning) stall, this is less than 1 second. Not enough at low altitude.

Every Challenger and Learjet 35A pilot that we have spoken to has told us: ‘the Stick Shaker/Stick Pusher comes on too late’. Anybody disagree? FAA?

I believe that the solution to this seemingly intractable situation is available and ready to be easily implemented at low cost. It’s called an effective Stall ALERTING System. FAA action is required.

Stall ALERTING Fundamentals:

In contrast to a stall WARNING system, adding an effective stall ALERTING system to an aircraft has no adverse effect on Performance, just a positive effect on Safety.

Most aviation safety experts agree with me and have advocated for this type of solution for more than 25 years.

A 1996 FAA/industry report titled “The Interfaces Between Flight Crews and Modern Flight Deck Systems” expressed concern about the history of Part 121 and Part 135 accidents involving lack of low-airspeed awareness in the context of the flight crews’ monitoring of automated systems. This report states:?

“Flight crews may not be provided adequate awareness of airplane energy state, particularly when approaching or trending toward a low-energy state…Transport-category airplanes are required to have adequate warnings of an impending stall, but at this point the airplane may already be in a potentially hazardous low-energy state. Better awareness is needed of energy state trends such that flight crews are alerted prior to reaching a potentially hazardous low-energy state.”

Accident reviews by the NTSB resulted in the following safety recommendation, first voiced in 2003, then again in 2010 and again in 2014.

“The Board concludes that a requirement for the installation of low-airspeed Alert systems could substantially reduce the number of accidents involving flight-crew failure to maintain airspeed.”

Exhaustive studies by the Commercial Aviation Safety Team (CAST) resulted in the following recommended Safety Enhancement in 2014:

“…in order to improve early flight crew awareness of a decreasing energy state, manufacturers should develop and implement multi-sensory low airspeed alerting at the caution level in existing and in-development airplanes, as practical and feasible. The focus of this Safety Enhancement is on low cost, low technology solutions with?ease of retrofit and production incorporation.”

In recognition of the potentially positive effect on safety, in 2010, the FAA issued an amendment to 14CFR Part 25.1322 (Flight Crew Alerting) that requires that all new transport aircraft have?multi-sensory low airspeed alerting systems?installed.

This sounds great for the future, but this mandate only applies to?forward fit?into aircraft seeking their first Type Certificate after 2010. Even though 40%+ of Commercial and Business/Corporate aircraft in service today have no Stall Alerting system (just an ineffective Stall Warning system), there is no FAA mandate for retrofit, just encouragement for voluntary adoption.

But it gets worse! The FAA actually sent a letter to the NTSB in 2021 explaining that they saw no need to do the work to create a retrofit mandate because 60% of aircraft today already have a Stall Alerting system and trends indicate that at the present rate of voluntary adoption, by 2035, 90% will have them.

Come on, FAA! What happens between now and 2035? What happens to the 10% that still won’t have a Stall ALERTING system 23 years from now but will be hauling passengers for money??

If you really want to save lives and make aviation safer, issue a mandate for installation of effective low airspeed/stall ALERTING Systems immediately, especially for aircraft operating under Part 121 and 135.

I believe installation of an effective Stall ALERTING system in the Challenger 605 and the Learjet 35A could have prevented these accidents and saved 10 lives.?

Is Angle-of-Attack the answer?

And for those of you that think I’m advocating for installation of angle-of-attack (AoA) displays or indexers in cockpits as a way to solve this problem, you could not be more wrong!??Don’t misunderstand. I’m a strong advocate of AoA. It provides essential information that every pilot needs. But it won’t solve this problem.

Take the T-38 as an example, an aircraft I know very well. This aircraft has been around for 60 years! It has had an AoA dial indicator on the panel and an indexer on the glareshield since day one. It is also the safest supersonic aircraft the USAF has ever operated – 1/6th?the accident rate of the F-16. In spite of this, one of the most common fatal accident categories in the T-38 is ‘traffic pattern stalls’.?

The T-38 AoA indexer is designed to give a ‘green doughnut’ on final at 155 kts (plus 1 kt for every 100 lbs of gas above bingo). It works well on Final. But that’s not where the accidents happen. They happen during the big sweeping turn from Downwind-to-Base-to-Final. If you’re flying the approach ‘by the book’, you’ve got gear and flaps down, your left hand on the throttles and at least 200 kts when you start this turn and nothing less than 175 kts during the turn (or maybe a little more). The indexer at that point is telling you that you’re fast. You’re not! If you’ve got a ‘green doughnut’ during the turn, you’re already in trouble.

Take a look at the accident report from the T-38C traffic pattern stall at Dannelly Field, AL in February 2021. Very similar elements to those that were seen in the Challenger and Learjet crashes later in the year:

·??????Maneuvering in the traffic pattern

·??????Task saturation

·??????Head out of the cockpit

·??????Interrupted/delayed instrument scan

·??????Inadvertent/un-noticed loss of airspeed

·??????Accelerated stall

·??????Strong roll off that was uncontrollable

·??????Impact with terrain

And having an AoA Dial Indicator and an Indexer didn’t help! And they didn’t help in at least 8 similar T-38 fatal traffic pattern accidents over the years.

What I have found is that the greatest value of an AoA display in the pattern is that it helps to reduce the dispersion around an intended touchdown point. That’s why Navy guys swear by it – they’re aiming for a spot between the #2 and #3 wires. Farmers that have a Piper Cub and an 800 ft grass strip next to the barn like it too. But that’s not the problem we are trying to solve!

IMHO, an amber band on the PFD, angle-of-attack dial indicators, moving tapes or indexers of various forms are NOT effective Stall Alerting systems. More on that in a future post.

Last word:

I didn’t write this to solicit ‘peanut gallery’ comments from the thousands of ‘experts’ out there that have the word ‘Safety’ prominently displayed on their business cards but whose main contribution to aviation safety is yearly attendance at seminars and conferences but never actually do anything. I wrote it to stimulate discussion and interaction among the small group that wants to make a safety difference. Feel free to raise the challenge flag, but let’s interact.

10 people died un-necessarily! 10 people!