What has the boot manufacturer said?

Stall strips DO alter the wing stall characteristics.

The rumble felt is exactly because the portion of the wing behind the stall strip is stalling earlier than the rest of the wing.

Missing, improperly placed, or asymmetric stall strips will cause very noticeable changes in how the plane stalls.

Mike C.

Somewhat related:

We once had a twin turboprop that would not pass the stall requirements in Production Flight Test.

Against our better judgement, we were directed to issue paperwork to shim the engine mount trusses to adjust the thrust angle.

We did so, it was a considerable pain requiring refitting of the cowling, and it didn't help a bit.

Problem was eventually traced to tooling for the outer wing to center wing attachment. I don't remember what the solution was for that particular airplane.

I wonder if that comment by the pilot would work on a ramp check by a knowledgeable FAA inspector. I also wonder if any of those planes were stalled. For me it would be a no go without documented relief.

A missing stall strip is a no fly issue due to it altering the original type design or type certificate with no supporting data. I doubt it would pass the ferry permit process because it changes the flight characteristics of the aircraft without supporting data in the AFM. For instance if the gear is not and would not retract you could get a ferry permit to relocate the aircraft to maintenance because gear down flight is part of normal flight operation. That would allow the FAA to set limitations on the flight that fall within normal operations.

As for stall strips being mislocated, that is down to inadequate maintenance. Each Citation maintenance manual gives the specifics on the location of the stall strips as long as they are using it.

Short answer:

A missing stall strip is an airworthiness item and the aircraft absolutely does NOT meet type design without it, in ways that are potentially dangerous and unrecoverable in some areas of the flight envelope.

It is not equipment in the context of an MMEL, and those here saying it is more like a wing or control surface that must be present are exactly right. The aircraft must be in the approved (by TCDS/STC/Field approval) aerodynamic configuration to operate without a special flight permit (aka ferry permit).

Long answer:

Fortunately, leading edge aerodynamics are fairly insensitive to deviations as long as a substantial margin from critical angle of attack is maintained. This is why people get away with missing/mislocated stall strips and odd ice shapes, and bird or hail impact damage, and even come away from such encounters with the impression it was no big deal and not that important.

This all changes significantly as the critical angle of attack is approached. Leading edge issues become dominant in how the stall progresses on the wing and can also dramatically affect control in all 3 axes.

The stall strip is there to make the stall characteristics meet the certification standards. For Part 25 airplanes like most of the legacy Citations (other than 501/551, but the aerodynamic configuration was initially certified Part 25), this means meeting the Flight Test guidance set out in Advisory Circular 25-7, specifically Chapter 8, as well as the regs themselves, such as 14CFR 25.173, 25.203 and 25.207.

So what is the stall strip really doing to help meet the rules? It may cause an aerodynamic rumble (which could be either turbulence on the wing itself, or wake over the tail {less so for T-tails}, or a combination of both) that can serve as part or even all of the stall warning (25.207). This isn’t true for most jets, but an older Citation may be an exception. If it has a stick shaker, the natural aerodynamic buffet was not deemed an adequate stall warning in at least one flight regime/configuration.

Second, it is forcing a section of the wing to stall at a lower angle of attack than it would have otherwise; almost always an inboard section. This makes the initial stall occur below the critical angle of attack for the outer wing which needs to be kept flying to maintain controllability. Most wings would otherwise tend to stall further outboard, which is unacceptable because it both can impart a large rolling moment and it reduces the effectiveness of the control surfaces. Many basic wing geometry features like wing twist and section thickness variation (thicker tends to delay stall) are built into the wing design to make the stall progress correctly. But these come at a cruise efficiency cost, so addressing all the stall characteristics this way handicaps the airplane. The stall strip has little penalty at cruise, and is more easily refined in development flight testing compared to basic wing shape parameters.

Third, the loss of lift is on the inboard wing (generally in front of the stabilizer, which is carrying a high downforce lift to pitch the aircraft nose up into the stall) and locally reduces the wing downwash which forms the flow field for the stabilizer. This reduces the local angle of attack* of the horizontal, which reduces the overall lift of the tail, and reduces the total tail downforce, creating a desirable nose-down pitching moment.

*In aircraft coordinates, reducing the wing downwash actually increases the horizontal tail’s absolute angle of attack, but as in this condition the horizontal is lifting upside down, this is a reduction in local angle of attack relative to the horizontal’s lift vector and is thus a corresponding reduction in lift. The aircraft sees this as a reduction in tail downforce and the aircraft gets a relative nose down pitching moment.

This pitch characteristic helps meet the cert rules 25.173 and 25.203 which do not allow the pitch force gradient to reverse. If it did, there would be a condition where the aircraft would pitch up to a higher AOA without the pilot commanding it.

Swept wing airplanes have an even stronger pitching moment effect, because the inboard stall strip is affecting lift further forward. This means when that area stalls, the aircraft center of lift moves aft, which again is a nose down pitching moment. This is good because the natural characteristics of a swept wing tends to be strongly the other way; the tips stall first, which would pitch the aircraft to a higher AOA uncontrollably. This is why autoslat stall protection systems like on some Falcons only extend the outboard slats. They want the inboard wing to stall well before the outer, so don’t extend the inboard slats. The Citation X doesn’t even have inboard slats, as their primary function was for stall characteristics.

On some early swept wing jets, especially with aft pylon mounted engines (because this both blocks airflow to the tail at high AOA and the aft engine weight tends to make the tail arm shorter than wing mounted engines), letting the stall occur too far outboard could pitch (even if the pilot was pushing nose down at the time) the aircraft up into a deep stall regime, where extremely high AOA’s occurred even if the aircraft pitch attitude was seemingly normal. The deep stall could become very stable and has limited control authority in any axis, so this usually resulted in a loss of the aircraft.

So what does all this mean if the stall strips are missing (symmetrically) and you go to critical AOA?

The airplane will tend to go to a higher AOA than normal entering the stall, and with potentially lighter (possibly even negative) pitch forces in that regime, with very poor or no lateral control at the actual stall. It will be very hard to control if one wing goes before the other. There may be no buffet warning before this happens.

What about if only one side is missing?

Same as above to a lesser degree for pitch control and warning, but now it is guaranteed one wing will stall substantially before the other. At that point, the differential lift and drag will make the plane want to snap roll and enter a spin, basically like stalling it with the controls crossed.

If you fly with one strip missing and go to critical AOA, you are in an area where the experimental test pilots haven’t gone. You really don’t want to be there!

So what to do if a stall strip is missing, and you can’t get it fixed at that location?

The aircraft usually can be ferried, but it is paramount that stall angles of attack be avoided. If Textron gives you a recommendation letter for a ferry permit, it would likely include wording that requires adjusting reference speeds up (and adjusting field requirements appropriately), and a prohibition against operating in flight below 1.2Vs for any configuration (this includes TO and landing). These limitations are also sometimes applied to temporary leading edge patches after bird strike damage, depending on location and extent of damage. In all these cases, the issue/damage is reviewed by an Aerodynamics engineer before they will issue the letter.

Hopefully this long (but actually very cursory) explanation gives you some insight into some of the more subtle aspects of stall behavior.

Phil

I had an FAA inspector try to ground my Aerostar due to missing stall strips on one wing. It was difficult to prove to him that it was designed that way. But yes, they do look at stall strips and are certainly willing to ground planes if they are improper.

Multi-engine prop airplanes with much of the wing in the prop wash are particularly challenging to get acceptable stall characteristics in all required conditions (which include power off and on, with huge asymmetric variations in the local angle of attack in the prop wash), so I'm not surprised the Aerostar is asymmetric by design.

Any truly knowledgeable inspector should understand that. That one would try to ground a plane without having detailed manufacturer data showing a non-compliance is only putting his ignorance on display, like the infamous inspector who red-tagged a plane with q-tip props.

It is literally the equivalent of telling the owner of an airplane to prove his wing airfoil is per type design or he'll ground it.

Phil

Then add to that on the ferry flight you take a large bird down one engine on takeoff after normal V1. What then? What single engine climb performance might you get with the ferry permit revised "single stall strip" or "no stall strips" V1 and V2 speeds? Those are rhetorical questions and my point is the maintainers saying it was ok to fly never thought it that far through.

Then add to that on the ferry flight you take a large bird down one engine on takeoff after normal V1. What then? What single engine climb performance might you get with the ferry permit revised "single stall strip" or "no stall strips" V1 and V2 speeds? Those are rhetorical questions and my point is the maintainers saying it was ok to fly never thought it that far through.

Yes, if they had the part. Apparently, none were to be readily found and the aircraft was AOG on a trip.

Yes the replacement stall strip is something that could be easily accomplished by the MSU or local maintenance shop to get the aircraft back in the air.

It looks like Textron has 37 stall strip PN 3D2487 in stock today but they may not have had them 10 days ago. Either way the AOG flight would have to wait for the stall strip to arrive and be installed, and the adhesive dry before the flight anyway. Probably would have made alternative plans for the passengers anyway.