Turbulence penetration speed
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Turbulence penetration speed
This is a question I thought I knew the answer too….
During a training detail, in the the decent I was around 270kts. I was quite happy since the turbulence limitation on type was 290-310KIAS /.82 -.85M.
I was told that I had to speed up to the turbulence penetration speed….ie 290-310kts.
This didn’t make sense to me and when I queried it later on the ground I was told that that is the penetration speed. I had never though about turbulence speed being an issue if you were below it and had a large margin on minimum stall speed. I have always though of it as a structural maximum speed. It still doesn't seem correct to me but I am often wrong……could someone please explain this to me and point me in the direction of a reference to an explanation of this situation.
Many thanks
During a training detail, in the the decent I was around 270kts. I was quite happy since the turbulence limitation on type was 290-310KIAS /.82 -.85M.
I was told that I had to speed up to the turbulence penetration speed….ie 290-310kts.
This didn’t make sense to me and when I queried it later on the ground I was told that that is the penetration speed. I had never though about turbulence speed being an issue if you were below it and had a large margin on minimum stall speed. I have always though of it as a structural maximum speed. It still doesn't seem correct to me but I am often wrong……could someone please explain this to me and point me in the direction of a reference to an explanation of this situation.
Many thanks
Yes, correct for "severe" turbulence. It protects you against stalls due to gusts (explanations can be found in the gust envelope diagram). On a B737 it will state you can only reduce to 250 in a descent if you are below max landing weight below 15000ft (250kts is not allowed during a climb). But I hope the person also applies the correct procedure for turbulence in cruise (on 737 classic that would mean flying in CWS and without AT, as ALT HOLD is not allowed).
The times I've encountered "severe" turbulence are very few... I think many use the speeds as a general limit for comfort reasons, but it is true you shouldn't reduce more as turbulence intensity increases.
The times I've encountered "severe" turbulence are very few... I think many use the speeds as a general limit for comfort reasons, but it is true you shouldn't reduce more as turbulence intensity increases.
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Agreed in the cruise, since the stall margin is much less. In the decent though, we were below MLW and had a large margin above stall. Accelerating seemed to be counter intuitive and surprised me.
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Let's remember that:
"Severe turbulence causes large and abrupt changes in altitude and/or attitude and, usually, large variations in indicated airspeed. The airplane may momentarily be out of control. Occupants of the airplane will be forced violently against their seat belts."
Anything less than the above will be at best moderate turbulence which can be flown within the normal airplane operating range of speeds as adequate margins exist.
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No, it was light to moderate at worst.
My FCOM Limitations makes no distinction with severe turbulence it states “ The turbulence penetration speed is 290-310KIAS/.82-.85M, whichever is lower”
So in the case of turbulence in my case, or in your case severe turbulence you would accelerate to the turbulence penetration speed? Where would that be referenced?
Many thanks
My FCOM Limitations makes no distinction with severe turbulence it states “ The turbulence penetration speed is 290-310KIAS/.82-.85M, whichever is lower”
So in the case of turbulence in my case, or in your case severe turbulence you would accelerate to the turbulence penetration speed? Where would that be referenced?
Many thanks
The main point is that you will be surprised … fundamentally surprised
The main point is that you will be surprised in severe turbulence … fundamentally surprised - something which you have difficulty in comprehending, believing, far from any expectations based on text or simulation.
Based on about 30sec experience from a preplanned, fully instrumented transit of a 'small' tropical cb in a 2nd gen commercial aircraft, the co-Captain described his experiences as similar to being shot at in a helicopter in 'Nam'.
From trimmed rough airspeed, the aircraft experienced sudden, rapid, and large pitch excursions, with speed and attitude changes ranging from stick-shake / push to overspeed alerting, both nose up and down, within a few seconds. n.b. unseen alpha changes, but recorded; very high rates of change. Normal accelerations (g) were up to but not exceeding the aircraft limits
The brief was to fly attitude - and nothing else. Nothing else would, could have been conceived due to mental freeze due to the severity of surprise, even after the briefed expectation and experience of 'mild' but 'severe' turbulence transits over the tops of cbs.
The most notable effect was the very high stick forces required to recover attitude - the maintain attitude task was essentially pitch upset recovery in both directions. The stick forces and displacement required were those associated with being out of trim between Vra and stall, and opposite at Vmo; very high forces - max manual effort, and large displacements, akin to full range checks.
It is assumed that in modern FBW aircraft the magnitude of force to some extent would be less, but the need for large control inputs might remain.
Then there was roll - at least the aircraft had roll stability vs the need, ability, for control input - judged similarly to be large like pitch, but not made.
The cb just spat the aircraft out of the side of the cloud into 'calm air'.
The safety concept is that the crew will be able to detect and avoid the most severe conditions, but those undetected or not avoided then …
Be prepared to be surprised, unbelievably surprised; fly attitude, do not trim.
Don't think of changing speed to Vra during an event; you will probably be incapable of thought, and any target or reference speed is meaningless. Fly constant attitude - know what these are.
The atmosphere does not read text books, it can throw events at you far in excess of theory.
Fly Vra after exit, be prepared for the next encounter.
As above:- "Severe turbulence causes large and abrupt changes in altitude and/or attitude and, usually, large variations in indicated airspeed. The airplane may momentarily be out of control. Occupants of the airplane will be forced violently against their seat belts."
+ The pilots might be momentarily out-of-control due to the effects of surprise.
Based on about 30sec experience from a preplanned, fully instrumented transit of a 'small' tropical cb in a 2nd gen commercial aircraft, the co-Captain described his experiences as similar to being shot at in a helicopter in 'Nam'.
From trimmed rough airspeed, the aircraft experienced sudden, rapid, and large pitch excursions, with speed and attitude changes ranging from stick-shake / push to overspeed alerting, both nose up and down, within a few seconds. n.b. unseen alpha changes, but recorded; very high rates of change. Normal accelerations (g) were up to but not exceeding the aircraft limits
The brief was to fly attitude - and nothing else. Nothing else would, could have been conceived due to mental freeze due to the severity of surprise, even after the briefed expectation and experience of 'mild' but 'severe' turbulence transits over the tops of cbs.
The most notable effect was the very high stick forces required to recover attitude - the maintain attitude task was essentially pitch upset recovery in both directions. The stick forces and displacement required were those associated with being out of trim between Vra and stall, and opposite at Vmo; very high forces - max manual effort, and large displacements, akin to full range checks.
It is assumed that in modern FBW aircraft the magnitude of force to some extent would be less, but the need for large control inputs might remain.
Then there was roll - at least the aircraft had roll stability vs the need, ability, for control input - judged similarly to be large like pitch, but not made.
The cb just spat the aircraft out of the side of the cloud into 'calm air'.
The safety concept is that the crew will be able to detect and avoid the most severe conditions, but those undetected or not avoided then …
Be prepared to be surprised, unbelievably surprised; fly attitude, do not trim.
Don't think of changing speed to Vra during an event; you will probably be incapable of thought, and any target or reference speed is meaningless. Fly constant attitude - know what these are.
The atmosphere does not read text books, it can throw events at you far in excess of theory.
Fly Vra after exit, be prepared for the next encounter.
As above:- "Severe turbulence causes large and abrupt changes in altitude and/or attitude and, usually, large variations in indicated airspeed. The airplane may momentarily be out of control. Occupants of the airplane will be forced violently against their seat belts."
+ The pilots might be momentarily out-of-control due to the effects of surprise.
No, it was light to moderate at worst.
My FCOM Limitations makes no distinction with severe turbulence it states “ The turbulence penetration speed is 290-310KIAS/.82-.85M, whichever is lower”
So in the case of turbulence in my case, or in your case severe turbulence you would accelerate to the turbulence penetration speed? Where would that be referenced?
Many thanks
My FCOM Limitations makes no distinction with severe turbulence it states “ The turbulence penetration speed is 290-310KIAS/.82-.85M, whichever is lower”
So in the case of turbulence in my case, or in your case severe turbulence you would accelerate to the turbulence penetration speed? Where would that be referenced?
Many thanks
“Whenever possible, avoid areas with known or forecasted severe turbulence. If turbulence is unavoidable, aim to keep the speed in the region of the target speed given in this section, so as to provide the best protection against the effect of gust on the structural limits, whilst maintaining an adequate margin above VLS.”
The whole section is focused around turbulence penetration in level flight but nowhere does it say that you can or should treat the penetration speed as a maximum rather than a target in descent.
Something else to consider is that your manufacturer has decided their turbulence penetration speed gives adequate margin to both high and low speed limits. If you are going to fly slower, how much slower? If you’re going to decide for yourself that your margin to the stall is adequate, how have you decided this? On what basis is 270 knots ok but 250 knots is not? Do you have a factor on the stall speed in mind, a fixed buffer of a certain IAS or Mach? Just a feeling? To me you either consider the turbulence to be bad enough, or to possibly become bad enough, to fly the target speed, otherwise you should just fly normal speeds.
You really need to look at the gust envelope and compare Va (design maneuvering speed), Vc (design cruise speed) and Vd (design dive speed). All 3 are combined with certain gust inputs of 2 different levels.
V-n diagram (gust envelope)
In turbulence it is clear you need to reduce speed as the biggest (regulatory defined) gust can overstress the aircraft. This is the case ie for Vc (and Vd). In the diagram you need to look at the Vc gust line, between the stall speed curve (with increasing g-load this is more commonly known as "accelerated stall") and the +n maneuver line (fixed structural g-load ie 2,5g). Between those two points is your "region of choice" for the turbulent penetration speed. ALthough you better keep it below Va to avoid overstressed pilots trying to recover and overstressing the airframe themselves (something like that...).
Of course there are many weights to take into consideration, but to make life easy there is one speed defined that sits on that line and works all around for different weights. It is fairly logical to state that with reducing weight, the ("accelerated") stall curve will move left and allow lower speeds. In the case of the B737, Boeing has tested and verified these margins and published the 250kts below MLW in the manuals. If your manufacturer has not stated this, this doesn't mean reducing speed is dangerous and you risk a higher-g-load stall, however, the manufacturer has not verified this and does not take responsibility for it.
For all these reasons the recommended approach is to fly attitudes starting from a recommended penetration speed: speed excursions are possible but if you've started at the penetration speed you should be protected against accelerated stall, you should be protected from structural overloads, and still retain some controllability (but don't exagerate as you might still "stall-it-out")
Anyway, it's been +20 years since engineering and I split paths, hard to imagine I once found this logical... feel free to add/correct/ban
PS as a frame of reference: if you've ever done a +2g landing, that's the turbulence level you're looking at... (according to the books: "can't read instruments" aka "can't read speed" aka if you manage to grab the button on the first try it's not enough intensity...)
V-n diagram (gust envelope)
In turbulence it is clear you need to reduce speed as the biggest (regulatory defined) gust can overstress the aircraft. This is the case ie for Vc (and Vd). In the diagram you need to look at the Vc gust line, between the stall speed curve (with increasing g-load this is more commonly known as "accelerated stall") and the +n maneuver line (fixed structural g-load ie 2,5g). Between those two points is your "region of choice" for the turbulent penetration speed. ALthough you better keep it below Va to avoid overstressed pilots trying to recover and overstressing the airframe themselves (something like that...).
Of course there are many weights to take into consideration, but to make life easy there is one speed defined that sits on that line and works all around for different weights. It is fairly logical to state that with reducing weight, the ("accelerated") stall curve will move left and allow lower speeds. In the case of the B737, Boeing has tested and verified these margins and published the 250kts below MLW in the manuals. If your manufacturer has not stated this, this doesn't mean reducing speed is dangerous and you risk a higher-g-load stall, however, the manufacturer has not verified this and does not take responsibility for it.
For all these reasons the recommended approach is to fly attitudes starting from a recommended penetration speed: speed excursions are possible but if you've started at the penetration speed you should be protected against accelerated stall, you should be protected from structural overloads, and still retain some controllability (but don't exagerate as you might still "stall-it-out")
Anyway, it's been +20 years since engineering and I split paths, hard to imagine I once found this logical... feel free to add/correct/ban
PS as a frame of reference: if you've ever done a +2g landing, that's the turbulence level you're looking at... (according to the books: "can't read instruments" aka "can't read speed" aka if you manage to grab the button on the first try it's not enough intensity...)
Last edited by BraceBrace; 15th Dec 2023 at 20:39.
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Then it begs the question , how do I slow to make an approach? Obviously in severe turbulence, I wouldn’t be making an approach, but my FCOM reference states ‘turbulence penetration speed’ not referring specifically to severe.
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You might want to have a look into the FCTM and or FCOM Supplementary Procedure. It clearly states that turbulence penetration speeds are
for severe turbulence only and speed modification is normally not required for moderate turbulence and below. At least Boeing wrote it that way.
for severe turbulence only and speed modification is normally not required for moderate turbulence and below. At least Boeing wrote it that way.
… THE question - to self
'Then it begs the question': not how, but should we be doing this.
After evaluating or taking a report as fact (turbulence, windshear, developing storms, ice), there will always be some uncertainty. What did the reporter experience - where, when. Is the report fact or forecast; how defined, why, by whom, when.
Safe flight depends on your interpretation, your definition, your choice, your judgement, decision, responsibility; and to remind yourself that you (flying), are matching your perception of personal skill and performance in the expected conditions. More often we think that we are better (before the fact) than we are - the reality of after the fact - not as in the book.
Situations rarely have a single factor. Thus the experience of flying through a storm has little relevance to the approach and landing - with the previously unseen, much larger storm following on; diversion had been rejected based on revised weather reports. Thus downwind landing on wet runway - judicious evaluation of landing performance - assume flooded runway - it was.
Small storm, mod turbulence low level; very large storm sever turbulence down to ground level, wind-shear, down-burst, floods; fortunately evaluated before-the-fact from a brick-built bar … but never again !
After evaluating or taking a report as fact (turbulence, windshear, developing storms, ice), there will always be some uncertainty. What did the reporter experience - where, when. Is the report fact or forecast; how defined, why, by whom, when.
Safe flight depends on your interpretation, your definition, your choice, your judgement, decision, responsibility; and to remind yourself that you (flying), are matching your perception of personal skill and performance in the expected conditions. More often we think that we are better (before the fact) than we are - the reality of after the fact - not as in the book.
Situations rarely have a single factor. Thus the experience of flying through a storm has little relevance to the approach and landing - with the previously unseen, much larger storm following on; diversion had been rejected based on revised weather reports. Thus downwind landing on wet runway - judicious evaluation of landing performance - assume flooded runway - it was.
Small storm, mod turbulence low level; very large storm sever turbulence down to ground level, wind-shear, down-burst, floods; fortunately evaluated before-the-fact from a brick-built bar … but never again !
I don't get this post.
The fact that some is avoidable is a different topic. Severe turbulence supplementary procedure is not an invitation to be less carefull about thunderstorms. It does however not eliminate the issue (ie CAT).
Engineers have agreed in these cases the "SOP" and standard trained procedures do not provide the safest tactic/approach. It is determined it requires a different operating procedure. Regulations have outlined requirements and manufacturers adhere to these regulations and outline a procedure crews have to follow.
The whole purpose of the supplementary procedure is to avoid everybody "inventing" their own stuff and ideas. Which is exactly why it is not wrong for a pilot to explain that in case of turbulence the idea is not to reduce speed with increasing turbulence, it is correct to explain it is a single speed connected to what is supposed to be a trained procedure.
The only question that remains here is the question of turbulence intensity which everyone agrees it is hard to quantify. However it does not change the "right of existence" of the procedure. It is a legal requirement to protect the aircraft.
The fact that some is avoidable is a different topic. Severe turbulence supplementary procedure is not an invitation to be less carefull about thunderstorms. It does however not eliminate the issue (ie CAT).
Engineers have agreed in these cases the "SOP" and standard trained procedures do not provide the safest tactic/approach. It is determined it requires a different operating procedure. Regulations have outlined requirements and manufacturers adhere to these regulations and outline a procedure crews have to follow.
The whole purpose of the supplementary procedure is to avoid everybody "inventing" their own stuff and ideas. Which is exactly why it is not wrong for a pilot to explain that in case of turbulence the idea is not to reduce speed with increasing turbulence, it is correct to explain it is a single speed connected to what is supposed to be a trained procedure.
The only question that remains here is the question of turbulence intensity which everyone agrees it is hard to quantify. However it does not change the "right of existence" of the procedure. It is a legal requirement to protect the aircraft.
Last edited by BraceBrace; 16th Dec 2023 at 12:45.
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Knowing safetypee's background, I would offer the comment that he knows a few things about certification matters ..... his counsel is well worth heeding.
SOPs are the 'Streetlights'; we operate in the 'Shadows'
The frustration is with my inability to understand why many responses in this thread, and similar cross the forum, resort to procedure or documents for factual solutions without considering that the situation may not be as that assumed in a simple if-then checklist.
Perhaps an issue of the industry's current view of safety management ?
In addition, the difficulty in explaining a way forward with required knowledge and skill, other than by example; particularly for Tacit Knowledge, awareness and judgement.
Resorting to explanations by others; the book 'Streetlights and Shadows', Gary Klein, is a good start - holiday reading.
Selective cherry picking the text, (only from part 1), is collated here for clarity:-
https://www.dropbox.com/scl/fi/7rdsx...nayvqnkfq&dl=0 Edit
Note that the experience referred to involves learning. Not necessarily from having being in every situation beforehand. Reading, considering what if, asking questions, and testing other potential situations, can build knowledge and experience.
There may be a gift under this tree:- https://epdf.tips/search/Streetlight...radford+Books)
Select book, and choose pdf Download as required.
Real life examples;-
QF 32 - extensive and lengthy use of checklists to determine what had failed, 'what was wrong'.
With hindsight, a search for 'what was still working' would provide a quick and relevant guide to 'rule 1' Fly The Aircraft.
The Hudson Accident - https://www.researchgate.net/profile...ication_detail
Note the need for a sacrificing choice of action - when to 'ditch' SOPs,
and the findings of the ACCOMPLI project.
"… with operating procedures shifting from detailed protocols for normal situations, to a generic action framework for emergency situations."
SOPs are the 'Streetlights'; we operate in the 'Shadows'
Perhaps an issue of the industry's current view of safety management ?
In addition, the difficulty in explaining a way forward with required knowledge and skill, other than by example; particularly for Tacit Knowledge, awareness and judgement.
Resorting to explanations by others; the book 'Streetlights and Shadows', Gary Klein, is a good start - holiday reading.
Selective cherry picking the text, (only from part 1), is collated here for clarity:-
https://www.dropbox.com/scl/fi/7rdsx...nayvqnkfq&dl=0 Edit
Note that the experience referred to involves learning. Not necessarily from having being in every situation beforehand. Reading, considering what if, asking questions, and testing other potential situations, can build knowledge and experience.
There may be a gift under this tree:- https://epdf.tips/search/Streetlight...radford+Books)
Select book, and choose pdf Download as required.
Real life examples;-
QF 32 - extensive and lengthy use of checklists to determine what had failed, 'what was wrong'.
With hindsight, a search for 'what was still working' would provide a quick and relevant guide to 'rule 1' Fly The Aircraft.
The Hudson Accident - https://www.researchgate.net/profile...ication_detail
Note the need for a sacrificing choice of action - when to 'ditch' SOPs,
and the findings of the ACCOMPLI project.
"… with operating procedures shifting from detailed protocols for normal situations, to a generic action framework for emergency situations."
SOPs are the 'Streetlights'; we operate in the 'Shadows'
Last edited by safetypee; 18th Dec 2023 at 06:52. Reason: V2 of link
The frustration is with my inability to understand why many responses in this thread, and similar cross the forum, resort to procedure or documents for factual solutions without considering that the situation may not be as that assumed in a simple if-then checklist.
Perhaps an issue of the industry's current view of safety management ?
In addition, the difficulty in explaining a way forward with required knowledge and skill, other than by example; particularly for Tacit Knowledge, awareness and judgement.
SOPs are the 'Streetlights'; we operate in the 'Shadows'
Perhaps an issue of the industry's current view of safety management ?
In addition, the difficulty in explaining a way forward with required knowledge and skill, other than by example; particularly for Tacit Knowledge, awareness and judgement.
SOPs are the 'Streetlights'; we operate in the 'Shadows'
The act of drowning is easy and self imposed once you jump in any kind of 'hostile' water. The act of swimming is taught step by step in a safe environment. The reaction to stop drowning is a revertion to what is taught before (try to float - passive or swim - active).
Humans don't survive in structure-less environments unless there is time available to adapt. Give the brain knowledge, awareness and judgement, and you still might end up drowning if no reference is made to previous taught behaviour to which one has as quickly as possible "hang-on" to. And as much as your examples refer to "out of the SOP" steps, the SOPs did give guidelines as to how not to drown.
In even in the shadows, the lights are points of reference giving trust and showing the way because of one very important SOP guideline: it shouldn't do more harm than not. To get back on topic... if turbulence becomes "uncontrollable" it becomes upset recovery. And hence we try no to drown with taught techniques.
On the topic of the "why this SOP", I completely agree, and the SOP is not king nor ultimate goal. However, in this world of aviation with the job of "pilot" being very common and pilots coming from all kinds of backgrounds, it is pretty impossible to state that the "darkness" is survivable for the majority of pilots when you decide to question the streetlight.
Last edited by BraceBrace; 17th Dec 2023 at 19:33.