AA Crash Jamaica
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The 737 QRP states that LDRs are upped by 15% for all surfaces except 'DRY'. Boeing do not otherwise segregate by wet, flooded or icy, but more sensibly, in my opinion, by reported BA - and it works!
This is the descriptive text: My highlight
Reference distance is for sea level, standard day, no wind or slope, VREF40 approach speed and two engine detent reverse thrust.
Max manual braking data valid for auto speedbrakes. Autobrake data valid for both auto and manual speedbrakes.
For max manual braking and manual speedbrakes, increase reference landing distance by 55 m.
Distances for GOOD, MEDIUM, and POOR are increased by 15%.
Includes distance from 50 ft above threshold (305 m of air distance).
This is the descriptive text: My highlight
Reference distance is for sea level, standard day, no wind or slope, VREF40 approach speed and two engine detent reverse thrust.
Max manual braking data valid for auto speedbrakes. Autobrake data valid for both auto and manual speedbrakes.
For max manual braking and manual speedbrakes, increase reference landing distance by 55 m.
Distances for GOOD, MEDIUM, and POOR are increased by 15%.
Includes distance from 50 ft above threshold (305 m of air distance).
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"...failed to decelerate as quickly as it should have and actually picked up speed slightly for a brief period. Even after maximum manual braking was applied."
"I think they want to question the veracity of their FDR data."
Anybody who has ever hydroplaned, whether in a car or an airplane, knows that indeed acceleration does take place at that moment. And anybody who thinks that isn't so and that claiming so invalidates the WSJ article apparently isn't a pilot. Or a very experienced one.
"I think they want to question the veracity of their FDR data."
Anybody who has ever hydroplaned, whether in a car or an airplane, knows that indeed acceleration does take place at that moment. And anybody who thinks that isn't so and that claiming so invalidates the WSJ article apparently isn't a pilot. Or a very experienced one.
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I haven’t had the time to read anything that may have been made available by the NTSB regarding the accident in Jamaica, so I can’t comment on Mr. Pasztor’s article. However, I’m more than willing to give anyone the benefit of the doubt until/unless I recognize a reason to change that opinion.
For whatever it may be worth, here is a list of approximations for additional runway distance required (after touchdown – where touchdown would have been planned to occur at the touchdown aiming point) for a typical transport category airplane, required to get the airplane stopped using normal landing procedures, not considering the use of reverse thrust. The calculation for each of the following factors is valid only if the factor involved is the only factor involved. If the following individual circumstances are combined, the increase would be at least the added values of the individual circumstances – and potentially more.
(1) For each additional 1000 feet of airport elevation above sea level, a 5% increase.
(2) For each 5 knots of tailwind, a 10% increase.
(3) For each additional 5% of airspeed above VREF, a 10% increase.
(4) For each additional 5% of airspeed above VREF and holding the airplane in the flare to decelerate, a 30% increase.
(5) For each additional 100 feet above the threshold at crossing, a 30% increase.
(6) For a wet runway, a 40% increase.
(7) For a snow covered runway, a 60% increase.
(8) For standing water or slush on the runway, a 100% increase.
(9) For an ice covered runway, a 250% increase.
A landing is, or should be, taught to touchdown at the touchdown aiming point (i.e., at 1000 feet beyond the approach threshold). However, generally, it is deemed acceptable to have the airplane on the runway within the touchdown zone, or the first 1/3 of the runway (whichever is shorter), with a normal rate of descent to the runway. Certainly, if the crew elects to overfly the first portion of the runway – for whatever reason – that runway surface cannot be included in the landing distance calculation.
________
(edit) I have been told that the US Flight Safety Foundation has a table available (on their website, perhaps?) that reflects numbers very similar to those above.
For whatever it may be worth, here is a list of approximations for additional runway distance required (after touchdown – where touchdown would have been planned to occur at the touchdown aiming point) for a typical transport category airplane, required to get the airplane stopped using normal landing procedures, not considering the use of reverse thrust. The calculation for each of the following factors is valid only if the factor involved is the only factor involved. If the following individual circumstances are combined, the increase would be at least the added values of the individual circumstances – and potentially more.
(1) For each additional 1000 feet of airport elevation above sea level, a 5% increase.
(2) For each 5 knots of tailwind, a 10% increase.
(3) For each additional 5% of airspeed above VREF, a 10% increase.
(4) For each additional 5% of airspeed above VREF and holding the airplane in the flare to decelerate, a 30% increase.
(5) For each additional 100 feet above the threshold at crossing, a 30% increase.
(6) For a wet runway, a 40% increase.
(7) For a snow covered runway, a 60% increase.
(8) For standing water or slush on the runway, a 100% increase.
(9) For an ice covered runway, a 250% increase.
A landing is, or should be, taught to touchdown at the touchdown aiming point (i.e., at 1000 feet beyond the approach threshold). However, generally, it is deemed acceptable to have the airplane on the runway within the touchdown zone, or the first 1/3 of the runway (whichever is shorter), with a normal rate of descent to the runway. Certainly, if the crew elects to overfly the first portion of the runway – for whatever reason – that runway surface cannot be included in the landing distance calculation.
________
(edit) I have been told that the US Flight Safety Foundation has a table available (on their website, perhaps?) that reflects numbers very similar to those above.
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Anybody who has ever hydroplaned, whether in a car or an airplane, knows that indeed acceleration does take place at that moment. And anybody who thinks that isn't so and that claiming so invalidates the WSJ article apparently isn't a pilot. Or a very experienced one.
I've (personally) have only experienced it only one time, in a 707 at the old Taipeh airport, runway 09.
I touched down, on speed, in the first 500 feet (in the driving rain) and applied maximum braking, spoilers, and maximum reverse, until stopping, at the very end of the nine thousand foot pavement.
A close call, certainly, and all FD retired to the bar for appropriate adult beverages.
I asked the very senior Flight Engineer (ex-Qantas) what I could have done better.
He mentioned...'Nothing, Captain, you did fine, it's the luck of the draw'.
So very true.
WhatsaLizad-II, I believe this to be the 15% (1.15 factor) from my post # 521, and thus relates to FAA in-flight planning. European wet data is based on a 1.92 factor (EU OPS-1.520).
BOAC, your reference in meters (305m vs 1000ft touchdown distance), appears to differ from mine – a European version?
Interesting if so, as few EU operators would require a 15% calculation, instead requiring EU-OPS fully factored data.
A correction to my reference – it is dated Dec 2005.
I urge caution in the use of Braking Action, particularly if given by PIREP.
Reported braking action based on an inspection of runway would be better, but what is the accuracy of the measurement, and what is the runway surface composition, or maintenance condition.
I will gladly expand on the issues, but first see EASA - Workshop Runway Friction and Aircraft Braking
Presentations, 2.2 FAA TALPA, slides 16 – 20.
Balance the US operator views in other presentations with the more muted views of PIREPS from the manufacturers.
How do pilots know what is normal or reduced braking; reduced from what?
Note that this accident involved a non grooved runway; see the issues discussed in http://www.pprune.org/rumours-news/4...rrun-cyqm.html
BOAC, your reference in meters (305m vs 1000ft touchdown distance), appears to differ from mine – a European version?
Interesting if so, as few EU operators would require a 15% calculation, instead requiring EU-OPS fully factored data.
A correction to my reference – it is dated Dec 2005.
I urge caution in the use of Braking Action, particularly if given by PIREP.
Reported braking action based on an inspection of runway would be better, but what is the accuracy of the measurement, and what is the runway surface composition, or maintenance condition.
I will gladly expand on the issues, but first see EASA - Workshop Runway Friction and Aircraft Braking
Presentations, 2.2 FAA TALPA, slides 16 – 20.
Balance the US operator views in other presentations with the more muted views of PIREPS from the manufacturers.
How do pilots know what is normal or reduced braking; reduced from what?
Note that this accident involved a non grooved runway; see the issues discussed in http://www.pprune.org/rumours-news/4...rrun-cyqm.html
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Runway 12, Norman Manley International Airport, Kingston, Jamaica is 8900 feet long. Wet or dry conditions, it would seem that if a pilot overflys the first 4000 feet of a 9000 foot long runway, and attempts to land from that point, he/she is doing a huge disservice to him-/her-self, the company for which he/she works, the other pilot in the cockpit, the passengers, the airport authority, the airplane manufacturer, and everyone else that may read with interest the report of the occurrence – and this list is not necessarily in an order of importance! Am I missing something?
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while there are extreme situations in which landing on the last half of the runway would be considered good, I agree with air rabbit's analysis.
I've spent my airline career landing on runways 7000' and shorter in the USA and every landing mattered.
I've spent my airline career landing on runways 7000' and shorter in the USA and every landing mattered.
Runway 12, Norman Manley International Airport, Kingston, Jamaica is 8900 feet long. Wet or dry conditions, it would seem that if a pilot overflys the first 4000 feet of a 9000 foot long runway, and attempts to land from that point, he/she is doing a huge disservice to him-/her-self, the company for which he/she works, the other pilot in the cockpit, the passengers, the airport authority, the airplane manufacturer, and everyone else that may read with interest the report of the occurrence – and this list is not necessarily in an order of importance! Am I missing something?
True, the crew touched down long. Obviously, that would seem to conflict with ops standards per the FAA and general airmanship standards. I would note some issues with KIN that I've observed from operating there. It is a little isolated from the typical night lighting at airports.
Water on the left, a few isolated light bulbs on the right and approach over water.
For whatever reason, the runway markings always seemed very dim to me at night even when dry. When wet they simply aren't what you'd see at a larger US or European airport when using them for reference.
Even landing 4000' from the approach end, I believe the aircraft should have stopped (not positive on numbers). If it went off the end at 60-80 kts, it's possible they still would have run off the end even if they touched down perfectly after flying to the GS intercept point (roughly at the 7800' remaining point), flared using Boeing procedures and guidance from the HUD.
Airport maintenance of the runway could be a factor. Were the drains along the runway clear, was the runway inspected for water pooling issues, was the vegetation cut back for water drainage, were there rubber deposits?
Also throw in a few 737 issues. Does the HUD guidance match accepted technique for flare/touch down? Is the aircraft more suspect to hydroplaning issues with a 150 Kt approach speed? Are the performance charts accurate?
Just throwing some other issues out that should also be examined. In no way shape or form giving "but,but,but" excuses for the accident.
PEI;
Thanks for the links.
If I recall correctly, there isn't a grooved runway in all of Canada.
Question, (if a thread diversion, please set aside): The Airbus FCOM provides landing distance numbers in the QRH for the A319/A320/321/A330 series for all factors...airport altitude, aircraft weight, tailwind component, forward CG, reversers operative as well as cross-referencing all modes of contamination. I seem to recall that the dry distances may also be co-related to the Transport Canada CRFI tables but that may be Ops-Specs specific and not apply elsewhere. Information on the CRFI is available in a number of places, along with in-depth discussions, on the TC website (and likely in some operators' manuals).
AirRabbit;
Not disputing your numbers but just curious as to the source, (aside from the FSF). They differ slightly (some more conservative, some less), from the Airbus numbers but I think for most of us the reasons for the differences are understandable in terms of specific test results vs general rules where actual distances and factors are not provided for the crew in the FCOM or QRH. Nevertheless I think they're appropriate.
Flight data analysis indicates a flat trend regarding long landings, some well beyond the TDZ, (3000' to 5000' beyond the 50' RA point). Further, such touchdown points are variously normalized (excused) by claiming that, "the runway was long" or, more creatively, "the wing is large and the airplane therefore floats more", and so on. We need not detain ourselves lamenting such silly excuses because we all know what the results of such ignorance are. The key is in providing both the data and the means to counter what the data is telling us, (and by "us" I mean this industry and not just one or another airline...from experience, this is a very common event), all in support of such excellent presentations as the one to which the link is provided.
PJ2
Thanks for the links.
Note that this accident involved a non grooved runway
Question, (if a thread diversion, please set aside): The Airbus FCOM provides landing distance numbers in the QRH for the A319/A320/321/A330 series for all factors...airport altitude, aircraft weight, tailwind component, forward CG, reversers operative as well as cross-referencing all modes of contamination. I seem to recall that the dry distances may also be co-related to the Transport Canada CRFI tables but that may be Ops-Specs specific and not apply elsewhere. Information on the CRFI is available in a number of places, along with in-depth discussions, on the TC website (and likely in some operators' manuals).
AirRabbit;
Not disputing your numbers but just curious as to the source, (aside from the FSF). They differ slightly (some more conservative, some less), from the Airbus numbers but I think for most of us the reasons for the differences are understandable in terms of specific test results vs general rules where actual distances and factors are not provided for the crew in the FCOM or QRH. Nevertheless I think they're appropriate.
Flight data analysis indicates a flat trend regarding long landings, some well beyond the TDZ, (3000' to 5000' beyond the 50' RA point). Further, such touchdown points are variously normalized (excused) by claiming that, "the runway was long" or, more creatively, "the wing is large and the airplane therefore floats more", and so on. We need not detain ourselves lamenting such silly excuses because we all know what the results of such ignorance are. The key is in providing both the data and the means to counter what the data is telling us, (and by "us" I mean this industry and not just one or another airline...from experience, this is a very common event), all in support of such excellent presentations as the one to which the link is provided.
PJ2
AirRabbit, #510, you perhaps unintentionally imply that the crew ‘deliberately’ overflew the first 4000ft. Deliberation would suggest awareness of the situation which might not have been the case.
Consider how a normal flare and landing is judged with respect to distance. This is probably by a sense of time - threshold to touchdown, time during the flare, particularly where ground markings are poor / night-time.
Distance, depends on ground speed, thus any tailwind might double the speed/distance difference for a given time, therefore for the perception of ‘normal timing’, the aircraft could have flown much further.
Add to this distance an increment due to a high approach – poor vis in heavy rain, poor lighting, and a tailwind.
Also, I wonder what effect a HUD flare profile has (see WhatsaLizad # 511). The difference between a manual landing and an auto land could be a 1400ft increase according to advisory landing data. If the HUD flare follows a similar profile as an autoland then a longer landing may result.
Thus, it is most likely that a long touchdown would result in these circumstances. In this case, safety would depend on correctly identifying the situation, knowledge of the risks in the situation, considering the aspects of a HUD guided flare and particularly tailwind, and then acting accordingly.
I suggest that this accident had its roots much earlier in the event than during the flare. Even in company SOPs if the HUD flare / tailwind were major contributors.
PJ2 – IIRC Airbus only deals with contamination via type (description), depth, and extent; Boeing relates to braking action.
CRFI is only used in Canada, and attempts to use it more widely as IRFI were not accepted – similar limitations as other friction methods due to poor correlation with the measuring devices, although within its limits (dry contaminants) CRFI appears to be better.
IMHO the success of CRFI (in Canada) stems from it triggering pilots to think about the distance increases – it helps identify (quantify) the risks and the distance margins to add.
Consider how a normal flare and landing is judged with respect to distance. This is probably by a sense of time - threshold to touchdown, time during the flare, particularly where ground markings are poor / night-time.
Distance, depends on ground speed, thus any tailwind might double the speed/distance difference for a given time, therefore for the perception of ‘normal timing’, the aircraft could have flown much further.
Add to this distance an increment due to a high approach – poor vis in heavy rain, poor lighting, and a tailwind.
Also, I wonder what effect a HUD flare profile has (see WhatsaLizad # 511). The difference between a manual landing and an auto land could be a 1400ft increase according to advisory landing data. If the HUD flare follows a similar profile as an autoland then a longer landing may result.
Thus, it is most likely that a long touchdown would result in these circumstances. In this case, safety would depend on correctly identifying the situation, knowledge of the risks in the situation, considering the aspects of a HUD guided flare and particularly tailwind, and then acting accordingly.
I suggest that this accident had its roots much earlier in the event than during the flare. Even in company SOPs if the HUD flare / tailwind were major contributors.
PJ2 – IIRC Airbus only deals with contamination via type (description), depth, and extent; Boeing relates to braking action.
CRFI is only used in Canada, and attempts to use it more widely as IRFI were not accepted – similar limitations as other friction methods due to poor correlation with the measuring devices, although within its limits (dry contaminants) CRFI appears to be better.
IMHO the success of CRFI (in Canada) stems from it triggering pilots to think about the distance increases – it helps identify (quantify) the risks and the distance margins to add.
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Originally Posted by PJ2
AirRabbit;
Not disputing your numbers but just curious as to the source, (aside from the FSF). They differ slightly (some more conservative, some less), from the Airbus numbers but I think for most of us the reasons for the differences are understandable in terms of specific test results vs general rules where actual distances and factors are not provided for the crew in the FCOM or QRH. Nevertheless I think they're appropriate.
Not disputing your numbers but just curious as to the source, (aside from the FSF). They differ slightly (some more conservative, some less), from the Airbus numbers but I think for most of us the reasons for the differences are understandable in terms of specific test results vs general rules where actual distances and factors are not provided for the crew in the FCOM or QRH. Nevertheless I think they're appropriate.
Originally Posted by PEI_3721
AirRabbit, #510, you perhaps unintentionally imply that the crew ‘deliberately’ overflew the first 4000ft. Deliberation would suggest awareness of the situation which might not have been the case.
Consider how a normal flare and landing is judged with respect to distance. This is probably by a sense of time - threshold to touchdown, time during the flare, particularly where ground markings are poor / night-time.
Distance, depends on ground speed, thus any tailwind might double the speed/distance difference for a given time, therefore for the perception of ‘normal timing’, the aircraft could have flown much further.
Add to this distance an increment due to a high approach – poor vis in heavy rain, poor lighting, and a tailwind.
Also, I wonder what effect a HUD flare profile has (see WhatsaLizad # 511). The difference between a manual landing and an auto land could be a 1400ft increase according to advisory landing data. If the HUD flare follows a similar profile as an autoland then a longer landing may result.
Consider how a normal flare and landing is judged with respect to distance. This is probably by a sense of time - threshold to touchdown, time during the flare, particularly where ground markings are poor / night-time.
Distance, depends on ground speed, thus any tailwind might double the speed/distance difference for a given time, therefore for the perception of ‘normal timing’, the aircraft could have flown much further.
Add to this distance an increment due to a high approach – poor vis in heavy rain, poor lighting, and a tailwind.
Also, I wonder what effect a HUD flare profile has (see WhatsaLizad # 511). The difference between a manual landing and an auto land could be a 1400ft increase according to advisory landing data. If the HUD flare follows a similar profile as an autoland then a longer landing may result.
It's been a long time since I've been into the Kingston airport in any capacity other than as a passenger, so I'm not aware of any marking anomaly or lighting or reflection deficiency that may currently exist; although I fully acknowledge the problems such circumstances could present. Be that as it may, however ... there are issues that remain puzzling.
Person Of Interest
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What about knowing your A/C and it's capabilities???
What about "flying the A/C"???
Quit relying on automation and do what you were hired for in the first place...FLY THE AIRPLANE!!!
This was a Boeing for gosh sake...not one of those "fully automated" can't do wrong airplanes...
What about "flying the A/C"???
Quit relying on automation and do what you were hired for in the first place...FLY THE AIRPLANE!!!
This was a Boeing for gosh sake...not one of those "fully automated" can't do wrong airplanes...
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Human's misperception during floating-flare
A comment on the typical "long" touchdown:
AAR85-06, pg 40; World DC10/ 23Jan82, off-end BOS 15R:
Still no information available to confirm AA331/22Dec09's final FLAP configuration during that mishap-landing (never confirmed in either of two rpts so far). Regarding the "normal" final FLAP setting, for TAILWIND approach to a wet rwy, mishap-pilots may have been taught, or became accustom to that "Lower Drag" F30 (rather than F40) configuration; perhaps that became their company's "standard" configuration (?). Recently, the manufacturer may have added to this Final FLAPs confusion with TBC's _Aero_ Qtr_2/2010, story encouraging the "Low Drag Approach" configuration:
"... overflew the first 4000ft... a normal flare and landing is judged ... by a sense of time ... during the flare, particularly where ground markings are poor / night-time...."
Several runway excursion investigations attempted to explain this float, & the human's misperception of a normal flare.AAR85-06, pg 40; World DC10/ 23Jan82, off-end BOS 15R:
"...The extended flare resulted in the touchdown about 2,800 feet beyond the displaced threshold.... That the flightcrew believed that the airplane touched down “about 1,500 feet” beyond the displaced threshold may be attributable to the night reduced visibility and the absence of good runway distance measuring references. Even had there been references, the crew would probably have believed that the ... runway remaining at the point of touchdown was adequate for stopping the airplane since it was more than twice that which would be required to stop the airplane on a dry runway. There was no indication that the flightcrew considered aborting the landing at touchdown, and the Board does not believe that the information and cues should have prompted such action...."
TSB/c Occurrence Rpt pg 102, AF A340 off-end 2Aug05 YYZ "... As they crossed the runway threshold, with the heavy rain, low visibility ... crew members became overwhelmed ... task saturated, making a normal landing difficult. The pilots, who were by this time both focusing primarily outside the aircraft, were not aware that a wind shift was also occurring. While in the flare and the initial float, the pilots did not appreciate how much of the runway was being used up. The tailwind component contributed to ... going above ... and to the overall landing distance required...." ...
Still no information available to confirm AA331/22Dec09's final FLAP configuration during that mishap-landing (never confirmed in either of two rpts so far). Regarding the "normal" final FLAP setting, for TAILWIND approach to a wet rwy, mishap-pilots may have been taught, or became accustom to that "Lower Drag" F30 (rather than F40) configuration; perhaps that became their company's "standard" configuration (?). Recently, the manufacturer may have added to this Final FLAPs confusion with TBC's _Aero_ Qtr_2/2010, story encouraging the "Low Drag Approach" configuration:
"... low-drag approach procedures can save an airline significant amounts of fuel over time ..."
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What about knowing your A/C and it's capabilities???
What about "flying the A/C"???
Quit relying on automation and do what you were hired for in the first place...FLY THE AIRPLANE!!!
This was a Boeing for gosh sake...not one of those "fully automated" can't do wrong airplanes...
What about "flying the A/C"???
Quit relying on automation and do what you were hired for in the first place...FLY THE AIRPLANE!!!
This was a Boeing for gosh sake...not one of those "fully automated" can't do wrong airplanes...
Gosh, you are sounding like 411A, and know what?
You are absolutely correct with your statements.
Yes, as I pointed out before, landing at near max weight on a wet runway, with possibly a slight tail wind, can be problem...therefore, if in doubt, divert.
However, if you do decide to land, at least don't overfly nearly half the runway whilst doing so.
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Anybody who has ever hydroplaned, whether in a car or an airplane, knows that indeed acceleration does take place at that moment. And anybody who thinks that isn't so and that claiming so invalidates the WSJ article apparently isn't a pilot. Or a very experienced one
this mistake a significant lack of deceleration as acceleration
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You're talking about personal anecdotal evidence. I suspect people who believe this mistake a significant lack of deceleration as acceleration
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Originally Posted by Murexway
Quote:
You're talking about personal anecdotal evidence. I suspect people who believe this mistake a significant lack of deceleration as acceleration
Nope. Acceleration is acceleration. What's being perceived, of course, is the decrease in the rate of deceleration, but another way of expressing a negative negative is positive. A decrease in deceleration is......acceleration.
You're talking about personal anecdotal evidence. I suspect people who believe this mistake a significant lack of deceleration as acceleration
Nope. Acceleration is acceleration. What's being perceived, of course, is the decrease in the rate of deceleration, but another way of expressing a negative negative is positive. A decrease in deceleration is......acceleration.
When you jump out of an airplane you descend toward the earth. In fact, as you descend, you accelerate toward the earth. Assuming you pull the rip-chord to deploy the parachute before you reach "terminal velocity," you change your downward acceleration. You continue down. However, your rate of speed toward earth is slower. You have changed your descent rate (remember, any change in "rate" or "speed" is called "acceleration") and therefore you have experienced an "acceleration;" an "acceleration in the negative direction," but you never "climbed," you only changed the rate at which you were descending.
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I was just pointing out that, to folks who hit a deep patch on a short, wet runway, and hydroplane, it feels like a slingshot effect - to them it IS acceleration, regardless of any physics involved.
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From my physics class manymany decades ago -
Position is also known as displacement
Rate of change of displacement per unit time (1st derivative) = velocity
Rate of change of velocity per unit time (2nd derivative) = acceleration
Rate of change of acceleration per unit time (3rd derivative) = jerk (I am not making this up...)
If you're braking w/ constant negative acceleration, then jerk = 0
But if you hit a slick spot so that acceleration suddenly goes to zero, then that's "positive jerk". Where you were against the tight shoulder straps, your torso now bounces back against the seat. This gives the PERCEPTION of positive acceleration!
Position is also known as displacement
Rate of change of displacement per unit time (1st derivative) = velocity
Rate of change of velocity per unit time (2nd derivative) = acceleration
Rate of change of acceleration per unit time (3rd derivative) = jerk (I am not making this up...)
If you're braking w/ constant negative acceleration, then jerk = 0
But if you hit a slick spot so that acceleration suddenly goes to zero, then that's "positive jerk". Where you were against the tight shoulder straps, your torso now bounces back against the seat. This gives the PERCEPTION of positive acceleration!