Is V1 called 'Decision Speed' or 'Action Initiation Speed'?
John, your general view (#15) summarises the operational practicalities, but more often the industry depends on a precise view of the subject which is assumed to apply to operations. Differences in viewpoint can represent a safety-gap between what is expected and what happens; the breeding ground for accidents.
Whilst the comments in parenthesis have good intent and are technically correct, they play on human weaknesses of false belief and inaccurate risk assessment which often dominate in sudden and surprising events.
I assume that you would agree that the decision re RTO is one of the few really critical time-dependant assessment in operations, and thus where possible should be couched as a simple ‘if then’ activity. This places emphasis on situation recognition which should be based on training and tech knowledge. Extremely remote (improbable) events can be discussed during training so that some pre-reasoning might dispel bias (strategic thinking), thus clearing the mind for a tactical decisions with a failure.
Unfortunately the FAA training guide introduced doubt by discussing ‘what if’ above V1, apparently without reference to certification and safety assumptions etc; from this we encounter these types of discussion.
aviatorhi, you can always find exceptions, both good and bad, but aviation safety is based on pre-assessed risk. The current probability of the exceptional events is far less than those of erroneous human judgement in situation assessment and choice of action. The required level of safety is maintained with aid of design and certification requirements, industry wide safety initiatives, and the assumption that pilots will act as accordingly.
Thus if you think (how do you know) that you have encountered one of your example situations you are almost certainly wrong, and thus expose yourself to a higher probability of an overrun. Also when considering the combined probabilities of failure, exceptional event, and speed, the exceptions do not warrant further concern.
Thus because of the human biases in assessing risk and situation recognition, extensive training and strategic reasoning is required to curb these deficiencies for this critical operation, and most others.
hawtin, I agree. The problem is not that there isn’t a suitable definition, but in the poor wording of the question. The examiner appears to have ignored context, and also pre-packaged some answers, expecting only one; whereas in reality V1 depends on how a pilot asks the question and knowledge of the subject before the event. These cannot be answered with a single exam question.
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For AIRBUS, V1 is Decision Speed, and you have 2 seconds more to initiate the reject. This is the "startle factor" included in Airbus performance computation. (Reference: Airbus Perormance Manual 9.4.3)
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From Airbus A318/A319/A320/A321 PERFORMANCE TRAINING MANUAL
Operating speed: V1
Definition: TakeOff decision speed chosen by the applicant.
V1 is the speed limit at which the pilot can interrupt TakeOff in case of failure.
The 2 seconds doesn't belong to the crew.
Operating speed: V1
Definition: TakeOff decision speed chosen by the applicant.
V1 is the speed limit at which the pilot can interrupt TakeOff in case of failure.
The 2 seconds doesn't belong to the crew.
and .......... the V1 speed calculation is variable depending upon whether full thrust is used, or reduced thrust, which is often the case. With reduced take off thrust the V1 speed will be lower, to take into account the slower deceleration to the "V1 speed" at a point further down the real estate.
but ......... in the case of an engine failure around V1 then using reduced thrust for take off, then the applicaton of full power on the remaining donkeys will alter the maths !
Nothing's ever easy ! "When in doubt, lash out " i.e. it is often better to get airborne and have time to think than end up a steaming wreck at the end of the runway ( or beyond it ! )
Not my problem anymore, best of luck chaps !
( just revert to Stick and Rudder - i.e. fly the beast, don't rely on Bill Gates' magic box. )
but ......... in the case of an engine failure around V1 then using reduced thrust for take off, then the applicaton of full power on the remaining donkeys will alter the maths !
Nothing's ever easy ! "When in doubt, lash out " i.e. it is often better to get airborne and have time to think than end up a steaming wreck at the end of the runway ( or beyond it ! )
Not my problem anymore, best of luck chaps !
( just revert to Stick and Rudder - i.e. fly the beast, don't rely on Bill Gates' magic box. )
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more often the industry depends on a precise view of the subject which is assumed to apply to operations
There are two views under consideration -
(a) certification, which varies according to the Standard relevant to a particular Type's certification. Refer TCDS info. As close as we can get, reasonably, to something relating to precision, whatever that might be.
(b) SOP driven practice. Main aims are repeatability and rule based simplicity. Scant consideration of precision in any aspect.
depends on a precise view of the subject which is assumed to apply to operations.
Absolutely the case - which is why (b) must be strongly rule based other than in the extreme situation where the commander has really good reason to believe that it isn't going to work on this occasion.
you have 2 seconds more to initiate the reject
Whoa, there. Absolutely NOT the case. A/L 42 acknowledged the reality that the previous accel-stop standards were a recipe for disaster on limiting runways in the real world. Rejection philosophy doesn't change ... the numbers just have a bit of a pad (and not all that much) built in to give the crew a chance of stopping on a REALWORLD limiting runway ... with all its contamination, pilot startle, etc., etc... The rejection still MUST have been started with respect to the speed schedules.
The 2 seconds doesn't belong to the crew.
Concur .. but it might just save their necks.
If you have a dual failure on the left side of a narrow body
.. and we can come up with a number of scenarios .. cross wind min V1 situation, for instance. If circumstances conspire to put you right outside the presumed certification envelope .. well, that was the day you shouldn't have dragged yourself out of bed ..
the V1 speed calculation is variable depending upon whether full thrust is used, or reduced thrust
Careful, I suggest .. For flex takeoff V1 min is based on rated thrust. For a derated takeoff, speed will be based on the lower derate thrust - one of the reasons for using derate on some critical runways. If, then, you add flex to derate, V1 min still reflects the derate thrust level.
in the case of an engine failure around V1 then using reduced thrust for take off, then the applicaton of full power on the remaining donkeys will alter the maths !
No, that's why V1 min is based on rated rather than flex thrust. Different ball game with a derated takeoff if you are able (and choose) to push the levers up rapidly and too far.
There are two views under consideration -
(a) certification, which varies according to the Standard relevant to a particular Type's certification. Refer TCDS info. As close as we can get, reasonably, to something relating to precision, whatever that might be.
(b) SOP driven practice. Main aims are repeatability and rule based simplicity. Scant consideration of precision in any aspect.
depends on a precise view of the subject which is assumed to apply to operations.
Absolutely the case - which is why (b) must be strongly rule based other than in the extreme situation where the commander has really good reason to believe that it isn't going to work on this occasion.
you have 2 seconds more to initiate the reject
Whoa, there. Absolutely NOT the case. A/L 42 acknowledged the reality that the previous accel-stop standards were a recipe for disaster on limiting runways in the real world. Rejection philosophy doesn't change ... the numbers just have a bit of a pad (and not all that much) built in to give the crew a chance of stopping on a REALWORLD limiting runway ... with all its contamination, pilot startle, etc., etc... The rejection still MUST have been started with respect to the speed schedules.
The 2 seconds doesn't belong to the crew.
Concur .. but it might just save their necks.
If you have a dual failure on the left side of a narrow body
.. and we can come up with a number of scenarios .. cross wind min V1 situation, for instance. If circumstances conspire to put you right outside the presumed certification envelope .. well, that was the day you shouldn't have dragged yourself out of bed ..
the V1 speed calculation is variable depending upon whether full thrust is used, or reduced thrust
Careful, I suggest .. For flex takeoff V1 min is based on rated thrust. For a derated takeoff, speed will be based on the lower derate thrust - one of the reasons for using derate on some critical runways. If, then, you add flex to derate, V1 min still reflects the derate thrust level.
in the case of an engine failure around V1 then using reduced thrust for take off, then the applicaton of full power on the remaining donkeys will alter the maths !
No, that's why V1 min is based on rated rather than flex thrust. Different ball game with a derated takeoff if you are able (and choose) to push the levers up rapidly and too far.
That 2 seconds....
I always thought the 2 seconds was the time you have to initiate the abort - having already made the decision to abort at V1 - and still stop before the end of the runway (or calculated distance).
If V1 is 150 kts you travel 77m a second.
If you aborted at exactly V1, the test pilots determined that the aircraft would stop in X metres. To account for our mortal reflexes and the "sh1t F#ck factor" as I call it, the performance calculations give you X plus 144m to stop.
If V1 is 150 kts you travel 77m a second.
If you aborted at exactly V1, the test pilots determined that the aircraft would stop in X metres. To account for our mortal reflexes and the "sh1t F#ck factor" as I call it, the performance calculations give you X plus 144m to stop.
Last edited by compressor stall; 26th Jul 2013 at 02:09.
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solipsist and compresser stall
V1 call is a GO call period. You are misinterpreting the accelerate stop distance definition after Amendment FAR 25-42
To summarize,
Amendment 25-42 required the accelerate-stop distance to include
two seconds of continued acceleration beyond V1 speed, before the pilot takes any action to stop the airplane.
That is when first action to reject is taken at V1 not two seconds afterwards.
V1 call is a GO call period. You are misinterpreting the accelerate stop distance definition after Amendment FAR 25-42
To summarize,
Amendment 25-42 required the accelerate-stop distance to include
two seconds of continued acceleration beyond V1 speed, before the pilot takes any action to stop the airplane.
That is when first action to reject is taken at V1 not two seconds afterwards.
Last edited by vilas; 26th Jul 2013 at 02:38.
Yes - a rereading of 25.109 states that the first action to reject is at the V1 - which is practically what you do anyway.
Last edited by compressor stall; 26th Jul 2013 at 03:51.
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Stallie .. the two seconds is intended to be hidden from the pilot's actions and represents money in the bank for the accel stop. You probably recall the equivalent Oz change to 101.6 at A/L 62.
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At ballanced field length for US was V1 meant you could stop at the end of the runway or be at 35 ft at the end of the runway. No 2 second buffer in the US. That is the way I was taught. The two second buffer to react seems pretty sloppy for an airline crew performance. I taught this in corporate jets and V1 was a go or abort speed, no buffer. My major airline was the same. No pilot I know would take 2 seconds to decide what to do. That is why V1 is there. It is not advisory, it is a decision speed. Some countries might do it differently but that is how we have always done it.
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bubbers 44
Your balanced field lengh understanding is correct the only change is at V1 you don't decide but first action to reject is taken. For that to happen it has to be before V1 call. V1 call is GO, actually according to Airbus GO call is only before V1 not afterwards. Many are mistaking the two seconds. There is no question of taking 2 seconds to decide after V1. V1 call is go call. Two seconds acceleration is added for ASDR calculations since it will take some time for decelaration to begin. It is not for pilot. This ammended definition of V1 is from FAA.
Your balanced field lengh understanding is correct the only change is at V1 you don't decide but first action to reject is taken. For that to happen it has to be before V1 call. V1 call is GO, actually according to Airbus GO call is only before V1 not afterwards. Many are mistaking the two seconds. There is no question of taking 2 seconds to decide after V1. V1 call is go call. Two seconds acceleration is added for ASDR calculations since it will take some time for decelaration to begin. It is not for pilot. This ammended definition of V1 is from FAA.
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Since this topic is not a matter of opinion but a regulatory and safety issue,to put an end to all arguments and opinions below is from Boeing document
"V1 and the GO No Go Decision"
V1- is the speed at which the takeoff should be continued unless the stopping maneuvre has already been initiated.
You can review this document on smartcockpit.com under FLT OPS.
"V1 and the GO No Go Decision"
V1- is the speed at which the takeoff should be continued unless the stopping maneuvre has already been initiated.
You can review this document on smartcockpit.com under FLT OPS.
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Since we ar talking about less than a knot of airspeed at V1 it really doesn't matter much, does it? Before 1 knot below V1 abort, at V1 continue. How many pilots have had a problem in that one knot range? Maybe none?
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Originally Posted by bubbers44
Since we ar talking about less than a knot of airspeed at V1 it really doesn't matter much, does it?
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The two second buffer to react seems pretty sloppy for an airline crew performance.
Anyway, the 2 seconds is built into the calculation of V1, so it is not an additional buffer. HN39 may confirm my memories?
Last edited by Owain Glyndwr; 28th Jul 2013 at 09:07.
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The 2 seconds of continued acceleration after V1 was introduced in the accelerate-stop distance specification of FAR 25.109 with FAR Amdt. 25-42, effective date March 1, 1978.
The current regulation is FAR Amdt. 25-92, effective date March 20, 1998.
P.S.
Advisory Circular AC 25-7C "Flight Test Guide For Certification Of Transport Category Airplanes" provides additional background on how the flight tests are conducted, and how the flight test data are 'expanded' to AFM distances.
Appendix 3 describes the Historical Development of Accelerate-Stop Time Delays
The current regulation is FAR Amdt. 25-92, effective date March 20, 1998.
P.S.
Advisory Circular AC 25-7C "Flight Test Guide For Certification Of Transport Category Airplanes" provides additional background on how the flight tests are conducted, and how the flight test data are 'expanded' to AFM distances.
Appendix 3 describes the Historical Development of Accelerate-Stop Time Delays
Last edited by HazelNuts39; 28th Jul 2013 at 13:05. Reason: P.S.
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No 2 second buffer in the US
Only for those aircraft for which the frozen design standard included A/L 42. TCDS gives that story but, given the use of grandfathering, one is looking at somewhat further down the date track.
The two second buffer to react seems pretty sloppy for an airline crew performance
A/L 42 was not intended to address crew incompetence. Rather it was to put in some conservatism to allow for real world things such as startle factor, surface contaminants in the stopping region of the runway (lots of rubber deposits from oppposite direction landings), etc
As Vilas and others note, there is a moderate to significant speed overshoot prior to the deceleration's actually resulting in the speed's (rather than the acceleration's) decreasing. For the AEO case, that overshoot can be quite significant.
the 2 seconds is built into the calculation of V1, so it is not an additional buffer
Actually, the recognition and reaction times must still be there (unless the current crop of TPs have, indeed, evolved to be superhuman) ... the A/L 42 two seconds is an additional pad added to the calculated (and is included in the demonstrated) accel-stop distance.
Only for those aircraft for which the frozen design standard included A/L 42. TCDS gives that story but, given the use of grandfathering, one is looking at somewhat further down the date track.
The two second buffer to react seems pretty sloppy for an airline crew performance
A/L 42 was not intended to address crew incompetence. Rather it was to put in some conservatism to allow for real world things such as startle factor, surface contaminants in the stopping region of the runway (lots of rubber deposits from oppposite direction landings), etc
As Vilas and others note, there is a moderate to significant speed overshoot prior to the deceleration's actually resulting in the speed's (rather than the acceleration's) decreasing. For the AEO case, that overshoot can be quite significant.
the 2 seconds is built into the calculation of V1, so it is not an additional buffer
Actually, the recognition and reaction times must still be there (unless the current crop of TPs have, indeed, evolved to be superhuman) ... the A/L 42 two seconds is an additional pad added to the calculated (and is included in the demonstrated) accel-stop distance.