V1 - That critical Sacred Cow.
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From: Australia
V1 - That critical Sacred Cow.
It has always bemused me to watch some captains during the takeoff roll, put their hands over the top of the thrust levers in an unnatural claw-like grip as if to emphasis how ready they were to rip the thrust levers back to idle up to V1. There is no need for that nonsense. For example, a tyre burst 10 knots below V1 on a limiting length runway (be it wet or dry) followed by an abort, would risk the chance of an over-run due to lack of maximum braking availability. V1 is not the sacred cow of all possible takeoff go/stop situations.
For example, I was jump seating on a night take off in a 737-200 at an island in the Central Pacific. Runway was 5600 ft in length with no overun - only a 30 feet drop over a sea wall to the ocean below. The take off weight was limted by accelerate or stop criteria with max take off thrust with bleeds off at 2.18 EPR giving 100% N1.
Unbeknown to us, and because engine covers had not been put in place overnight, both of the engines Pt2 sensors were blocked by insects and dust from a nearby phosphate mine. The result was both EPR instruments over-read. This meant when the EPR showed 2.18 EPR, the real power was only around 2.08. That meant the N1 read about 85% N1 - not the 100% we expected. The engine gauges in the 737-200 were quite small and the difference between 100% N1 and 85% N1 was diificult to distinguish at night. with dim instrument lighting.
Early in the take off roll, both the captain who was PM and myself on the jump seat, had an uneasy feeling that something was not quite right as there wasn't the strong kick in the bum feeling we would have expected at 2.18 EPR. The fact that all gauge needles in both engines read the same, probably lulled us in the same sense of security. It wasn't until late towards the end of the take off roll it dawned on both the captain and myself there was no way the aircraft was going to get airborne by the end of the runway. With about three runway lights remaining and 10 -15 knots below V1, the captain urgently called "taking over" and slammed both thrust levers hard up against the mechanical stops and rotated to 15 degrees nose up. The acceleration increased markedly and we climbed into the totally dark night on instruments. Even after gear and flap retraction and climb power of 1.94 EPR set, the rate of climb was well below expected.
With 1400 nm to go to our destination of Guam in the Central Pacific, the captain made the decision to return to our departure base. Before that, a comparison made between the Operations Manual climb engine parameters and what our instruments showed, revealed a marked discrepency. In other words, we were considerably down on power on both engines. After the landing back on base, engineers discovered both Pt2 senors in the front of the cowls were blocked by insects and phosphate dust. We had experienced a similar problem as to the ill-fated Air Florida 737-200 that went into the Potomac River. In that instance the cause was traced to iced up Pt2 sensors and thus over-reading EPR gauges because the engine anti-ice had not been switched on.
What saved us was the decision by the captain not to abort even though we well below V1 and in theory should have beeen able to stop by the end of the runway. The decision to firewall the engines and thus get maximum power and rotate to 15 degrees early on rotation was the key to survival. When the sun rose in the morning after the take off, debris from footpaths over the road that passed a mere 20 feet from the runway end, was scattered down the runway. It was the closest shave I have ever experienced in my flying career.
Despite his quick actions in continuing the take off below V1, the captain was castigated by management for failure to pick up the discrepency between indicated EPR and N1 indications at the very start of the take off roll. Yet I too, had not picked up the discrepency although not wearing my glasses didn't help either. That incident was a classic example of being wise after the event.
For example, I was jump seating on a night take off in a 737-200 at an island in the Central Pacific. Runway was 5600 ft in length with no overun - only a 30 feet drop over a sea wall to the ocean below. The take off weight was limted by accelerate or stop criteria with max take off thrust with bleeds off at 2.18 EPR giving 100% N1.
Unbeknown to us, and because engine covers had not been put in place overnight, both of the engines Pt2 sensors were blocked by insects and dust from a nearby phosphate mine. The result was both EPR instruments over-read. This meant when the EPR showed 2.18 EPR, the real power was only around 2.08. That meant the N1 read about 85% N1 - not the 100% we expected. The engine gauges in the 737-200 were quite small and the difference between 100% N1 and 85% N1 was diificult to distinguish at night. with dim instrument lighting.
Early in the take off roll, both the captain who was PM and myself on the jump seat, had an uneasy feeling that something was not quite right as there wasn't the strong kick in the bum feeling we would have expected at 2.18 EPR. The fact that all gauge needles in both engines read the same, probably lulled us in the same sense of security. It wasn't until late towards the end of the take off roll it dawned on both the captain and myself there was no way the aircraft was going to get airborne by the end of the runway. With about three runway lights remaining and 10 -15 knots below V1, the captain urgently called "taking over" and slammed both thrust levers hard up against the mechanical stops and rotated to 15 degrees nose up. The acceleration increased markedly and we climbed into the totally dark night on instruments. Even after gear and flap retraction and climb power of 1.94 EPR set, the rate of climb was well below expected.
With 1400 nm to go to our destination of Guam in the Central Pacific, the captain made the decision to return to our departure base. Before that, a comparison made between the Operations Manual climb engine parameters and what our instruments showed, revealed a marked discrepency. In other words, we were considerably down on power on both engines. After the landing back on base, engineers discovered both Pt2 senors in the front of the cowls were blocked by insects and phosphate dust. We had experienced a similar problem as to the ill-fated Air Florida 737-200 that went into the Potomac River. In that instance the cause was traced to iced up Pt2 sensors and thus over-reading EPR gauges because the engine anti-ice had not been switched on.
What saved us was the decision by the captain not to abort even though we well below V1 and in theory should have beeen able to stop by the end of the runway. The decision to firewall the engines and thus get maximum power and rotate to 15 degrees early on rotation was the key to survival. When the sun rose in the morning after the take off, debris from footpaths over the road that passed a mere 20 feet from the runway end, was scattered down the runway. It was the closest shave I have ever experienced in my flying career.
Despite his quick actions in continuing the take off below V1, the captain was castigated by management for failure to pick up the discrepency between indicated EPR and N1 indications at the very start of the take off roll. Yet I too, had not picked up the discrepency although not wearing my glasses didn't help either. That incident was a classic example of being wise after the event.
Last edited by Centaurus; 30th September 2025 at 11:28.
Joined: Dec 1998
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From: UK
I put very light pressure on the back of the thrust levers during the takeoff roll so I don’t inadvertently pull them back (with my fingers lightly wrapped round the front ready to reject, and thumb on the A/T disconnect). You don’t stop for a tyre burst after 80kts. But appreciate a bang and a swing can feel like an engine failure.
However, V1 is still important and accelerating for takeoff isn’t the time for in depth cognitive decision making - it’s rule based for a reason. However I accept that there may be very rare occasions when you have to break the rule - such as your example above.
However, V1 is still important and accelerating for takeoff isn’t the time for in depth cognitive decision making - it’s rule based for a reason. However I accept that there may be very rare occasions when you have to break the rule - such as your example above.
Joined: Apr 2003
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From: Europe
Indeed, a beautiful example of how things are often not that black and white. This however does not negate the fact that V1 is to be treated with the respect it deserves. I think you were at least partly lucky that night as performance wise you were really in no-man's land!
Joined: Apr 2010
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From: IRS NAV ONLY
Great story. Shows very well how EPR is more suited for the engineers in a laboratory environment, rather than the harsh reality of the real world flying, especially with modern near real-time engine condition monitoring.
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From: New Zealand
Debatable. Crews have come to similar grief from flap mis-selections, weight errors, takeoff starting point errors etc. N1 isn't the be-all end-all.
I would argue it's not the EPR that's the issue; it's the lack of position/acceleration monitoring.
A couple of manufacturers have started pushing Take-Off Monitoring that does a better job than just waiting for V-1 but I would argue they could go further. And given that take-off margins are being sliced increasingly thinly, probably should go further.
I would argue that you could have a system that continually calculates current position, stopping distance, climb performance etc. and can then tell you, continuously:
All that said, there's definitely a lot of human factors reasons why we have a clear go/no-go and I'm not necessarily saying it should be dropped. But it might be better served by a go/no-go point, and/or some further nuance.
I would argue it's not the EPR that's the issue; it's the lack of position/acceleration monitoring.
A couple of manufacturers have started pushing Take-Off Monitoring that does a better job than just waiting for V-1 but I would argue they could go further. And given that take-off margins are being sliced increasingly thinly, probably should go further.
I would argue that you could have a system that continually calculates current position, stopping distance, climb performance etc. and can then tell you, continuously:
- What the stopping margin would be if you aborted now (runway remaining, brake energy remaining etc.) - given the discussions around the Russian A321 that hit birds a few years back, this could even tell you if you're OK to abort after rotation/liftoff given sufficient runway remaining.
- What the rotation/climb margins would be if you suffered an engine failure now; could you continue.
- Expected duration of the overlap between safe abort and safe continue. If this is decreasing, something is reducing your performance below expectations. If it's negative, there will be a period where you couldn't safely handle an engine failure.
All that said, there's definitely a lot of human factors reasons why we have a clear go/no-go and I'm not necessarily saying it should be dropped. But it might be better served by a go/no-go point, and/or some further nuance.
Joined: Aug 2007
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From: Sydney
Excellent point Compressor Stall. If the acceleration is below the charted rate due to a number of reasons you will end up further down the runway at V1 and therefore a rejected take-off may well result in a runway over run if it is conducted near V1. Dragging brakes, reverted rubber aqua planning and reduced power etc., are all risks. Slightly less obvious is a rise or hump in the runway. Rwy11/29 at Darwin is a good example. ERSA lists it as level since the elevation of both thresholds is the same. You can imagine what happens in a heavy commercial aircraft. The initial acceleration will clearly be slower since it is uphill with V1 reached further down the runway. Any abort will be down hill. None of this is considered in basic performance calculations. I’m not sure how the latest computer calculated take-off data goes with this such as the Boeing Lap Top Tool and others. One advantage of MilSpec 3011 and 5011 is that an acceleration check time is utilised to a speed somewhat below V1. If you don’t make the speed within the time (with a bit of a buffer) you are still well short of V1 and can safely abort. This strategy would have saved the Emirates A340 in Melbourne from tail striking on take-off after a much lower takeoff weight was put into the FMS.
Beez
Beez
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From: Australia
I recall that at Townsville in the 1960's I think, Distance to Run markers for Runway01 were set up on the flight strip. . I believe these were for the use of visiting USAF B66 twin engine jet bombers. If a certain airspeed wasn't reached by the time the aircraft was passing a specific marker i.e 100 knots by the 4000 ft marker, the pilot would abort the take off.
In fact I witnessed that happen when we watched a B66 start his take-off roll only to abort around the time it had passed the 3000 ft marker. The pilot taxied back and tried another go but this time kept going and climbed away normally. Of course we didn't know the real cause of the first abort or whether he hadn't reached the minimum required speed by the time it had reached the 3000 ft marker. Some weeks later the distance-to- go marker boards were taken down and to my knowledge never used again.
In fact I witnessed that happen when we watched a B66 start his take-off roll only to abort around the time it had passed the 3000 ft marker. The pilot taxied back and tried another go but this time kept going and climbed away normally. Of course we didn't know the real cause of the first abort or whether he hadn't reached the minimum required speed by the time it had reached the 3000 ft marker. Some weeks later the distance-to- go marker boards were taken down and to my knowledge never used again.
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From: UK
I have always "struggled" with V1, since it isn't a fixed point on the runway. It's physical location depends on many variable factors. So how can we be sure that we have sufficient distance left to emergency stop from just before V1 ? - that depends on factors such as acceleration, which is not measured.
In theory, it should be fairly simple with modern GPS equipped aircraft to write a routine in the FMS to use GPS data, the take-off distance calculation and aircraft airspeed, to determine a physical location on the runway for a true, dynamic V1. This could be relied on and used to provide a green GO indication on the PFD, or whatever sort of alert was thought to be appropriate.
This would take into account slow acceleration, incorrect ZFM, incorrect thrust setting, incorrect thrust achieved, etc.
In theory, it should be fairly simple with modern GPS equipped aircraft to write a routine in the FMS to use GPS data, the take-off distance calculation and aircraft airspeed, to determine a physical location on the runway for a true, dynamic V1. This could be relied on and used to provide a green GO indication on the PFD, or whatever sort of alert was thought to be appropriate.
This would take into account slow acceleration, incorrect ZFM, incorrect thrust setting, incorrect thrust achieved, etc.
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From: Wherever I go, there I am
I have always "struggled" with V1, since it isn't a fixed point on the runway. It's physical location depends on many variable factors. So how can we be sure that we have sufficient distance left to emergency stop from just before V1 ? - that depends on factors such as acceleration, which is not measured.
In theory, it should be fairly simple with modern GPS equipped aircraft to write a routine in the FMS to use GPS data, the take-off distance calculation and aircraft airspeed, to determine a physical location on the runway for a true, dynamic V1. This could be relied on and used to provide a green GO indication on the PFD, or whatever sort of alert was thought to be appropriate.
This would take into account slow acceleration, incorrect ZFM, incorrect thrust setting, incorrect thrust achieved, etc.
In theory, it should be fairly simple with modern GPS equipped aircraft to write a routine in the FMS to use GPS data, the take-off distance calculation and aircraft airspeed, to determine a physical location on the runway for a true, dynamic V1. This could be relied on and used to provide a green GO indication on the PFD, or whatever sort of alert was thought to be appropriate.
This would take into account slow acceleration, incorrect ZFM, incorrect thrust setting, incorrect thrust achieved, etc.
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From: Everett, WA
It's worth noting that EPR on modern FADECs is a huge improvement over the old hydromechanical engines (e.g. JT8D).
Modern FADECs cross compare the inlet total pressure with the aircraft, and the exhaust is modeled using rotor speeds, etc. - in both cases if the measured pressure fails the crosscheck, EPR will be invalided in plenty of time to allow a (relatively) low speed RTO.
IF everything is working properly, EPR is more closely associated with thrust than N1 (and doesn't require a temperature correction) - as a result N1 engines 'give away' some thrust since the thrust settings must reflect a 'minimum engine'. That's why some manufacturers prefer EPR to N1.
All that being said, given we now have the ability to easily measure things like ground speed and position on the runway, perhaps it's time for a better "Go-NoGo' decision guide than V1.
Modern FADECs cross compare the inlet total pressure with the aircraft, and the exhaust is modeled using rotor speeds, etc. - in both cases if the measured pressure fails the crosscheck, EPR will be invalided in plenty of time to allow a (relatively) low speed RTO.
IF everything is working properly, EPR is more closely associated with thrust than N1 (and doesn't require a temperature correction) - as a result N1 engines 'give away' some thrust since the thrust settings must reflect a 'minimum engine'. That's why some manufacturers prefer EPR to N1.
All that being said, given we now have the ability to easily measure things like ground speed and position on the runway, perhaps it's time for a better "Go-NoGo' decision guide than V1.
Joined: Jun 2002
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From: east ESSEX
Well you could all go back to `skool`,relearn Mr Newtons `Laws of Motion` again,about time ,acceleration ,distance,speed,etc
or,for simpler beings,
`if you reach 50% distance of the runway available,before reaching 71%Vr,you`d better stop...
or,for simpler beings,
`if you reach 50% distance of the runway available,before reaching 71%Vr,you`d better stop...
Joined: Feb 2000
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From: 500 miles from Chaikhosi, Yogistan
Some flight planning programs do spit out the TORR and TODR. While this doesn't tell you about V1, it does give you some idea of when you should be airborne or when you should start to be concerned that you might not become airborne. Regardless, I agree with you that having this displayed to us in the flight deck as some form of indication on the PFD is a no-brainer. Surely it has to be more simple than the programing required in a ROPS.
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From: Australia
I put very light pressure on the back of the thrust levers during the takeoff roll so I don’t inadvertently pull them back (with my fingers lightly wrapped round the front ready to reject, and thumb on the A/T disconnect). You don’t stop for a tyre burst after 80kts. But appreciate a bang and a swing can feel like an engine failure.
However, V1 is still important and accelerating for takeoff isn’t the time for in depth cognitive decision making - it’s rule based for a reason. However I accept that there may be very rare occasions when you have to break the rule - such as your example above.
However, V1 is still important and accelerating for takeoff isn’t the time for in depth cognitive decision making - it’s rule based for a reason. However I accept that there may be very rare occasions when you have to break the rule - such as your example above.
I can understand the reason why Boeing (for example) mandate disconnecting the A/T before thrust lever closure in order to cover the low speed abort case and thus ensure the one standard procedure for a low or high speed abort. On the other hand, every second delayed on a high speed abort nearing V1 could be vital to the aim of stopping before the end of the runway.
Closing the thrust levers first saves that vital second it takes to press the A/T thumb switch and hoping it disconnects the A/T. .
Joined: Nov 2010
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From: USA
I have always "struggled" with V1, since it isn't a fixed point on the runway. It's physical location depends on many variable factors. So how can we be sure that we have sufficient distance left to emergency stop from just before V1 ? - that depends on factors such as acceleration, which is not measured.
In theory, it should be fairly simple with modern GPS equipped aircraft to write a routine in the FMS to use GPS data, the take-off distance calculation and aircraft airspeed, to determine a physical location on the runway for a true, dynamic V1. This could be relied on and used to provide a green GO indication on the PFD, or whatever sort of alert was thought to be appropriate.
This would take into account slow acceleration, incorrect ZFM, incorrect thrust setting, incorrect thrust achieved, etc.
In theory, it should be fairly simple with modern GPS equipped aircraft to write a routine in the FMS to use GPS data, the take-off distance calculation and aircraft airspeed, to determine a physical location on the runway for a true, dynamic V1. This could be relied on and used to provide a green GO indication on the PFD, or whatever sort of alert was thought to be appropriate.
This would take into account slow acceleration, incorrect ZFM, incorrect thrust setting, incorrect thrust achieved, etc.
I had a discussion with a tech pilot/check airman who thought of V1 as a ground speed because of how it was measured. Maybe in theory, but what is the instrument we use when we call V1. With current technology we should be doing way better..
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From: Bremen
So instead of a current readout of stopping distance etc., the computer should take the acceleration measurements it has, compare it to the expected acceleration, and then warn/alert if it's below a certain threshold so the crew can abort and troubleshoot long before they're near V1.
The question isn't "how fast can we get with engine problems and still stop", the question is "how quickly can we recognize them reliably so we can abort ASAP".
(could also be incorrect weight/temperature/altitude problems, ofc)