The V1 conundrum
Thread Starter
Joined: Aug 2011
Posts: 438
Likes: 13
From: Australia
The V1 conundrum
Years ago I was a deading jump seat pilot in a Boeing 737-200 that experienced a dual EPR blockage while taking off at night on a runway limited length of 5600 ft.
It transpired later that both Pt2 tubes serving the engines had been blocked well before take off because the engine covers had not been fitted during an overnight stop.
Looking back at that event years later both the captain and myself had the same sixth sense feeling at the start of the take off roll that the acceleration wasn't what we expected for a max thrust take off. Yet all the engine readings were normal at a quick glance. With four runway lights to go to the departure threshold it dawned on both the pilot and myself we were never going to be airborne by the end of the runway.
We were 15 knots below V1 by then and an abort would have been fatal due to no over-run safety area . There were boulders for a sea wall only 50 yards from the departure end which would have torn us to pieces had an abort taken place. When we saw that an abort would be fatal, the captain fire-walled both engines and pulled back hard on the control column into the black void of the night. Later FDR read outs showed the aircraft maintained 50-100 feet above the water for one mile before we climbed away. Airspeed was V1 minus 15 knots when the captain took those actions. In theory the 737 was runway limited for take off over the sea which is why the take off was 2.18 EPR bleeds off. The V speeds on the take off chart confirmed this - except we were not to know the EPR readings were faulty and the real EPR we experienced was more like 2.10 - well below the expected 2.18 EPR.
The N1 readings would have shown the actual power but there was barely 2mm difference between the needles of the expected 100% N1 and the 92% N1 the engines actually achieved. Being at night and with the instrument lighting turned low, the 2 mm difference was difficult to notice especially on both engines at the same time
Later investigation of the engine Pt2 sensors located at the bottom of the engine cowls showed both were blocked with a combination of phosphate dust from a nearby mine and insects.
While the crew including myself in the jump seat, could be blamed for not monitoring the engine instruments more closely (the lower than expected N1 readings was the main clue) the lack of acceleration feeling after brakes release was another missed opportunity. The actions of the captain when at the last few seconds he realised that we were never going to get airborne in the remaining runway length, was probably the best decision he had ever made in his flying career.
For my part, that experience made me have a long standing distrust of V1 as a stop/go decision point. I made a private promise to myself that in future I would make a decision to go on passing 15 knots below V1. My thinking was that the chances of a real V1 cocked up abort for whatever reason, was greater than the continue case. Fortunately I have long since retired knowing my theory will never be put to test.
It transpired later that both Pt2 tubes serving the engines had been blocked well before take off because the engine covers had not been fitted during an overnight stop.
Looking back at that event years later both the captain and myself had the same sixth sense feeling at the start of the take off roll that the acceleration wasn't what we expected for a max thrust take off. Yet all the engine readings were normal at a quick glance. With four runway lights to go to the departure threshold it dawned on both the pilot and myself we were never going to be airborne by the end of the runway.
We were 15 knots below V1 by then and an abort would have been fatal due to no over-run safety area . There were boulders for a sea wall only 50 yards from the departure end which would have torn us to pieces had an abort taken place. When we saw that an abort would be fatal, the captain fire-walled both engines and pulled back hard on the control column into the black void of the night. Later FDR read outs showed the aircraft maintained 50-100 feet above the water for one mile before we climbed away. Airspeed was V1 minus 15 knots when the captain took those actions. In theory the 737 was runway limited for take off over the sea which is why the take off was 2.18 EPR bleeds off. The V speeds on the take off chart confirmed this - except we were not to know the EPR readings were faulty and the real EPR we experienced was more like 2.10 - well below the expected 2.18 EPR.
The N1 readings would have shown the actual power but there was barely 2mm difference between the needles of the expected 100% N1 and the 92% N1 the engines actually achieved. Being at night and with the instrument lighting turned low, the 2 mm difference was difficult to notice especially on both engines at the same time
Later investigation of the engine Pt2 sensors located at the bottom of the engine cowls showed both were blocked with a combination of phosphate dust from a nearby mine and insects.
While the crew including myself in the jump seat, could be blamed for not monitoring the engine instruments more closely (the lower than expected N1 readings was the main clue) the lack of acceleration feeling after brakes release was another missed opportunity. The actions of the captain when at the last few seconds he realised that we were never going to get airborne in the remaining runway length, was probably the best decision he had ever made in his flying career.
For my part, that experience made me have a long standing distrust of V1 as a stop/go decision point. I made a private promise to myself that in future I would make a decision to go on passing 15 knots below V1. My thinking was that the chances of a real V1 cocked up abort for whatever reason, was greater than the continue case. Fortunately I have long since retired knowing my theory will never be put to test.
Joined: Dec 2003
Posts: 2,349
Likes: 841
From: Tring, UK
...For my part, that experience made me have a long standing distrust of V1 as a stop/go decision point. I made a private promise to myself that in future I would make a decision to go on passing 15 knots below V1. My thinking was that the chances of a real V1 cocked up abort for whatever reason, was greater than the continue case. Fortunately I have long since retired knowing my theory will never be put to test.
Modern performance software can show your margins for TO with AEO, OEI and reject so you can do a bit of what-if preparation for these kind of scenarios, should something rare/unusual come to pass. We don’t train it and we don’t check it, so up to you on the day if you think unorthodox action will save it.
Joined: Jan 2025
Aviation Qualifications: Non-Aircrew
Posts: 640
Likes: 782
From: New Zealand
Are you sure this isn't you (or one of the other crew on the same flight): V1 - That critical Sacred Cow.
Joined: Jul 2003
Posts: 1,323
Likes: 54
From: An Island Province
conundrum:-
A riddle in which a fanciful question is answered by a pun.
A paradoxical, insoluble, or difficult problem; a dilemma.
In aviation no pun or difficulty; more often the framing the question or in understanding the situation.
If judgement was sufficient to detect a shortfall in distance remaining for an RTO (how known - accel, distance), then why would similar judgement not be made during initial acceleration - again how knowable, what cues - acceleration alone?
To change a worldwide approved procedure due to one instance of a 'feeling' is fraught with danger.
If in the event of an engine failure before V1, then choosing to go is more likely a decision to 'go off' the side of the runway - the 'chances of a real V1 cocked up abort' start with the choice and application of procedures, 'by you, your thinking, your misplaced feeling' (us, humans).
As described, the actions - full thrust, suggest that the judgement was that distance remaining was insufficient to achieve lift off; that is not a V1 issue, its more likely how to determine adequate acceleration which has been discussed previously.
The 'problem' is how to detect blocked sensors, incorrect thrust, aircraft acceleration.
In modern EFIS aircraft, note the normal range of speed trend - an acceleration cue; a quick cross check by PM as required early in the takeoff run.
A riddle in which a fanciful question is answered by a pun.
A paradoxical, insoluble, or difficult problem; a dilemma.
In aviation no pun or difficulty; more often the framing the question or in understanding the situation.
If judgement was sufficient to detect a shortfall in distance remaining for an RTO (how known - accel, distance), then why would similar judgement not be made during initial acceleration - again how knowable, what cues - acceleration alone?
To change a worldwide approved procedure due to one instance of a 'feeling' is fraught with danger.
If in the event of an engine failure before V1, then choosing to go is more likely a decision to 'go off' the side of the runway - the 'chances of a real V1 cocked up abort' start with the choice and application of procedures, 'by you, your thinking, your misplaced feeling' (us, humans).
As described, the actions - full thrust, suggest that the judgement was that distance remaining was insufficient to achieve lift off; that is not a V1 issue, its more likely how to determine adequate acceleration which has been discussed previously.
The 'problem' is how to detect blocked sensors, incorrect thrust, aircraft acceleration.
In modern EFIS aircraft, note the normal range of speed trend - an acceleration cue; a quick cross check by PM as required early in the takeoff run.
Joined: Jul 2013
Aviation Qualifications: Non-Aircrew
Posts: 5,681
Likes: 3,343
From: Everett, WA
That technology simply wasn't available on the JT8D which resulted in a few takeoff accidents (the most famous being the 737-200 that ended up in the Potomac.) Which is why it's been implemented on FADEC.
Several years ago, I got a couple of 'oh-god-thirty' calls from Boeing when a Singapore 747-400 experienced multiple RTO's due to EPR on one engine repeatedly being invalidated prior to 80 knots. They eventually found a partially plugged P5 probe (turbine exit pressure). It wasn't completely plugged, so during a normal engine ground run-up (which are done with a gradual accel) it read OK, but during TO power set, the partially plugged probe would add enough of a lag to the P5 measurement would fail the synthetic cross check and invalidate EPR.
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
Posts: 7,448
Likes: 310
From: various places .....
both the captain and myself had the same sixth sense feeling at the start of the take off roll that the acceleration wasn't what we expected for a max thrust take off
That report comes from a colleague who related the tale to me long ago over coffee. Rather hear about it than watch it from the jump seat, methinks.
There have been monitoring systems available in the past. Several bits and pieces at random may provide interest reading to some eg
20080005969.pdf
Takeoff Surveillance & Monitoring Functions – Safety First (I have no AB background so can only take the linked material as read)
1499066060199533181-06175315
content
18pos11-take-off-performance-monitoring-system.pdf
How to make takeoff safer | Airbus
NPA 2025-01 (A)
In the distant past, we used speed/time against distance gone (or similar) data (using runway board style information during the takeoff). Easy enough to figure out such data if not superbly precise out on the line. It provided a useful bit of comfort (that things were progressing reasonably according to Hoyle) or discomfort (should things be heading grossly south) and gave the Commander a bit more information for decision processing assessment.
As always, the real problems are risk and risk/benefit analyses. Not much point in having such information unless the overall accuracy/reliability gives us a benefit when compared to the risk of excessive instances of nuisance warnings leading to grossly excessive reject rates.
That report comes from a colleague who related the tale to me long ago over coffee. Rather hear about it than watch it from the jump seat, methinks.
There have been monitoring systems available in the past. Several bits and pieces at random may provide interest reading to some eg
20080005969.pdf
Takeoff Surveillance & Monitoring Functions – Safety First (I have no AB background so can only take the linked material as read)
1499066060199533181-06175315
content
18pos11-take-off-performance-monitoring-system.pdf
How to make takeoff safer | Airbus
NPA 2025-01 (A)
In the distant past, we used speed/time against distance gone (or similar) data (using runway board style information during the takeoff). Easy enough to figure out such data if not superbly precise out on the line. It provided a useful bit of comfort (that things were progressing reasonably according to Hoyle) or discomfort (should things be heading grossly south) and gave the Commander a bit more information for decision processing assessment.
As always, the real problems are risk and risk/benefit analyses. Not much point in having such information unless the overall accuracy/reliability gives us a benefit when compared to the risk of excessive instances of nuisance warnings leading to grossly excessive reject rates.
Joined: Oct 2019
Posts: 234
Likes: 12
From: SW1A 2AA
V speeds are only any good if everything is working as planned. As soon as something is "out" they are pretty much useless. Wrong thrust settings, wrong flap settings, wrong data input and taking off from the wrong part of the runway are all commonly seen mistakes that can really spoil your day. Fudging the speeds by calling V1 early doesn't really solve your problem. We had an A330 where the crew entered the ZFW into the EFB and took off with a V1, VR and V2 all about 40 knots too low. They did exactly as your guys did, firewalled the levers as they approached the end of the runway and managed to get airborne and somehow saved the day.
I guess trusting your instincts is as good a protection as anything else when something has gone Pete Tong.
I guess trusting your instincts is as good a protection as anything else when something has gone Pete Tong.
Thread Starter
Joined: Aug 2011
Posts: 438
Likes: 13
From: Australia
Are you sure this isn't you (or one of the other crew on the same flight): V1 - That critical Sacred Cow.
Joined: Jan 2025
Aviation Qualifications: Non-Aircrew
Posts: 640
Likes: 782
From: New Zealand
You could even do what autobrake does: set a target acceleration, and adjust thrust (instead of brakes) to maintain that desired acceleration.
There's a decent number of accidents that have been caused by incorrect thrust or weight settings/calculations; MK1602 is the poster child. A proper accurate acceleration check would pick those up, too.
