Perf Group A history

[Centaurus]

The early raising of the nose during the takeoff roll seemed to be a common technique in those days. At night, especially with the short nose on the Comet and also the DH Vampire series and the Canberra, meant it was a guess by the pilot how much nose up showed on the artificial horizon compared to the view outside seen from the cockpit when the nose-wheel was lifted off the runway at speeds well below the recommended main lift off speed.

For example, RAF Pilot's notes for the Vampire (another de Havilland type like the Comet) required: "As soon as the aircraft reaches a speed of 60-70 knots IAS, lift the nose-wheel just clear of the ground, then at 82-87 knots IAS ease the aircraft off the ground".

So a fair amount of back pressure would be needed to lift the nose-wheel at 60-70 knots and if that same back pressure was maintained during the remainder of the takeoff roll, it was inevitable with increasing airspeed the elevator was steadily becoming more effective. By the time the aircraft reached lift-off speed of 82-87 knots the nose would be considerably higher than "lift the nose-wheel just clear of the ground"

Is it any wonder the induced drag could have increased significantly while all this was going on, resulting in a longer take off run and a sense the aircraft was not accelerating fast enough.

There have been reports of this happening to civilian owned Vampires where the pilot held the nose-wheel too far off the ground instead of just clear while accelerating towards published lift off speed. During this writer's time on Vampires (both single and two seat versions) it was common pilot technique to "feel" the aircraft off the ground at speeds well in excess of the Pilots Notes recommended lift off speed because it was an uncomfortable squashing feeling to lift off at the PN speeds.

I have often wondered since those days, why it was seen necessary to lift the nose wheel relatively early in the take off run in the early jets, rather than at the rotation speed used in todays jet transports. Was it because a precaution against slush build up on the nose-wheel in winter weather in those climes?

[Vessbot]

I was wondering this myself 30 seconds ago while reading your post describing the history. The possibilities I came up with were lower speed limit on the nosewheel, or structural weakness wrt. bouncing, etc. And soft fields.

All those factors aside, my ideal conception of a tricycle takeoff is to rotate so as to reach the liftoff attitude at the same time as you reach the liftoff speed. Any sooner, and it's unnecessary induced drag. Any later, and you're using more runway than you need to.

[JW411]

kenparry:

Certainly not captain's discretion. I think we are talking AOC here. Load was usually local troops and equipment to Salalah. IFR safety height was impossible to achieve nor could we turn right and go down the coast via Masirah. Not enough fuel for that. So the answer was to turn left and follow the Bid Bid to Izki valley VFR out on to the plain. It would not have been good to do this every day of the week.

[kenparry]

I have often wondered since those days, why it was seen necessary to lift the nose wheel relatively early in the take off run in the early jets, rather than at the rotation speed used in todays jet transports. Was it because a precaution against slush build up on the nose-wheel in winter weather in those climes?

Me too. The JP, Vampire (T11 only for me) and Hunter all had that technique written in. Don't think slush came into it, as we did not fly from contaminated runways. Your Vampire speeds are lower than I recall; I do remember 100kt as the threshold speed for landing, and there's a vague memory of about 110kt for lift-off. This was the T11 with no external stores, and internal fuel of about 3300 lbs. (Was it? Long ago now. Might have been closer to the 2500lb quoted in the link below)

JW411: thank you for that. From what you say I conclude you were Aden based. My Gulf time was later, at Muharraq, where we had a standing detachment of Argosies from Benson.

[Goldenrivett]

why it was seen necessary to lift the nose wheel relatively early in the take off run in the early jets

Was it due to "nose wheel shimmy"? (like many supermarket trolley wheels)

The nose wheel does not seem to steerable from this description.
de Havilland Vampires - Aircraft Data
"The undercarriage was hydraulically operated with a castoring nose-wheel. Brakes were the typical DH pneumatic style, fed by a pre-charged accumulator and using a single control column actuator and rudder application for asymmetric braking."

[kenparry]

Was it due to "nose wheel shimmy"?

Possibly. The types I listed in the previous post all had castoring nosewheels. Shimmy could be a problem on the JP at times. The JP (or Vampire - or both - it's long ago and memory is fading) had nosewheel tyres with a tread on each shoulder and a gap between on the wheel's central plane, described as "anti-shimmy" tyres.

[Sidestick_n_Rudder]

I remember David Beatty’s „The Naked Pilot” refering to the FCO Comet accident. He indeed quoted nosewheel shimmy as the reason for this technique.

[JW411]

kenparry:

I was on 105 in Aden 1966/67 and then did 5 x 3.5 month Ardet detachments in Muharraq finishing in October 1971.

Moving on; I have a friend who flew the Comet 4B with BEA. He talks about a technique called "skimming" the nosewheel at 80 knots on take-off. In other words, raising the nosewheel just off the runway. The object of the exercise was apparently to prevent water/snow/ice etc from going down the intakes.

[Centaurus]

The nose wheel off the ground technique is still taught by numerous Australian flying schools in several types of light aircraft including the Cessna 150/172 and their ilk. This is on runways. Reasons often include "take the weight off the nose-wheel because the nose wheel is the weakest part of the aircraft." Other reasons offered was minimising the chances of nose-wheel shimmy. Crazy ideas yet still taught by new instructors for some reason..

Several years ago I wrote to the Cessna company about this strange technique and they replied they had never heard of it before. Different if a soft field take off but unnecessary on hard surface. I once saw a Cherokee Six with a full load of passengers depart from a long runway at Essendon, Melbourne. As it taxied I noticed it was very tail down in attitude. The nose wheel was raised off the runway early in the take off run and the aircraft lifted off in a most frightening nose up attitude squashing while trying to gain speed. I was later told it was still nose up when it pranged into gently rising ground a mile from the end of the runway. No one hurt though.

[megan]

The procedure in the Comet airliner was to lift the nose wheel off at a speed of 80 knots, and hold it off while accelerating. V1 and V2 were used in the planning eg at the maximum permissible a.u.w. of 105,000 lb the critical speed at London Airport on the E.W. runway is 95 kt I.A.S. and the unstick speed 100 kt I.A.S.

The practise of lifting the nose wheel early lead to the first two crashes of Comets, both write offs. Inability to accurately assess attitude when holding the nose wheel off lead to the possibility of the aircraft either becoming airborne in a stalled state, or not being able to accelerate to sufficient speed within the confines of the runway, all due to excessive induced drag. I maybe wrong, but I seem to recall it was these two accidents that lead to the current procedures with introduction of the Vr call.

[Shrikered]

And the US version of takeoff regulations?

Originally Posted by kenparry

I have been trying to establish, out of idle curiosity, the history of the development of aircraft Performance Groups, particularly Perf “A”, with its rigorous treatment of engine-out operation. Web searches have produced little info.

I have seen a claim that Perf A originated with FAR25, which was first published in 1965, and that the first type certificated to FAR25 was the B737. Is that correct

I have read this thread with great interest and am chasing the references. I am trying to figure out how the V1, V2, MTOW and related calculations changed over time, and I'm mainly studying US practices including FAR 25. For example:

• When was V1 first conceived? VR, V2?
• When and where were they first made official?
• What was done before then, for large prop aircraft?
• Multiple accidents have shown that runway friction was not being measured consistently. According to AeroSafetyWorld, Nov. 2012, the FAA is investigating better ways to evaluate it. What other changes have been made over theyears in the details of calculations used for takeoffs and landings?
• Do all airlines and pilots make the calculations consistently (presumably because of legal requirements)? Or, do some have what they consider improved methods?

Thanks to everyone who has contributed, and please PM or respond here if you might have additional information.

[aterpster]

I can only comment about V1 and jet transports. I did not fly piston transport airplanes. I went with TWA in January, 1964. V1 was already in use on the jets. Some years later the industry decided to reduce V1 down to Vmcg with long runways and light weights, on the premise the accident rate of high speed aborts had proven it was safer to continue the takeoff with an engine failure than attempt a high-speed abort.

[kenparry]

Do all airlines and pilots make the calculations consistently (presumably because of legal requirements)? Or, do some have what they consider improved methods?

I can only answer from my experience of one UK airline in the 80s/90s. We had a Performance Section within the Nav Dept, and they produced analyses for every runway on our route network to give V1/VR/V2 and RTOW for a range of temperatures (typically 0 to about +40 deg C) and wind components from 10kt tail to 20kt headwind. There were corrections available for pressure variations. Some runways had several pages for different flap settings. The resulting manual was about 3 ins thick on US letter-sized pages. It satisfied the regulators and gave the operating crews the data we needed.

Was it consistent across airlines? I don't know - we (UK) all worked to the same regs, but I suspect some Perf Sections had better analyses than others. These days, the same airline has a different system with all pilots having the manuals on iPads; tap in the numbers for your take-off, and the system produces V speeds, flap setting, trim setting etc in the blink of an eye.

After a couple of years on the line I had thought that I had a good grip of Perf "A", but at each Winter refresher course our man from the Perf Section would open my eyes to something new.

Regarding V speeds, look at CAR4b: here http://www.theiplgroup.com/CAR-PART4b.pdf

4b.114 lists takeoff speeds as V speeds in this 1953 document - and many of the other performance considerations of FAR25 are found elsewhere, though the requirements appear to be less demanding than those of FAR25. CAR4b is long - I have not yet read all of it.

[oxenos]

Can't give chapter and verse on the regs, but I can add a few memories

Mid sixties, the nose wheel was raised on the Mk 3 Shackleton at Rotate -10 knots. Certainly was not for nose wheel shimmy, we had a steering nosewheel. There was indeed a long gap between stop speed and rotate with no guarantees. On the Mk 2 Shackleton there was also a max brake speed. If an engine failed before stop speed but after max brake, it was close the throttles, wait for the speed to drop to max brake speed, then brake. Brake too early, and the pneumatic brake sacs would burst and leave you with no brakes at all. Happy Days.
1970, went on to the Nimrod. It was operated to Perf A, but we were told that in war we would be issued with Military Operating Standards.
The Nimrod performance was originally done entirely on D and X graphs, but later some simpler graphs were produced for our "regular" airfields.
I was on the same conversion course as one of the Flight Commanders, had no problems with perf, and so when we got back to the Squadron and he decided that there should be a performance "expert" on the squadron, guess who he chose? I went on the splendidly named "Long Performance Course" at Hullavington. Turned out I was the only person on the course who was not leaving the Air Force. All the others were, and were treating the course as a way to passing the performance exam for their licences.
The course and the exam were indeed based on the Britannia. It was necessary to extract figures from a set of graphs, and then use these figures to DRAW a graph from which you got your MTOW and V1/VR ratio.
6 years later I was the performance instructor on the Nimrod OCU, which in turn made me performance advisor to HQ Maritime. They asked what contingency plans I had for issuing the MOS to the Squadrons. None, says I, you've got them. Oh no we haven't says they. Turns out Hawker Siddeley had never been asked to produce them.

[JW411]

Centaurus:

Out of curiosity I've just got my Pilots Notes for the Vampire T11 out (I only ever flew the T11 and that was in 1961). It says:

Raise the nose wheel at 80-85 knots, maintain this attitude and fly the aircraft off at 110-115 knots at typical service load. At maximum all-up weight raise the nose wheel at 105-110 knots, the aircraft then becomes airborne at 120-125 knots.

It doesn't say why and I can't remember!

[kenparry]

JW: in that case, you are just the chap to revive my failed memory of the T11's fuel capacity - please?

[john_tullamarine]

Some thoughts ..

Some years later the industry decided to reduce V1 down to Vmcg with long runways and light weights

Fine, on first reflections. However, a poor overall risk balance due to the adverse effect on Vmcg of crosswind.

The problem can be mitigated, in the case of non-limiting runways, by increasing the V1 a bit to account for the crosswind. This, typically, varies from around 05kt/kt for twins to a bit in excess of 1kt/kt for quad jets.

By pushing up the V1, the handling should remain something similar to the nil wind (7kt for the old BCARs, as I recall) case.

they produced analyses for every runway on our route network

The bog standard approach for manual charts (whether produced as graphs - usually - or tabulations). Done by hand, a pain in the neck, so we generally crossplotted from the AFM to start with easier (read quicker) charts for analysis. When the use of electronic computers arrived, it was a tedious, but very useful, exercise to run a heap of regressions (or set up lookup tables) and then have the computer do in minutes what the human took many hours to produce.

but I suspect some Perf Sections had better analyses than others.

Presuming the basic ops engineering competence was similar, it really came down to how much money the bean counters would let be spent on the exercise. Very much a case of diminishing returns as one chased the last few kilos. If the exercise were done with a bit of care, the delta between the AFM and the RTOW chart data could be kept down to a very small figure .. but that took time and money so some skimped a bit here and there.

These days, the same airline has a different system with all pilots having the manuals on iPads

Essentially the same as earlier .. depending on source, the computer software will have been developed from the paper AFM (as we did in the old days) or (if from the OEM) will use software similar to that which would be used to produce the AFM data. Six of one, half dozen of the other.

There now arise two very significant problems -

(a) the crews tend to end up dumbed down (as in, they become adept at pressing buttons in the absence of understanding what the button might be doing) in the knowledge stakes and may have little basic knowledge of what it is the computer is doing for them. Particularly, for the older BCAR AFMs, the charts generally were a delight as each separate limitation was presented separately, making it a doddle to get a really good feel for what the aircraft could do.

(b) getting the obstacle data. The backroom folk spend (or should be) a lot of time and effort on this. Obviously, the line crew can't/don't so the question now becomes what does the tech services group provide for the exercise ?

but at each Winter refresher course our man from the Perf Section would open my eyes to something new.

A good airline that saw the value in investing a few extra dollars in crew training. We did the same with the IPEC (now a long-gone operator) boys.

The Nimrod performance was originally done entirely on D and X graphs

For those not familiar with these, the technique was a means of combining data sets rather than keeping things separate. Typically used for TOD/ASD analyses. Not as intuitive for the user's getting a feel for the aircraft but, in essence, just a different way of painting by numbers.

IGh's post is useful as it points to the good sources of superseded data on the web relating to the US rules, in particular. While it can get a bit tedious tracking things down, the results can be useful for those with the interest.

[aterpster]

Originally Posted by john_tullamarine

Presuming the basic ops engineering competence was similar, it really came down to how much money the bean counters would let be spent on the exercise.

Have I previously told you about the fiasco at TWA that some obnoxious pilots got resolved in the 1980s?

[john_tullamarine]

.. c'mon, c'mon ... tell us all about it ....

[aterpster]

Originally Posted by john_tullamarine

.. c'mon, c'mon ... tell us all about it ....

A bit incredible since that had a performance and engineering department that sold performance data to some other operators:

They taught: with an engine failure just above V1 you could fly the takeoff profile straight-ahead to 1,500 feet, afe, anywhere, anytime. That made no sense at Las Vegas to the west or San Francisco 19L/R. The union tech committee questioned it and were told "no problem, have a nice day."

We didn't buy that, so the head of the union went above the guy who ran the p and e department and gave a specific question: 727-231 taking off Ls Vegas Runway 25 at a temperature that just allowed MGTOW, engine failure just above V1, please provide altitude in one mile increments until reaching 1,500 afe. It took 31 miles. The airline would have crashed into the first ridge 8 miles west of the airport going straight ahead.

The result was pink takeoff performance sheets for airports where straight-ahead wouldn’t hack it. The pink performance sheet had an OEI flight track to avoid offending terrain.

This was in the early 1980s. TWA started operating jets some 20 years earlier. They were fortunate that an engine failure just above V1 didn’t bite them somewhere.

The solution was still the lousy +/- 300’ wide corridor. No one does that these days. Instead they use AC 120-91.