I suppose I should add: If you don’t want to tare the altimeter, then you should consider that the previous pilot probably did. So you have to check it and adjust as needed. :smack:
ETA: Although, if you’ve been carefully taught (as I was), you can live without it. On my second solo, the needle got stuck and I noticed halfway through downwind. Some good training on the part of my instructor saved the day!
For example, modern motorcycles have a carburetor or direct fuel injection.
Going from Galveston to the top of Pikes peak with a carburetor requires re-jetting the carb as you move higher for the best power and to prevent way over rich mixtures. Same with simple aircraft but since they do it almost every flight, they are equipped with a mixture control.
With direct fuel injection, the brain box does it for you in the delivery system. New cars can go from a cold day in Galveston to a very hot day at Pikes Peak with no worry or adjustment from the driver. The FAA is not real fast and the demand is very low for little planes to be retrofitted and to do it to new airplanes requires Mega-Dollars for all the tests that would be required.
This is about small piston or old piston aircraft, J-3 Cub to Douglas A1 Skyraider stuff.
About quick turn around or get in and go:
While I was doing pipeline patrol, I used the same place many times ( 3-4 ) times a week and I trusted the lineman to properly fill and replace the caps on the fuel tanks.
95% of the days, I could stuff the old 150 with manual 40° flaps onto the straight taxiway to the gas pumps. I would shut down, go in the office & wait for the fuel numbers to be called in. I would pay and head to the airplane. While I was doing that, the line man was double checking his work.
I would go clockwise around the plane hitting the drains and climb into the cockpit. I always faced the plane back the way I came so I could hit the master switch & mag switch, shoving the mixture in and steering with my feet to finish doing the seat belt. About then was the time to grab 40° of flaps and I would be off the ground before I got to the runway.
From when I touched the first fuel drain to when I broke ground, I could do it in under 15 seconds, just barely, but I tried to time it a few times and that seemed the best I could do. 
Other aircraft around, wind direction a few times, a different lineman other than the one I trusted and knew what I wanted, those things would slow me way down. But, small place off the beaten path, it was very uncommon for that to happen.
Years later, while flying the C-310, sitting at 12, to 17K above ground many times, I would dive for the place that I got better and quicker service even if it was a bit farther away. A slow refill burned more sun angle and we needed to be on the flight lines as much as we could.
After I left, they finally added the extra tanks that I had been telling them for 10 years would up the daily miles on the job which = greater profits. After I LEFT !!! The bums… Grrrrrrrr :smack:
I think you may be overthinking the UI thing here a little. I guess the only real requirement is to not have too much “dead zone” but in general the pilot moves the thrust levers to whatever position they need to get the result they want, they wouldn’t know if that position was 50% TL movement or 60%, they just know it’s somewhere between closed and full open. Another requirement is to have a reliable idle position. If you move the thrust levers back to the idle stop then you want it to be an accurate idle, to low and the engines might bog down and flame out, too high and you are forced to carry excess thrust during descent and landing.
The turbine (whether it be prop or jet) engines I’ve operated have had excess power at the “open” end of the throttle/ power lever / thrust lever range, that is, in all normal operating cases you never move the thrust levers fully forward because if you do you get thrust or power in excess of the engines limits. Setting take-off thrust is a process of fine tuning the thrust lever position to get the thrust you require for that particular take-off, no more, no less. If you have a FADEC and/or an auto-throttle then the fine tuning is handled for you, but the principle is the same.
From a piloting point of view though, you probably have some numbers in your head that will give you an appropriate power setting for various phases of flight. The actual thrust lever position is not one of those things you remember, you just move the lever until you get the numbers you want. As long as the range of movement of the thrust levers is useful then it will work fine.
As noted by LSL, this already happens and is called trend monitoring. We do a manual trend each flight in our company where we note all of the engine parameters in the cruise, but there is also an automatic trend that is sent to the company post flight.
What trend monitoring doesn’t do is monitor engine limits right here and now and stop an engine failure from occurring now, that’s up to us. Even for a fairly automated start with a FADEC equipped engine you still need to closely monitor the start temps for signs of a hot start, failure to light up, etc so that you can step in if required. One of the big ones to watch out for is a failure of the FADEC itself during a start. In my type that requires us to abort the start. On a four engined aeroplane doing four sectors a day there is a danger of becoming a bit complacent with the engine starts as they are very reliable. Back when I was flying a twin engined aeroplane doing 9 - 10 sectors per MONTH it was easier to give engine starts the respect they deserve.
That is standard procedure for us. The flaps aren’t set until we have taxied clear of the manoeuvring area. Then flaps are set, trims checked, radar turned on, and the before take-off checklist done while taxiing.
One of those things that is type specific I suspect. We can’t start any more than one engine at a time, and if we could somehow trick the system into letting us do it I think the APU would fail. Our starts are done using electrical power and it puts a fair bit of load on the APU.
Our engine starts take about 30 seconds each plus another 20 seconds or so to check the engine is stable, turn the engine anti-ice off, and select the next engine for starting. If we are in a “power out” bay and therefore need all engines running prior to moving and recording our departure time, I know I need about 4 minutes to have all engines running, say thanks to the engineer, and complete the after start checklist. If we are not close to starting engines by 4 minutes prior to departure time then we will be late.
We need 3 minutes at ground idle between starting and applying take-off power. It is very rare that we would be in danger of breaking that limit though.
Do you have an airspeed derivative indicator? If your takeoff speed ground-speed for this particular aircraft with this particular load and a particular wind condition is 100 knots, and you know the distance to the end of the runway, you can calculate the rate of acceleration needed to take off before you reach the end. (or better still, only use 60% of the total pavement leaving room for an abort)
So if you need to gain 5 knots/second^2, do you just adjust the throttle lever during the takeoff roll until it reads 5 knots being added every second?
The original microsoft flight simulator only had a gauge that measured your current speed, don’t remember an indicator for how fast you were picking up speed.
And I just realized there’s another problem with my “airspeed derivative” indicator. Since kinetic energy is proportional to velocity squared, at a given engine power setting you will gain speed more slowly during the end of your takeoff roll than at the beginning.
Also, drag increases as the aircraft goes faster and since Fnet = (thrust-drag), and the mass of the aircraft is constant, acceleration drops off.
On the other hand, as you get higher airspeed the amount of air available to your jet engines also increases, so…
This is vastly more complex a problem than it appears at first glance. Go figure.
So how do pilots find a throttle setting that will result in them lifting off the ground with plenty of runway to spare, consistently, under real world conditions including wind and aircraft mass differences?
In the video games you just firewall the throttle and off you go, but in the real world where you have to pay for engine maintenance bills, I take it you want to use a throttle setting that doesn’t push the engine any harder than necessary (but still results in you leaving the runway with plenty of room for an abort)
Ground speed is never necessary to take off. In fact, you can theoretically take off with 0 ground speed. You can theoretically fly with -10 ground speed. Because it’s airspeed that influences the wings to create lift, which it completely independent of the speed over the ground.
Here’s a video of a Piper Cub flying backwards in a strong wind.
Duh, but ground-speed determines how much runway you have left. So what you’ve done in this situation Vground = Vair_stall + -Vwind (vector sums). That tells you how fast under the current wind conditions you need to be traveling along the ground to take off, and that in turn lets you calculate how fast you need to accelerate along the ground in order to take off with a given amount of runway remaining.
Airspeed determines how much runway you have left.
No it doesn’t, you could be moving at 0 m/s along the ground and lift off the ground if the aircraft is very light and the wind is strong enough. You have a positive air speed and an infinite amount of time before you reach the end of the runway.
With ground-speed, you use the equation d = d0 + v*t + 1/2 (A * t^2). Velocity is your V_ground, d0 is your initial position on the runway, d is the distance along the runway you want to lift off at, and you calculate acceleration by also using the formula v = v0 + at, where V is your ground speed needed to lift off. 2 equations, 2 unknowns, it tells you the acceleration (relative to ground speed, again) needed to lift off the runway before you hit a given point on it. Note that if the wind shifts on you during the takeoff roll this changes the answer.
Never mind. This is going no where.
? I have quoted basic newtonian mechanics and vector math. Where it’s going is you are factually, objectively wrong and are not decent enough to admit it. If I’m incorrect, show your work, I did.
Basically, you look at your takeoff performance chart. In the linked case, a Cessna 172 at 2,300 pounds at a pressure altitude of 3,000 feet and a temperature of 20ºC (68ºF) with no wind needs 1,100 feet of runway to take off and an additional 870 feet after it leaves the ground to clear a 50-foot obstacle.
ETA:
Pilots don’t need Newtonian mechanics and vector math. That work has already been done for them and published in the charts.
ETA 2:
Morgenstern is correct in practice. The chart I linked earlier has notes on how to account for headwinds and tailwinds, as well as different runway conditions.
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These charts are at full throttle. The question was how to optimize your thrust to take off at a given position on the runway while using as little throttle as possible.
Why would you want to use less than full throttle? The powerplants are rated for it. You might have a ‘maximum continuous power’ time limit, but it’s plenty of time to get off of the ground. Another reason to throttle back is for noise abatement, but this is after you’ve taken off. If you’re concerned about shortening the life of your (piston) engine, one thing you can do is advance the throttle slowly. For example, the 1968 Cessna 182 Skylane POH advises two seconds from idle to full throttle. If you’re operating from a shorter runway, there’s no law that says you have to be moving forward before you’re at full power.
This is for commercial jets, something mentioned earlier in this very thread. The engines need less expensive repairs if they are not pushed to their limits as often (also they consume less fuel)
Since it depends on actual performance and wind conditions during the actual takeoff roll, it sounds like you would need to adjust throttle position during the actual roll to get liftoff at a given distance without any more throttle than necessary.
In new and old jets we have printed tables or special purpose slide rules or algorithms in the company computer and in the aircraft computers which determines the maximum permissible power output given atmospheric pressure and temperature. Before pushback and then again during taxi out we input the local physical conditions and obtain the maximum safe engine power settings. Then …
In old jets on takeoff we advance the throttles to the pre-computed setting derived from the sources above. This will be less than full mechanical travel of the physical knobs. And yes, you can overtemp, damage, or even detonate an engine by simply stuffing the throttles forward to the stops.
In new jets on takeoff we advance the throttles to the pre-computed setting derived from the sources above. This will be less than full mechanical travel of the physical knobs. But there’s a computerized limiter inside the engine’s fuel control (equivalent to automotive fuel injection computer) that will prevent over temping or over pressurizing the engine. So after you’ve set the maximum permissible power the remaining couple of inches of throttle travel don’t do anything.
A few modern engines have an extra “emergency power” range which will allow the normal maximums to be exceeded for at least a couple of minutes by going full forward with the knobs. This is actually an example of the engines being “detuned” n normal use: They’re limited to less than true physical maximum power for daily use for life extension (read as cost-savings) reasons, with the willingness to bypass that lower limit when truly necessary for survival.
Not only does a piston aircraft pilot select full power for takeoff, but he HOLDS the throttle in with one hand to insure it doesn’t creep out (reduce) while climbing.
As Johnny was saying in small piston airplanes you always take off with maximum available power. The reasons why get a bit deep & I’ll let the light plane folks explain if they can. I can come back to it later if nobody is game.
In airline jets the situation is quite different. We rarely use maximum power to take off. Instead we tailor the power to the minimum necessary (defined below) to hoist the weight we have from the runway we’re using under the atmospheric conditions present.
There are a host of factors which affect the takeoff power requirement. We have a whole department that does nothing but evaluate runways all over the world and maintain the data tables which drive these decisions.
Ultimately we need to be able to:
You can clearly see how a lower thrust setting makes each of those maneuvers have less margin versus a higher power setting.
You can also see where a headwind, a cold day, a high atmospheric pressure, a sea-level airport, a long runway, a level or downsloping runway, flat nearby terrain with no obstacles, or a light load would be helpful.
Conversely a tailwind, a hot day, a low atmospheric pressure, a high altitude airport, a short runway, an uphill runway, nearby hills, mountains, buildings, antennas, etc, a heavy load, or a wet or snowy runway would be unhelpful.
So how do we decide? …
In effect we iterate a calculation backwards starting with current conditions and full power and determine whether we can accomplish all 3 takeoff criteria safely. If not, it’s time for another long-winded post by me. 
If we can get off the ground under maximum power, we back off the power a smidgen & recalculate. Iterate until we first can’t make one of the 3 success criteria. Then add back a smidgen of power & we’re good to go.
As a practical matter all of that is pre-computed so the manual process is done by consulting a data table for the particular airport & runway and entering it with the various weather variables to determine what power setting meets the most restrictive of the 3 safety criteria.
Typically this is all handled automatically by our HQ computer systems, and we’re responsible to ensure the weather inputs used match current reality, verify the reasonableness of the results, ensure the settings are correctly transferred into the aircraft, and then bet all our lives on it actually being correct.
And yes, there are safety margins built into the three takeoff criteria. But less than you might imagine.
To answer the questions somebody is sure to ask: Why use less than full thrust if it represents an erosion in performance and in some sense an erosion of safety margin?
The answer is three-fold. 1) Less than full thrust is much quieter for the neighbors. Noise regulations have real teeth worldwide. Billions of dollars of perfectly good airplanes have been scrapped because they’re noisier than folks living near airports are willing to live with. 2) Less than full thrust makes engines last longer, saving money. Which means lower fares, which makes customers happier. 3) Engines almost never fail mechanically except at maximum thrust which is maximum thermal, centrifugal, vibrational, etc., stress. About 98% of the industry-wide “kablooey” engine failures happen on < 10% of takeoffs done at full thrust. Kablooey’s are real rare. By avoiding max thrust we make them much rarer yet.
You could adjust this in realtime. Aircraft presumably has wheel sensors and other ways to measure ground speed. It has air speed sensors - those infamous pitot tubes, so you don’t have to rely on weather reports for wind from the tower, you can measure the wind on your wings right now.
It knows how heavy it is, and it knows the barometric pressure, and it knows how far you want your takeoff roll to be.
As you are rolling down the runway, the computer could notice that the indicated airspeed is not as high as expected relative to the amount of ground covered (that’s why you need to know ground speed accurately) because the wind decided to shift right before you started the run. It might also notice that barometric pressure is a little low, etc.
It could realtime recalculate if you need to increase power, and nudge power up a bit to make sure you’ll meet your target. The calculations are involved are too much for human judgement, however. (sure a salty airline pilot could notice that he’s going a little slow halfway down the runway and nudge up the power, but that’s not as accurate as a computer figuring this out in the first 10% of the takeoff roll)
