17
$\begingroup$

It makes sense to me, that under normal circumstances the engines shouldn´t be pushed to their maximum, to reduce wear and tear. But why isn´t TOGA thrust, which is the maximum selectable thrust setting, not corresponding to the maximum possible thrust, thereby 100% N1 or N2 whichever is the limiting factor.(In the picture TOGA is limited to 88.9% N1) picture

$\endgroup$
  • $\begingroup$ Does TOGA always correspond to maximum possible thrust? It makes sense for a go-around to use all the thrust you can get, because the aircraft is in a high drag, low lift configuration (flaps, slats, spoilers, landing gear, whatever else you've got to slow it down and force it toward the ground...) coming in for a landing and needing to regain altitude and speed as quickly as possible, but for take-off (other than the landing gear) the aircraft is in a low drag, high lift configuration already. Quite different scenarios... $\endgroup$ – a CVn Dec 13 '17 at 12:15
  • $\begingroup$ Imagine you are on your takoff run, and suddenly there is an obstruction down the runway, you are above V1 so you can´t reject the takoff so what you wanna do is get maximum thrust to make it over the obstacle. So what you, as the pilot would do, is set the thrust from FLX to TOGA to hopfully make it. So even if you are on takeoff there are cases where you want the maximum thrustsetting to really be the maximum the engine can give you $\endgroup$ – duesensegler Dec 13 '17 at 12:29
  • 4
    $\begingroup$ Depending on the situation max power is limited by Vmcg/Vmca. If an engine fails at 100% the rudder may not have enough authority to counter the asymmetric thrust. $\endgroup$ – TomMcW Dec 14 '17 at 1:14
  • $\begingroup$ @TomMcW, that appears to be the correct answer here. The accepted one almost certainly isn't. $\endgroup$ – Jan Hudec Dec 17 '17 at 8:34
  • $\begingroup$ Is TOGA thrust the same as cerification MAX TO thrust? $\endgroup$ – user7241 Dec 26 '17 at 7:55
14
$\begingroup$

TOGA thrust is not always 100% N1 (or N2), because TOGA thrust is actually not about delivering the absolute maximum thrust that can be provided. Rather, it's about delivering a guaranteed large amount of thrust, to ensure the performance of the aircraft is what the pilot expects, and what the flight manual states will occur. The level of thrust needs to be predictable. So it's got to be the same with a new aircraft, as one 5 years old (for example), and the engines are about to be overhauled.

To achieve this, engines are "flat rated". See What is a Flat Rated Engine?

enter image description here

To repeat that answer a bit, a flat rated engine, produces a constant, or the same amount of thrust, up to a certain ambient temperature. This is achieved by having a margin (gap) between the maximum thrust the engine provides, and what it can really produce. This also results in a temperature margin between the engines actual turbine temperature, and the red line temperature. This means as the engine degrades, it can increase its operating temperature without hitting the red line (until it has degraded a lot), and so still provide the same level of thrust.

When a gas turbine's engine control is designed to produce the same thrust at different ambient temperatures, it means N1 speed will be higher on a hot day, than a cold day. See how in the figure, for a constant thrust, N1 is increasing as ambient temperature goes up? N1 has to go up as ambient temperature increases, because hotter air at the intake is less dense than colder air, so the engine needs to run at a faster RPM to make up for the lower density.

$\endgroup$
  • 1
    $\begingroup$ Flat rating is not to provide uniform performance—the performance calculation needs to be done before every take-off anyway—but to avoid exceeding other limiting factors inside the engine (since temperature is lower in that range, the limiting factor is likely pressure). $\endgroup$ – Jan Hudec Dec 17 '17 at 16:35
  • 1
    $\begingroup$ Flat rating is a responsibility of the FADEC. However, the pictured limit looks like a FMC one. That would most likely be due to Vmcg limitation that @TomMcW mentioned i nthe comment to the question. $\endgroup$ – Jan Hudec Dec 17 '17 at 16:37
  • 2
    $\begingroup$ @JanHudec, flat rating is to provide a TIT margin for deterioration. That causes engine performance to be independant of age, i.e. predictable. The pre take-off performance calc assumes this level of thrust - the calculation is not tailored to each aircraft (by registration number). If thrust needs to be reduced to avoid exceeding rudder authority, that is another (seperate, additional) factor. (As TomMcW says "depending on the situation") $\endgroup$ – Penguin Dec 18 '17 at 12:12
7
$\begingroup$

The N1 and N2 values are simply RPM speeds, expressed as percentage of some nominal value. (It’s easier to talk about 95.6% N1 than 10,250 N1 RPMs.) But thrust for modern jet engines, and limitations for that thrust, are defined by the engineers as actual thrust, which is not a value directly observed on the flight deck. And due to factors such as altitude and temperature, you don’t have a set relationship between N1 and thrust. The thrust you get at 100% N1 in mile-high Denver on a hot summer day, you may get at 90% N1 at a sea-level airport mid-winter.

So a performance computer does the math & tells you what the N1 value is, given the conditions here & now, that will give you the specified thrust.

There is nothing special, or necessarily limiting (in many engines, at least) about “100%” N1. It isn’t uncommon on the CFM-56 to see N1 values above 100% for max takeoff thrust. That 100% is simply an RPM setting that matches the chosen nominal value, so if TOGA thrust today is 100.2% N1, that’s what you’ll set. In the photo in the OP, there is a redline (actually red arc) on the N1 indicator, but it is well above 100%.

N2, being engine core speed, has even less relationship to thrust than N1 does.

$\endgroup$
  • $\begingroup$ This doesn´t explain why the TOGA thrust setting isn´t always the maximum N1 RPM. There might not be a set relationship between N1 and thrust, but the both are definitely proportional. $\endgroup$ – duesensegler Dec 17 '17 at 20:25
  • 2
    $\begingroup$ TOGA power could be defined in various ways... by RPM or EGT or EPR, OR by the actual thrust being produced. Modern airliners performance uses the latter definition, and the pilots set the N1 that will in these conditions produce that XX,XXX lbs of thrust. TOGA is not “the most power the engine can possibly produce” (i.e. maximum N1 or EGT or EPR), but rather a set value. Performance charts (and engine lifespan) work better with a set value, rather than always having the engine going to “giv’in ‘er all she’s got, Capt’n” power (see Scotty in Star Trek). $\endgroup$ – Ralph J Dec 17 '17 at 20:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.