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Not full power, just enough to keep the plane moving at 4km altitude.

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    $\begingroup$ This borderline qualifies as a valid question, but please add some context. Are you referencing a specific event? $\endgroup$
    – Therac
    Dec 30, 2020 at 17:17

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The QRH (Quick Reference Handbook) contains a few tables for flight with unreliable airspeed. The one for holding comes closest to the situation you asked about:

Holding Table
(Boeing 747-400 QRH 20.2 - Performance Inflight - CF6 Engines)

As you can see, holding at 10000 ft altitude requires between 59.7% and 77.4% N1 depending on weight. You asked about 4 km, which is about 13000 ft, so a little more will be required.

There is also a table for cruise speed at various altitudes:

Cruise Table
(Boeing 747-400 QRH 20.1 - Performance Inflight - CF6 Engines)

You can extrapolate the values shown here between 10000 and 20000 ft altitude to get the required thrust at 4 km. Note however, that the speed flown here is higher (290 KIAS or Mach 0.84) than the one required for maintaining altitude.

If you combine the information from both tables, you should get a good idea of the required N1 in your situation. The number will however strongly depend on the weight of the aircraft.

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    $\begingroup$ Minimum thrust is at vbg, does the qrh have a section on that as well? Might be interesting to compare to the speeds given here. $\endgroup$
    – Sanchises
    Dec 30, 2020 at 16:14
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    $\begingroup$ @Sanchises No, not as far as I could find. Vref30+80 (reference speed for flaps 30° + 80 kt) should be relatively close to the safe maneuvering speed in clean configuration. It is definitely possible to fly slower, but probably not that much. $\endgroup$
    – Bianfable
    Dec 30, 2020 at 16:39
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    $\begingroup$ There should be min drag speed tables which represent best L/D for cruise. They may not be in the QRH however. If not, I'd look for the driftdown tables which are best L/D for engine out descent. $\endgroup$
    – John K
    Dec 30, 2020 at 18:18
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    $\begingroup$ @JohnK Yes, I saw the driftdown tables, but they don't go down to 13000 ft. With just one engine inop even at the heaviest weight, the driftdown altitude is still 23000 ft (at ISA+15). The driftdown speed would be 309 KIAS in that case. $\endgroup$
    – Bianfable
    Dec 31, 2020 at 11:17
  • $\begingroup$ @Bianfable could you help me and say how much steam and at what pressure is required to have 2KN of thrust for 20 seconds? I post it as a new question but was closed and said that not valid question for this forum.Thanks $\endgroup$
    – Thomas
    Jan 8, 2021 at 12:36
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An rough estimation follows: in s/l flight, lift = weight and thrust = drag. If we assume a mass of 350000 kg and an L/D of 15, the thrust will be (350000 · 9,8)/15 = 228700 newton...

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  • $\begingroup$ This 220kn (approxi) is around 15~25% of total thrust? $\endgroup$
    – Thomas
    Dec 31, 2020 at 17:41

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