Spool up time

By lex, on November 8th, 2007

Turbojet engines like the one installed in the A-4 Skyhawk series typically run at about 33,000 RPM or so when firewalled – what we call “mil” power, or military rated thrust. They idle at around 61-64% of that speed on deck and a few percentage points higher in flight due to the “ram” effect of high velocity airflow – say 65-67%.

One of the curious features about turbojet engines is the non-linearity of thrust response. You get not much more than a hot breeze from idle to say 70% RPM, and just enough to taxi with between 70-80%. From 80-90% you get good, useable thrust for steady state cruise, but anytime you want to really accelerate the airplane you’re probably talking a setting of above 95% throttle. The net effect is that a disproportionate amount of thrust is generated within the relatively narrow band of 94-98% RPM (most engines are slightly “de-tuned” to ensure that they don’t over speed, so you rarely see 100% RPM on the gauge).

Not only is the thrust response quicker in the higher RPM band, engine response times decrease as well. Unlike a piston or radial engine prop aircraft, which gives a gratifyingly immediate and proportional response to a given throttle input, spooling a J52-P6A engine from ground idle to mil could take up to 13 seconds. This is mostly a function of shaft inertia and compressor lag, but also because fuel flow increases are carefully metered at lower power settings to avoid over temping an engine that’s not yet up to speed. While response times are slightly faster from flight idle there is still a power band on the back side of the curve that a survival-oriented pilot would avoid if he thought he might need more power in a hurry – when he was low and slow, for example. Or in the landing pattern.

Most of the time throttle response time isn’t an issue in carrier aircraft, since naval aviators are taught to fly constant angle-of-attack, power-on approaches. But sometimes it matters.

You: Why do you want to make my head hurt, Lex?

Me: Which I was just getting to the point, wasn’t I?


Besides being a sporty little crate in its own right, the TA-4J Skyhawk was the student naval aviator’s first exposure to anything like a “war bird” – although it was a training aircraft, it had many of the same characteristics as very high performance combat aircraft: Swept wing design, nimble aerodynamics, high subsonic top end and guns – actual guns! – in the wing. It was to drool.

As a single-engine airplane, the list of engine-related emergency procedures was also rather short and easy to memorize. If the only engine stopped making go-noises, you had some little time (depending on altitude) to try to get it going again, or – barring that – to go for a walk. There were also procedures for engines that, while they might have been running rough or had lost oil pressure, had not yet pooped out entirely – these were called precautionary approaches, or PAs.

One form of PA was flown from a position about a mile and a half or so abeam the intended point of landing on downwind (i.e., heading the opposite direction from the landing runway). Pattern altitude was around 2,000 feet above ground level and airspeed was whatever speed you could get out of a clean (wheels up) jet at 87-89% throttle – usually around 200 knots. Turned out that damaged A-4 engines had run for prolonged periods at that RPM without seizing, and it was a pretty much optimal ejection airspeed if you had to step out: Fast enough to trade a little speed for altitude, not so fast as to tear off important appendages in the wind blast.

Instead of the staid, power-on, exquisitely precise approach and landing that carrier pilots spend so much time practicing, the PA was an exciting, g-on, swooping turn to final that really got the heart beating – it felt more like a low-angle bombing or strafing run than a normal landing. Once safely established on final approach the pilot would throw the gear down, as well as some increment of flaps. Once the landing was assured he would retard the throttle to idle – in an actual emergency, he’d shut that baby off. Since he’d be coming in hot the pilot would have to point the nose well short of the touchdown zone and “flare” to land – another innovation for a carrier guy. Rather than on the rubber-blackened strip abeam the simulated carrier deck, anywhere on the first third of the runway was considered acceptable.

I had an early stage solo one day in training, and decided to practice my PAs – not because I needed the practice so much as that they were just fun to fly. The pattern was clear, so tower cleared me to zoom and swoop to my heart’s content. I was starting to innovate a little on my third approach – never a good idea for a student pilot with all of maybe 200 hours total time, but what did I know? – a little higher abeam, a little tighter on the runway prior to the approach turn. The combination found me turning to final carrying a whole bag o’ knots, maybe 220 indicated or so. I was also a bit high, so getting the landing gear and flaps down early seemed like a good idea. Approaching the fence I was still cooking on high heat, so I pulled the throttle to idle a bit earlier than usual and even feathered out the speed brakes.

Time came to flare and I was still going pretty fast, 165 knots maybe, rather than the target 120 kts. The excess airspeed caused me to float over the runway, delaying my touchdown. With the acceptable first third of the runway rapidly running out behind me, I finally – and somewhat ingraciously – manhandled the jet on deck. Which is when my trusty scooter – which apparently had thoughts of its own about high speed landings – bounced back into the air, 40 or 50 feet off the ground.

It is one thing, gentle reader, to be 40 or 50 feet off the ground after a touch-and-go landing, climbing out at full power and looking for traffic on downwind. It is another thing entirely to be hanging suspended in the air at 50 feet with the speedbrakes out and the engine at idle.

It’s quiet. Very, very quiet.

You can hear yourself squirm.

She was slowing down just fine at that point too, so much so that we were about to go below optimum angle of attack and come falling out of the sky like a turd off a tall moose. And I’d easily used up the approach end of the runway, leaving only the running off the other end portion in front of me – an academic distinction, given that I wasn’t entirely sure we wouldn’t break off important parts of the landing gear when the whole thing came clattering back to earth. And even if I wasn’t running out of prepared surface upon which to place the jet, there wasn’t altitude enough to dump the nose again for knots and still reset to a proper landing attitude.

Bit of a bind. And I’d been doing so well.

Well, there’s bad and there’s worse, and faced with no particularly good options but diminishing airspeed I rammed the throttle up against the mil stops, closing the speedbrake as I did. And sat there in the cramped and stultifying silence, waiting for the engine to spool back up from flight idle. Seconds can seem like years when the angle of attack gauge is tickling at stall, and the runway starts to fill up the windscreen. It can seem like an eternity.

A hideously. Quiet. Eternity.

But, because I’m here and not pushing sub-prime mortgage derivatives somewhere in Manhattan, you had to know that it all worked out for the best in the end. Just as I started to wonder how truly bad it was going to be, the engine caught up with my desperate need and I carefully rotated the jet to a fly-away attitude just as the main mounts kissed the runway about the midfield marker. I joined downwind set up for a normal approach to a full stop landing. Not much older, but quite a bit wiser.

I learned about flying from that.


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Filed under Best of Neptunus Lex, by lex, Uncategorized

4 responses to “Spool up time

  1. Have to leave a comment, Lex was of a generation that didn’t get to train in the T-2A. The “spool up time” thing.
    Which was something in the T-2A.
    Gotta refresh the brain cells here, the first T-2 in the Navy inventory was the “A” model, it was single engine. An engine of the earliest sort in the jet engine world, a centrifugal flow jet, much like the original Whittle engine. I’ll spare you all the details, suffice it to say that the T-2A engine had serious spool up time problems.
    Me, I was going through the basic jet training in the 70’s and all of my initial flights were in the T-2A, I had no frame of reference to compare the jet engine on that jet to any other.
    It was all I had.
    One would take the runway with all the checklists complete, and when cleared for takeoff you would jam the throttle to the stops and punch the clock.
    Then do something to pass the time. I used to joke about writing a letter home to mom, or something like that.
    The engine took a while to get from idle to max power.
    I believe if the time to get to max was around 14-16 seconds the engine was working properly. Not good if you were really high on approach and pulled the power to idle.
    My first flight in the 2 engine T-2C, with more thrust and near instant throttle response, was funny in retrospect. Cleared for takeoff, jammed the throttles to the stops and the jet was airborne while my head was still on the runway numbers waiting for the spool up. Not the engines, my brain.

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