The Hydromechanical Fuel Control Unit of the Solar T62-T2A1: Principles of Speed and Power Management
A Technical Summary for Maintenance Personnel
by David Botich
Purpose and Scope
This paper describes how the hydromechanical Fuel Control Unit (FCU) on the Solar T62-T2A1 single-spool, direct-drive gas turbine manages engine speed and power output, from cranking through governed steady-state operation. It is written for a mechanic with broad machinery experience but no prior exposure to this specific fuel control architecture. The goal is to trace the chain of cause and effect through each stage of the system, so that the interrelationships between components — not just their individual functions — are clear.
This document was assembled from a combination of the engine’s technical manual (TM 55-2835-204-24, Figure 1-4, Fuel System Schematic), the Depot Maintenance Work Requirements overhaul manual (DMWR 55-2835-100, Chapter 6, Acceleration Control Adjustment and Test Requirements), direct physical inspection and measurement of a disassembled unit, and reasoned mechanical analysis. Where a conclusion rests on inference rather than confirmed documentation, it is noted as such. This paper is not a substitute for approved maintenance data, rigging procedures, or manufacturer specifications, and should not be used as the sole basis for adjustment or repair decisions.
1. Engine Context
The T62-T2A1 is a single-spool, direct-drive turbine — the compressor, turbine wheel, and output shaft all rotate as one assembly at rated speed (100% N = 56,700 RPM). There is no free power turbine and no pilot-operated throttle. This has one critical consequence for everything that follows: shaft speed (N) is a direct, real-time indicator of the balance between turbine power produced and load demanded. There is no mechanism to decouple output speed from gas generator speed the way a free-turbine engine can. Any imbalance between available fuel/power and commanded load shows up immediately as a change in N.
The FCU’s entire job is to keep N at its governed setpoint by continuously adjusting fuel flow — and to do so safely, without overfueling (overtemp, surge) or underfueling (flameout, hung start) at any point in the operating envelope.
2. Overall Fuel Path
Fuel moves through the system in this order:
Fuel tank → gear-type fuel pump → filtration/relief → primary (governor) metering valve → differential pressure (ΔP) regulating valve → injectors
Two additional subsystems act on this path rather than being part of the straight-through flow: the altitude/start compensator, which physically forces the ΔP valve open under certain conditions, and the minimum fuel flow orifice, which provides a guaranteed fuel path independent of the primary metering valve’s position.
Each stage is described below in the order fuel encounters it.
3. Pump Section
A gear-type positive-displacement pump draws fuel through an inlet filter and delivers it through an outlet filter toward the governor/metering section. A relief valve protects the pump section from over-pressure by recirculating excess flow internally when downstream demand is low — this is internal bypass, not a return path to the aircraft fuel tank, and does not affect total fuel consumption measurements taken upstream of the FCU. A fuel pressure switch monitors delivery pressure for cockpit/monitoring purposes.
Pump output feeds a regulated high-pressure rail (approximately 300 psi in this installation) that becomes the reference pressure for the ΔP regulating valve described in Section 5.
4. The Governor and Primary Metering Valve
This is the heart of the speed-governing function, and its internal geometry is worth understanding in detail because it is easy to misread from the outside.
4.1 Mechanical Arrangement
A flyweight-type governor is mechanically driven off the engine’s accessory gearbox, so its flyweights spin at a speed directly proportional to N. As N rises, centrifugal force on the flyweights increases, driving an inner metering tube axially against the resistance of a governor control spring. The spring’s preload — and therefore the N at which flyweight force and spring force balance — is set by the Rated Speed Adjusting Screw. Turning this screw in (clockwise) compresses the spring further, requiring higher N to reach balance, which raises the governed speed setpoint. This is a standard flyweight-governor arrangement, directly analogous to mechanical governors used on many other constant-speed machinery applications.
The inner tube rides inside an outer metering tube (valve body). The axial position of the outer tube — not the inner tube — is set by the Acceleration Adjustment Screw.
4.2 The Metering Geometry
Physical measurement of the unit revealed the following:
The inner tube has six small feed holes, approximately 0.010 inch in diameter, arranged around an annular undercut groove machined into the tube — 0.030 inch long axially, 0.020 inch deep, with sharp (non-radiused) edges.
The outer tube has two fixed metering ports, approximately 0.015 inch in diameter, drilled directly through the tube wall with no chamfer or relief.
Because the six inner holes feed a continuous circumferential groove, their exact rotational alignment with the two outer ports is irrelevant — fuel reaches the groove from any of the six holes and is available at any point around the circumference. The actual metering restriction is the sharp step where the undercut groove ends, sliding axially past the two fixed 0.015-inch outer ports as the inner tube moves.
Because the groove (0.030 in.) is longer than the port diameter (0.015 in.), there are effectively three phases of inner-tube travel relative to a given outer port: 1. Fully closed — port sits entirely outside the groove, land blocks it completely. 2. Transition (metering) band — roughly one port diameter (≈0.015 in.) of travel, during which the groove’s edge progressively uncovers the port. Flow area — and therefore fuel flow — varies continuously through this band, even though the total travel involved is small enough to appear “on/off” to the eye without close measurement. 3. Fully open plateau — port sits entirely within the groove; flow is limited only by the port’s own area, roughly constant across this portion of travel.
This is functionally a miniature spool valve, and it provides genuine, continuous, proportional metering — not a binary on/off valve, despite its very small physical dimensions.
4.3 The Role of the Acceleration Adjustment Screw
The Acceleration Adjustment Screw sets the axial position of the outer tube body against a spring, independent of governor/flyweight action on the inner tube. Shifting the outer body’s position changes where the fixed ports sit relative to the inner tube’s groove for any given governor (inner tube) position — effectively re-biasing the fuel schedule, particularly through the low-to-mid speed acceleration range before the governor reaches its rated-speed control band.
Per the engine manual’s guidance and confirmed operationally: turning this screw clockwise (in) enriches the acceleration schedule (uncovers more port area at a given speed) and is the correction for a hung start; turning it counterclockwise (out) leans the schedule and corrects a start overtemp, hot start. This is consistent with the physical geometry: advancing the outer body moves its fixed ports further into overlap with the groove at any given inner-tube position, exposing more flow area.
4.4 Function Summary
The primary metering valve’s job is to establish the desired open area — the fuel schedule — as a function of governed speed error (steady state) and the acceleration schedule (transient, low-speed operation). It does not, by itself, guarantee a specific flow rate, because flow through any orifice depends on both area and the pressure drop across it. That second variable is controlled by the next stage.
5. The Differential Pressure (ΔP) Regulating Valve
5.1 Why It Exists
The relationship governing flow through an orifice is approximately:
Q ≈ Cd · A · √(2ΔP/ρ)
Flow depends on both open area (A) and pressure differential (ΔP) across that area. If the primary metering valve’s area were the only variable in the system, flow would still swing with any change in pump output pressure or downstream demand — meaning valve position would no longer reliably correspond to fuel flow, defeating precise governing. The ΔP regulating valve exists to control the pressure drop across the primary metering valve’s ports, so that its position translates predictably into flow.
5.2 Mechanical Arrangement
The ΔP valve is a spring-loaded piston (fuel metering valve, with its matched sleeve) operating inside a matched cylinder — lapped parts to fine tolerance:
The top of the piston is exposed to governor-metered fuel pressure — the pressure downstream of the primary (governor) metering valve, which reflects how restricted that valve’s opening currently is. This is an enriching (piston-down) force.
The bottom of the piston is exposed to the high-pressure fuel rail (~300 psig), taken from pump discharge downstream of the pump’s regulating bypass valve. This is a leaning (piston-up) force.
The piston also carries the rod that extends up into the altitude/starting compensator assembly described in Section 6 — so the piston’s position is really the resolution of a larger, single-rod force balance that includes that assembly, not just the two fuel pressures acting on the piston itself.
As the piston moves, it itself acts as a second metering stage, porting fuel onward to the FCU outlet and the injectors. Piston up = leaner (less port overlap); piston down = richer (more port overlap).
5.3 Function — a Scheduled Differential, Not a Fixed One
Overhaul manual DMWR 55-2835-100 (Chapter 6, Acceleration Control Adjustment and Test Requirements) shows that the ΔP valve does not hold a single constant differential across the full operating range — it runs a differential pressure that is itself scheduled to rise with flow/load, set through two adjustable elements: a diaphragm preload adjusting screw and a diaphragm ratio adjusting screw. Per the overhaul manual’s acceptance criteria:
Operating point | Specified differential pressure |
Acceleration setting (2000 rpm, 0 psig CDP) | 8–15 psig |
Reference at zero CDP | ~12.9 psig nominal |
Maximum fuel flow (rated speed, 37 psig CDP) | 40–50 psig |
So rather than a single setpoint the piston holds constant everywhere, the differential itself is a ratio schedule — low at low flow/acceleration conditions, rising substantially toward high power. The purpose described in 5.1 still holds — the mechanism still exists so that metering valve position translates predictably into flow rather than drifting with pump output or downstream demand — but the reference it holds is a function of operating point, not a fixed number. This is a meaningful basis for validating field data: if a fuel-flow-meter/rail-pressure test shows the actual differential deviating from this schedule at a given flow condition, that points directly at the diaphragm/piston assembly rather than requiring guesswork.
In practical terms: the primary/governor valve decides how much the engine should be fueled; the ΔP valve ensures that decision is executed accurately, against a differential target that itself rises with commanded flow.
6. Altitude Compensation and Starting/CDP Enrichment
The following description was made possible by a hand sketch of the actual assembly and direct confirmation of port locations and fluid pressures. This unit consists of the altitude bellows and the CDP diaphragm that operates a lever that then drives a separate rod to the ΔP piston. The bellows-and-lever assembly, the diaphragm/spring/CDP chamber, and the ΔP piston all share one continuous rigid rod — this is a single, three-station force balance, not two mechanisms handing off to each other.
6.1 The Rod and Its Force Stations, Top to Bottom
Bellows and lever (top). The altitude bellows sits at one end of a pivoting lever, contacting it through a touch contact only (it can push, not pull). The lever’s other end bears on the top of the rod. When the bellows is advanced into contact (as ambient pressure drops with altitude), it pushes down on its end of the lever, which pivots the rod-side end up — a leaning force.
Diaphragm, spring, and CDP chamber (middle). The rod is in contact with the lever and the top of the lever is in contact with the bottom of the diaphragm. A spring bears on top of the diaphragm, and CDP is ported into the same chamber as the spring, above the diaphragm — so CDP pressure and the spring act together, both pushing the diaphragm, and with it the lever/rod, down. This is an enriching force, and it means rising CDP enriches the schedule rather than relieving it.
ΔP (fuel-regulating) piston (bottom). The same rod continues down and bears on the piston. Governor-metered fuel pressure on top of the piston adds to the down (enrich) side; the 300 psig high-pressure rail on the bottom of the piston adds to the up (lean) side.
Enriching (down) forces, all acting on the same rod: diaphragm spring preload, CDP. Leaning (up) forces, all acting on the same rod: the 300 psig rail pressure (on the piston), and the bellows-via-lever once advanced into contact.
The rod’s resting position — and therefore how far open the ΔP piston’s metering ports are — is wherever these forces balance. The CDP reinforces the spring.
6.2 Why This Is the Correct Behavior, Not a Contradiction
At first this seems backward — shouldn’t rising CDP, a proxy for engine speed and power demand, relieve a starting enrichment as the engine spools up? But the overhaul manual’s own acceptance data (Section 5.3) shows required fuel flow rising with CDP throughout the operating range — 28–30 phr at 0 psig CDP, up to 115–170 phr at 37 psig CDP. Fuel demand genuinely increases with CDP across the whole envelope, not just at the moment of start. A mechanism where CDP adds enrichment, opposed by rising rail pressure and (once engaged) the bellows, is exactly the right behavior for a continuously CDP-scheduled fuel curve — not a start-only override that has to “fade out.” The diaphragm/CDP chamber isn’t a start-enrichment device that hands off to the governor once spooled up; it’s a standing contributor to the fuel schedule for as long as the engine runs, working in concert with the ΔP piston’s own pressure balance and the governor’s metered-pressure input.
At zero psi CDP, only the spring is pushing the rod down, and (per direct bench test) that spring preload alone is enough to hold the diaphragm bottomed against its stop, with the rod fully down and the ΔP piston forced fully open — maximum enrichment, appropriate for a cold, unpressurized start. As fuel pressure comes up and the engine begins running, the 300 psig rail (on the piston) and CDP (on the diaphragm, reinforcing the spring) both come into play simultaneously, and the rod finds a new equilibrium reflecting the real-time balance of all these inputs together — not a simple one-directional handoff.
6.3 A Mechanical Explanation for the Original Hung-Start Symptom
This corrected model offers a genuine physical explanation for the hang-at-~70%-N-then-sudden-catch-up behavior that opened this investigation, without requiring a contamination or restriction theory. As N climbs through the acceleration range, CDP is rising continuously, and — since CDP reinforces the spring rather than opposing it — the enrich-side force on the rod is building the whole time. Nothing dramatic needs to happen until the accumulated enrich-side force (spring + CDP + governor pressure) finally exceeds the accumulated lean-side force (300 psig rail, plus the bellows once engaged) by enough to move the rod suddenly rather than smoothly — producing exactly the hang-then-rush pattern observed. Whether contamination in a passage is also contributing to where that threshold sits remains a fair question, but it is no longer necessary to explain the basic shape of the symptom.
6.4 The Bellows’ Role, Confirmed by Bench Test
The overhaul manual’s rigging procedure (DMWR 55-2835-100, paragraph 6-45) independently confirms the bellows’ direction of action:
Rigging sequence, as specified: 1. The bellows is first backed out of contact with the lever entirely, and the sea-level acceleration schedule is set using the diaphragm’s preload and ratio screws alone. 2. The bellows is then advanced back into light preloaded contact with the lever. 3. At a fixed bench condition (control running, 15 psig CDP), turning the bellows screw clockwise — advancing it further into the lever — measurably reduces fuel flow.
This matches the corrected model directly: the bellows, once in contact, adds a leaning force, opposing the enrich-side spring+CDP force on the rod. As ambient pressure drops with altitude, the bellows expands and pushes harder, trimming the schedule back — which is how the system keeps the altitude-corrected schedule to the specified +2 to +8 phr above sea level (paragraph 6-43(3)) rather than letting CDP’s continuously enriching influence run further unchecked as true air density falls.
This altitude bellows is the physical, real-time hydromechanical implementation of the same density-altitude correction principle (δ = inlet pressure ÷ 14.7) documented on the engine’s performance chart — the chart’s tabulated correction and the bellows’ mechanical action are addressing the same physics by different means (one on paper, one in hardware).
7. Minimum Fuel Flow Orifice
A small, fixed-size orifice provides a guaranteed minimum fuel path to the injectors regardless of the primary metering valve’s position. This prevents flame-out or an excessively lean condition at idle, during light throttle transients, or in the event the primary valve is fully closed, ensuring a floor of fuel delivery is always present.
8. The Full Chain of Events
Bringing the components together, a typical start-to-flight sequence proceeds as follows:
Cranking / Light-off. Starter motor turns the engine; N, CDP, and fuel pressure are all near zero. With no fuel pressure yet to move the ΔP piston and no CDP present, the diaphragm’s own spring preload holds the single shared rod fully down against its mechanical stop — forcing the ΔP piston open for a rich starting schedule, largely independent of the primary valve’s minimal opening at near-zero N. The minimum fuel flow orifice guarantees a baseline delivery even before the primary valve contributes meaningfully. The start fuel solenoid and start injector deliver this flow; ignition establishes combustion.
Acceleration (light-off through ~70–100% N). As N climbs, the flyweight governor begins driving the inner metering tube, and the primary valve’s effective opening — shaped by the Acceleration Adjustment Screw’s setting of the outer tube’s position — determines how much fuel is scheduled at each point along the ramp. Simultaneously, fuel pressure is building (300 psig rail, acting on the bottom of the ΔP piston) and CDP is rising (reinforcing the diaphragm spring, per Section 6.1) — both enrich- and lean-side forces are increasing together on the shared rod. The rod’s position reflects the running balance of all these inputs, and where that balance sits relative to the primary valve’s own governor-driven schedule determines the actual fuel delivered through this phase. As documented in Section 6.3, this accumulating, simultaneously-rising force balance is a plausible mechanical explanation for a hang-then-sudden-catch-up pattern during acceleration, without requiring a restriction or contamination.
Governed steady state. Once at 100% N with stable load, the governor spring/flyweight balance holds the primary valve at a position corresponding to steady fuel demand. The ΔP valve (and the diaphragm/CDP/bellows assembly sharing its rod) settles to the differential appropriate for that flow condition (per Section 5.3 — roughly 8–15 psig near low-CDP conditions, rising toward 40–50 psig near rated power/high CDP), translating governor valve position into a precise, repeatable flow rate. The altitude bellows continues to make small, ongoing corrections for ambient pressure, opposing the CDP-driven enrichment as needed to hold the altitude schedule within its specified band.
Load increase (e.g., collective pull on a helicopter installation). Increased torque demand tends to slow N slightly, registering as a speed error at the flyweight governor. The governor responds by repositioning the primary metering valve toward greater open area. The ΔP valve continuously re-balances to the higher differential appropriate for that greater flow, allowing more fuel through to the injectors. Power output and EGT rise as a result, and N recovers toward its setpoint — provided the additional fuel and resulting power are within what the engine can support thermally and aerodynamically at that ambient condition. If commanded load exceeds available capacity, N stabilizes at a new, lower value (droop) rather than continuing to recover.
9. Interpreting Droop: A Diagnostic Framework
Because N is a direct torque-balance indicator on this single-spool architecture, droop under load can arise from more than one cause, and distinguishing between them is largely a matter of watching EGT behavior alongside N:
EGT continues rising and settles near the engine’s maximum operating temperature line as N stabilizes low — the engine has reached a genuine thermal/aerodynamic power limit for that ambient condition. This is expected behavior, not a fault.
EGT rises but plateaus with meaningful thermal margin remaining (well below the maximum operating temperature) while N stabilizes low — this pattern suggests something in the fuel delivery chain is capping flow before the engine’s true thermal or aerodynamic limit is reached. Candidates include: the primary metering valve’s travel/port condition, the ΔP regulating valve’s piston/spring/metering holes, insufficient high-pressure rail pressure feeding the ΔP valve, or (as explored separately) a fuel with different flow characteristics than the unit was calibrated against.
A fuel flow meter placed in the supply line, combined with a pressure tap on the high-pressure rail, allows direct comparison between commanded fuel flow (inferred from the performance chart’s fuel-flow parameter at a given EGT, corrected for ambient δ) and actual measured flow at the same operating point. A shortfall between the two, with rail pressure confirmed steady, narrows the search to the metering stages themselves rather than the supply pressure or a true engine limit.
10. Acceptance Specifications (per DMWR 55-2835-100, Chapter 6)
These are the overhaul manual’s calibration targets for a serviceable unit. They are useful both as rigging references and as diagnostic checkpoints — actual field-measured values that deviate meaningfully from these targets point at a specific stage of the system rather than requiring guesswork.
Parameter | Specification |
Governor speed droop | 200–300 rpm change, for a nozzle flow change from 50 to 108 phr |
Governor hysteresis | ≤0.25% of governed speed, referenced to any fuel flow point on the rated droop line |
Relief valve setting | 365–385 psig |
Minimum fuel flow | 29–35 phr |
Maximum fuel flow | 115–170 phr (86 phr nominal set point, at 4200 rpm / 37 psig CDP) |
Acceleration set point | 28–30 phr at 2000 rpm, 0 psig CDP |
ΔP valve differential — acceleration condition | 8–15 psig (2000 rpm, 0 psig CDP) |
ΔP valve differential — nominal reference | ~12.9 psig at zero CDP |
ΔP valve differential — maximum fuel flow | 40–50 psig (rated speed, 37 psig CDP) |
Altitude schedule vs. sea-level schedule, same CDP | +2 to +8 phr higher at 20,000 ft |
Total control leakage (pump seal drain) | ≤4 drops/minute |
Leakage from ambient bellows port | Zero — any leakage here indicates a fault |
11. Summary of Adjustable Elements
Adjustment | Mechanism | Effect of Clockwise (in) rotation |
Rated Speed Adjusting Screw | Compresses governor control spring | Raises governed N setpoint |
Acceleration Adjustment Screw | Repositions outer metering tube body | Enriches acceleration fuel schedule (corrects hung start) |
Diaphragm Preload Adjusting Screw | Sets spring preload on the shared rod (CDP-reinforced) | Raises the enrich-side baseline (restores/raises differential pressure setting) |
Diaphragm Ratio Adjusting Screw | Sets how differential pressure scales with flow/CDP | Increases differential pressure (used to correct low max-flow) |
Altitude Compensator Bellows (screw/locknut) | Sets bellows preload/contact with lever | Advances bellows into lever, reducing flow at a given bench CDP |
Minimum Fuel Flow Adjusting Needle Valve | Sets fixed minimum-flow orifice restriction | Adjusts guaranteed baseline fuel flow |
12. Closing Notes
This description reflects a combination of documented reference material — the engine’s -24 technical manual and the DMWR 55-2835-100 overhaul manual’s Chapter 6 acceleration control adjustment and test procedures — and hands-on mechanical reasoning developed through direct inspection, disassembly, and a hand sketch of a specific unit. The overhaul manual confirmed several conclusions reached independently through teardown (acceleration screw direction, the bellows’ push-only, leaning action) and corrected one (the ΔP valve runs a scheduled differential rather than a fixed one). The hand sketch and bench observation of the diaphragm assembly corrected a more fundamental error in earlier drafts: the altitude/starting compensator, the diaphragm/CDP chamber, and the ΔP piston are not separate mechanisms handing off to one another, but three force stations on a single shared rod, with CDP reinforcing the enrich-side spring rather than opposing it. This kind of correction — arrived at through direct hands-on verification overturning a plausible-sounding but wrong inference — is exactly why the paper flags reasoning from documentation and geometry alone as provisional, and why bench and physical confirmation take precedence over it wherever the two conflict. Readers using this paper to guide adjustment, troubleshooting, or repair should cross-check conclusions against approved technical data for their specific engine and FCU serial configuration wherever possible, and should verify any adjustment’s effect on the bench before relying on it in flight.
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