Rotor disk (turbine and compressor disk)

01

Set the life limit and the three plans that protect it

14 CFR 33.70 requires, by an FAA-approved procedure, operating limitations giving the maximum allowable number of flight cycles for each engine life-limited part — rotor and major static structural parts whose primary failure is likely to cause a hazardous engine effect, including disks, spacers, hubs, shafts, high-pressure casings and non-redundant mounts. Integrity is established by an engineering plan (validated analysis, test or service experience of loads, material properties, environment and operating conditions, plus damage tolerance assessment of material, manufacturing and service-induced anomalies), a manufacturing plan that fixes the manufacturing constraints needed to produce the required attributes consistently, and a service management plan that sets maintenance processes and repair limits; the approved lives are published in the Airworthiness Limitations Section. [1]

For stationary turbines and expanders outside aviation, the same questions — approved life, controlled manufacture, defined repair limits — are the ones to put to the supplier and the machine's maker, even where no regulation sets them.

Give the supplier the machine and stage, the original part number and drawing, operating speed, temperatures and load cycles, material and forging specification, the life limit or life basis required, inspection and traceability requirements, the certifying authority or customer approvals, and any repair or rework limits.

02

Plan damage tolerance and inspection

FAA AC 33.14-1 (Damage Tolerance for High Energy Turbine Engine Rotors, change 1, 2017) gives an acceptable — not the only — means of showing compliance for titanium alloy high-energy rotating parts. It defines critical rotating parts as rotor structural parts such as disks, spools, spacers, hubs and shafts whose failure could be hazardous, and damage tolerance as the part of life management that recognises possible imperfections from material structure, processing, design, manufacture or usage and addresses them through fracture-resistant design, fracture mechanics, process control or non-destructive inspection. [2]

The AC explains that service experience shows material and manufacturing anomalies do occur and can degrade the integrity of high-energy rotors, that the conventional safe-life method assumes nominal material and manufacturing conditions, and that after the 1989 Sioux City accident the FAA asked engine makers to add damage tolerance to rotor life management. It defines inspection terms used in that process, including probability of detection (POD), focused and full-field inspection, and hard- and soft-time inspection intervals. [2]

03

Specify the blade-attachment slots and how they are machined

A 2023 review from Rzeszow University of Technology sets out the tolerances and machining routes for fir-tree slots in turbine aero-engine discs. It gives slot widths of 20 to 40 mm, shape accuracy of 5 to 25 µm, surface roughness of 0.8 to 1.25 µm, a minimum inside radius of 0.3 mm and two to seven pairs of protrusions, and states that disc manufacturing should reach Ra ≤ 0.8 µm with no cracks, discoloration or non-parent material permitted. [4]

The review groups metallurgical changes from slot machining, starting with surface micro-anomalies, which links slot finishing to the manufacturing-induced anomalies that FAA's damage tolerance guidance addresses. [4] [2]

Editorial calculation: a 5–25 µm shape accuracy on a 20–40 mm wide slot is 0.0125–0.125% of the slot width. Ask the disk supplier for slot form tolerance and roughness, the finishing route (broach, mill, grind or EDM), how surface integrity is checked after machining, and whether shot peening is applied and to what intensity. [4]

04

Learn from a disk failure: manufacturing anomaly and missed inspection

NTSB report AAR-98/01 investigated the 6 July 1996 uncontained failure of the left JT8D-200 engine on Delta Air Lines Flight 1288, an MD-88, at Pensacola. The front compressor fan hub, a Ti-6Al-4V forging holding 34 fan blades, fractured from a fatigue crack at one of 24 tierod holes (0.5175 in diameter, 2.91 in deep). The Board found the crack began in altered microstructure created during hole drilling at manufacture and was not detected by Delta's fluorescent penetrant inspection (FPI). [3]

The report cites the engine maker's testimony that the 20,000-cycle limit was based on testing showing no more than 1 in 1,000 hubs would have a minute crack indication at that life, and lists insufficient redundancy in the in-service inspection programme as a contributing factor. This is the failure mode the life-limit, manufacturing and in-service inspection plans described above are meant to prevent. [3] [1] [2]

Editorial calculation: the hub failed at 13,835 of its 20,000 allowed cycles (about 69% of life), and 1,142 cycles after an inspection that should have found a crack about 13 times the 0.10 in minimum detection length.

Ask the disk supplier for the hole-making process controls and post-machining inspection of bores and holes, and agree in-service inspection methods and intervals with redundancy for critical features.

05

Find rotor disk manufacturers

The companies in the manufacturer list below show this product on their own websites. Confirm the legal seller, factory and scope for your order.

Ask any supplier for the material and melting route, forging and heat-treatment records, mechanical test results, ultrasonic and surface inspection methods and their detection capability, dimensional inspection of slots and bores, traceability, the life basis or approval held for the part, and their repair or replacement capability.

06

Trace what each part does

This is a functional reference built from the sources cited; it is not one maker's specification.

07

Fill the specification fields with values for your order

A rotor disk quotation needs values for your machine. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

08

Send the machine, part and life data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Machine: [type; stage]
Part: [number; drawing]
Duty: [speed; temperature; cycles]
Material: [alloy; melt; forging]
Life: [approved life or basis]
Inspection: [methods; POD; records]
Approvals: [authority; customer]
Quantity: [disks]

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