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Eastman – SKYDROL-LD-4
Eastman - SKYDROL-LD-4
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Eastman SKYDROL® LD-4 Fire-Resistant Aviation Hydraulic Fluid

SKYDROL® LD-4 is a phosphate-ester based, fire-resistant aviation hydraulic fluid engineered for modern aircraft hydraulic systems operating under high pressure and extreme temperature conditions. Introduced in 1977, it was developed to address valve erosion, thermal instability, and deposit formation seen in earlier fluids, and has since become a benchmark Type IV hydraulic fluid in commercial and military aviation.

Designed for MRO teams, aerospace engineers, and procurement managers, SKYDROL LD-4 delivers excellent thermal stability, low density, and reliable contamination control while significantly reducing fire risk compared to mineral-oil based fluids. It is suitable for critical flight systems such as flight controls, landing gear actuation, and braking systems where fluid performance, cleanliness, and compliance to recognized aerospace standards are non-negotiable.

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SKYDROL® LD-4 Product Overview

SKYDROL® LD-4 is a fire-resistant aviation hydraulic fluid based on phosphate ester chemistry. It is engineered specifically for aircraft hydraulic systems where high temperature performance, fire resistance, and long-term system cleanliness are critical. Since its introduction in 1977, it has been widely adopted across commercial and military aviation and is regarded as a benchmark Type IV hydraulic fluid.

How It Works (High-Level Principle)

As a phosphate ester fluid, SKYDROL LD-4 resists ignition at temperatures where mineral oils would readily burn. Its molecular structure provides inherent thermal stability, while its formulation minimizes valve erosion and internal deposit formation within hydraulic components. The fluid maintains controlled viscosity across a wide temperature range, ensuring consistent hydraulic response from extreme cold conditions to high-temperature operation.

Why This Matters in Real Aircraft Operations

Aircraft hydraulic systems power safety-critical functions such as flight controls, landing gear actuation, and braking systems. Using a fire-resistant, thermally stable fluid like SKYDROL LD-4 reduces fire risk, helps protect precision components from wear, and supports reliable system performance in demanding environments such as high-altitude flight, rapid cycling operations, and hot climate conditions.

Technical Information

  • Fluid Type: Phosphate ester-based, Type IV aviation hydraulic fluid
  • Primary Function: Fire-resistant hydraulic medium for aircraft systems
  • Thermal Performance: Excellent real-world thermal stability with strong resistance to degradation
  • Valve Protection: Formulated to prevent valve erosion and internal deposits
  • Cleanliness Control: Designed to meet strict particle contamination limits (NAS 1638 / AS4059F)
  • Appearance: Clear, oily liquid with purple coloration
  • Moisture Control: Maximum allowable moisture content of 0.20%
  • Operating Behavior: Maintains stable viscosity across extreme temperatures

Technical Specifications

Parameter Typical Value / Limit Notes
Appearance Clear, oily liquid Visual
Color Purple (standard hue) Manufacturer standard
Acid Number 0.10 mg KOH/g (max) TDS limit
Autoignition Temperature 400°C (752°F) minimum ASTM D 2155
Flash Point (COC) 160°C (320°F) minimum Cleveland Open Cup
Fire Point (COC) 177°C (350°F) minimum Cleveland Open Cup
Pour Point -62°C (-80°F) maximum Low-temperature flow limit
Specific Gravity (25°C) 1.003 – 1.013 Density range
Viscosity @ 38°C (100°F) 10.65 – 11.65 cSt Normal operating viscosity
Viscosity @ 99°C (210°F) 3.66 – 4.00 cSt High-temperature stability
Viscosity @ -54°C (-65°F) 2000 cSt maximum Cold-start performance

Elemental Content (Maximum Limits):

  • Calcium: 10 ppm
  • Chlorine: 50 ppm
  • Potassium: 30 ppm
  • Sodium: 10 ppm
  • Sulfur: 1185–1540 ppm

Certifications & Standards

  • Recognized as a Type IV aviation hydraulic fluid
  • Specified under widely accepted SAE aerospace hydraulic fluid frameworks
  • Approved for use in aircraft systems designed for phosphate ester fluids

Applications

  • Commercial Aviation: Flight control systems, landing gear actuation, and aircraft braking systems where fire resistance is critical.
  • Military & Defense Aircraft (where approved): High-performance hydraulic systems operating under severe environmental and thermal conditions.
  • Aircraft MRO & Maintenance: Fluid top-ups, system servicing, and scheduled hydraulic fluid changes.
  • Ground Support Equipment (GSE): Compatible hydraulic systems that require phosphate ester, fire-resistant fluids.

How to Use (Practical Guidance)

  1. Confirm that the hydraulic system is designed or approved for phosphate ester fluids.
  2. Use only clean, dedicated transfer equipment to avoid cross-contamination.
  3. Ensure the system meets cleanliness requirements before adding or replacing fluid.
  4. Fill or top-up strictly according to the aircraft or OEM maintenance manual.
  5. Monitor moisture levels and fluid condition during routine service intervals.

Handling, Storage & Regulatory Notes

  • Store in sealed, original containers in a cool, dry area away from direct sunlight.
  • Minimize exposure to air and moisture, as phosphate ester fluids are moisture-sensitive.
  • Avoid contact with incompatible materials or mineral oils.
  • Use dedicated pumps, hoses, and containers for SKYDROL products.
  • Always consult the Safety Data Sheet (SDS) before handling.
  • Shelf life and batch details are available on product labeling and documentation.

Alternative Product Numbers & Codes

  • Eastman Product Code: 71093409 (SKYDROL® LD-4)
  • Fluid Category: Type IV aviation hydraulic fluid
  • Functional Class: Fire-resistant phosphate ester hydraulic fluid
  • Important Note: Not interchangeable with mineral-oil fluids such as MIL-PRF-5606.

Key Features & Benefits

  • Phosphate ester chemistry: Inherently fire-resistant, lowering ignition risk in high-temperature systems.
  • Low density formulation: Supports efficient hydraulic system response and performance.
  • Excellent thermal stability: Maintains fluid integrity under prolonged high-temperature operation.
  • Valve erosion prevention: Protects precision hydraulic components from premature wear.
  • Deposit control: Helps keep hydraulic systems cleaner over extended service intervals.
  • Wide temperature viscosity stability: Reliable performance from extreme cold to high heat.
  • Strict contamination limits: Supports compliance with aviation cleanliness standards (NAS 1638 / AS4059F).

Packaging & Commercial Details

  • Available Pack Sizes: Quart, 1-gallon, 5-gallon pail, and 55-gallon drum (subject to availability).
  • Supply Model: Quotation-based depending on quantity and destination.
  • Batch Traceability: Each shipment can be supported with technical documentation.

Shipping Information

Shipping for SKYDROL® LD-4 is arranged based on order quantity, packaging requirements, and delivery location. As this product is supplied on a quotation basis, shipping details are confirmed after order review and technical verification. Delivery timelines, packaging format, and logistics method are communicated at the time of quotation.

Our team works with approved logistics partners to ensure safe and compliant handling for industrial and aviation-related shipments. For shipping estimates and availability, please submit a Request a Quote. Our team will respond with complete commercial and delivery details.

Why Buy From Us

  • Aviation chemical supply expertise: Regular supply of critical hydraulic fluids to MROs and aerospace operators.
  • Batch documentation & traceability: Full technical records available for quality audits and maintenance logs.
  • Reliable availability: Consistent sourcing of SKYDROL LD-4 to minimize aircraft downtime.
  • Application knowledge: Clear understanding of compatibility, contamination control, and handling requirements for phosphate ester fluids.

Frequently Asked Questions

Q1. Can SKYDROL LD-4 be mixed with mineral-oil hydraulic fluids?
No. It is not compatible with mineral-oil fluids and requires a dedicated, cleaned system.

Q2. What happens if water contaminates the fluid?
Moisture reduces performance and must be controlled through monitoring and maintenance procedures.

Q3. Is this suitable for all aircraft hydraulic systems?
Only for systems specifically designed or approved for phosphate ester fluids.

Q4. How long can the fluid be stored after opening?
It should be used promptly and kept tightly sealed to prevent moisture absorption.

Q5. Why is the fluid purple?
The color is a manufacturer standard identifier and does not affect performance.

Q6. Is SKYDROL LD-4 fire-resistant?
Yes. Its phosphate ester chemistry provides significantly higher fire resistance than mineral oils.

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