Lubricants and aviation fuels – property priorities For aircraft engines and aviation service, which of the following choices reflects the preferred property direction among the listed options?

Introduction / Context:
Aviation places stringent demands on both fuels and lubricants due to wide ambient temperatures and critical reliability. While many properties matter, the options here contrast two lube-oil viscosity-index choices and an obviously undesirable aviation-fuel attribute, testing whether you can identify the most suitable direction.

Given Data / Assumptions:

• Engines experience large temperature swings during climb and cruise.
• Viscosity index (VI) expresses viscosity–temperature stability of lubricants.
• For fuels, freezing point must be low, not high, to avoid wax crystallisation at altitude.

Concept / Approach:
A higher VI lubricant changes viscosity less with temperature, ensuring adequate film at high temperatures yet acceptable flow during cold starts at altitude. Conversely, a low VI oil thins excessively when hot and thickens excessively when cold. For fuels, the requirement is a low freezing point; therefore “high freezing point” is explicitly undesirable.

Step-by-Step Solution:

Verification / Alternative check:
OEM recommendations emphasise multigrade/high-VI oils for aircraft piston engines and stable viscosity behaviour for turbines (with specific specs), supporting the general preference for higher VI in challenging thermal profiles.

Why Other Options Are Wrong:

• Low VI oil: Poor viscosity stability across temperatures.
• High freezing point fuel: Risks waxing/flow blockage at altitude.
• None of these: Incorrect because one option is clearly preferred.

Common Pitfalls:
Assuming “thicker is always better”; in aviation, controlled viscosity across temperature is the priority, not maximum thickness at one point.

Final Answer:
High viscosity index lube oil