hello,
those two all means very important and emergency. how can I tell them? can use mutually?
here is a sentence: De-icying was essential for all aircraft.
thank you for your answers 
I agree that you can use these interchangeably. By the way, De-icying should be de-icing.
Hi Diverhank,
Could you please tell us the meaning intensity of ‘important’, ‘essential’ and ‘crucial’? Does ‘essential’ sound more intense than ‘important’ and less than ‘crucial’?
Many thanks,
Nessie.
Hi Nessie,
It’s probably useful to put these adjectives in phrases/sentences to give you some idea of the meanings: It is important to read as much as you can in the language you are learning. It is essential to acquire a basic understanding of the grammar of the language. It is crucial that you make time either to listen to the language being spoken or ideally to try to speak it yourself.
Some ideas
Alan
To me, "essential"and “crucial” are about the same and they tend to carry more weight than “important”. The caveat: somebody will find exceptions to what I just wrote! 
I agree that they have the same level of importance/weight. But what I sense is that “essential” is on-going (essential vitamins and minerals), while “crucial” seems more immediate. That’s not to say they cannot be used either way – it’s just my overall sense.
The first 30 collocates of each in the BNC are:
1 ESSENTIAL PART 332
2 ESSENTIAL OILS 155
3 ESSENTIAL ELEMENT 91
4 ESSENTIAL FEATURE 77
5 ESSENTIAL INGREDIENT 77
6 ESSENTIAL OIL 74
7 ESSENTIAL SERVICES 63
8 ESSENTIAL ELEMENTS 58
9 ESSENTIAL FEATURES 57
10 ESSENTIAL POINT 56
11 ESSENTIAL INFORMATION 49
12 ESSENTIAL COMPONENT 47
13 ESSENTIAL DIFFERENCE 43
14 ESSENTIAL ROLE 43
15 ESSENTIAL CHARACTERISTICS 39
16 ESSENTIAL READING 39
17 ESSENTIAL ITEMS 35
18 ESSENTIAL PREREQUISITE 33
19 ESSENTIAL NATURE 32
20 ESSENTIAL TOOL 29
21 ESSENTIAL INGREDIENTS 28
22 ESSENTIAL CHARACTER 25
23 ESSENTIAL COMPONENTS 25
24 ESSENTIAL NUTRIENTS 25
25 ESSENTIAL REQUIREMENT 24
26 ESSENTIAL ASPECT 22
27 ESSENTIAL STEP 22
28 ESSENTIAL WORK 22
29 ESSENTIAL POINTS 21
30 ESSENTIAL SUPPLIES 21
…
1 CRUCIAL ROLE 169
2 CRUCIAL IMPORTANCE 135
3 CRUCIAL FACTOR 104
4 CRUCIAL PART 99
5 CRUCIAL POINT 74
6 CRUCIAL QUESTION 71
7 CRUCIAL ISSUE 60
8 CRUCIAL DIFFERENCE 51
9 CRUCIAL ELEMENT 48
10 CRUCIAL STAGE 44
11 CRUCIAL TIME 42
12 CRUCIAL MOMENT 41
13 CRUCIAL ISSUES 27
14 CRUCIAL STEP 26
15 CRUCIAL TEST 26
16 CRUCIAL PERIOD 24
17 CRUCIAL THING 22
18 CRUCIAL DISTINCTION 21
19 CRUCIAL DECISIONS 19
20 CRUCIAL QUESTIONS 19
21 CRUCIAL ASPECT 18
22 CRUCIAL DECISION 18
23 CRUCIAL EVIDENCE 18
24 CRUCIAL AREA 17
25 CRUCIAL VOTE 17
26 CRUCIAL DIFFERENCES 16
27 CRUCIAL MOMENTS 16
28 CRUCIAL AREAS 15
29 CRUCIAL FEATURE 15
30 CRUCIAL INFORMATION 15
You may have something there, Barb. I get the feeling of “essential = necessary/important” and “crucial = necessary/important and urgent”.
I agree. Barb_D made a very good point.